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This chapter explains the features, technical details and syntaxes of the C++ programming language. I assume that you could write some simple programs. Otherwise, read 'Introduction To C++ Programming for Novices and First-time Programmers'.

To be a proficient programmer, you need to master two things: (1) the syntax of the programming language, and (2) the core libraries (i.e., API) associated with the language.

Introduction to C++

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C++ Standards

C++ is standardized as ISO/IEC 14882. Currently, there are two versions:

  1. C++98 (ISO/IEC 14882:1998): First standard version of C++.
  2. C++03 (ISO/IEC 14882:2003): minor 'bug-fix' to C++98 with no change to the language. Commonly refer to as C++98/C++03 or First C++ standard.
  3. C++11 (ISO/IEC 14882:2011): Second standard version of C++. Informally called C++0x, as it was expected to finalize in 200x but was not released until 2011. It adds some new features to the language; more significantly, it greatly extends the C++ standard library and standard template library (STL).
  4. C++14: Infomally called C++1y, is a small extension to C++11, with bug fixes and small improvement.
  5. C++17: informally called C++1z.
  6. C++2a: the next planned standard in 2020.
C++ Features
  1. C++ is C. C++ supports (almost) all the features of C. Like C, C++ allows programmers to manage the memory directly, so as to develop efficient programs.
  2. C++ is OO. C++ enhances the procedural-oriented C language with the object-oriented extension. The OO extension facilitates design, reuse and maintenance for complex software.
  3. Template C++. C++ introduces generic programming, via the so-called template. You can apply the same algorithm to different data types.
  4. STL. C++ provides a huge set of reusable standard libraries, in particular, the Standard Template Library (STL).
C++ Strength and Pitfall

C++ is a powerful language for high-performance applications, including writing operating systems and their subsystems, games and animation. C++ is also a complex and difficult programming language, which is really not meant for dummies. For example, to effectively use the C++ Standard Template Library (STL), you need to understand these difficult concepts: pointers, references, operator overloading and template, on top of the object-oriented programming concepts such as classes and objects, inheritance and polymorphism; and the traditional constructs such as decision and loop. C++ is performance centric. The C++ compiler does not issue warning/error message for many obvious programming mistakes, undefined and unspecified behaviors, such as array index out of range, using an uninitialized variable, etc, due to the focus on performance and efficiency rather than the ease of use - it assumes that those who choose to program in C++ are not dummies.

Basic Syntaxes

Revision

Below is a simple C++ program that illustrates the important programming constructs (sequential flow, while-loop, and if-else) and input/output. Read 'Introduction To C++ Programming for Novices and First-time Programmers' if you need help in understanding this program.

Program Notes

using namespace std;
The names cout and endl belong to the std namespace. They can be referenced via fully qualified namestd::cout and std::endl, or simply as cout and endl with a 'using namespace std;' statement. For simplicity, I shall use the latter approach in this section. I will discuss the significance later.

return 0;
The return value of 0 indicates normal termination; while non-zero (typically 1) indicates abnormal termination. C++ compiler will automatically insert a 'return 0;' at the end of the the main() function, thus, it statement can be omitted.

Instead of using numeric value of zero and non-zero, you can also use EXIT_SUCCESS or EXIT_FAILURE, which is defined in the cstdlib header (i.e., you need to '#include <cstdlib>'.

Comments

Comments are used to document and explain your codes and program logic. Comments are not programming statements and are ignored by the compiler, but they VERY IMPORTANT for providing documentation and explanation for others to understand your program (and also for yourself three days later).

There are two kinds of comments in C/C++:

  1. Multi-line Comment: begins with a /* and ends with a */, and can span several lines.
  2. End-of-line Comment: begins with // and lasts till the end of the current line.

You should use comments liberally to explain and document your codes. During program development, instead of deleting a chunk of statements permanently, you could comment-out these statements so that you could get them back later, if needed.

Statements and Blocks

Statement: A programming statement is the smallest independent unit in a program, just like a sentence in the English language. It performs a piece of programming action. A programming statement must be terminated by a semi-colon (;), just like an English sentence ends with a period. (Why not ends with a period like an english sentence? This is because period crashes with decimal point - it is hard for the dumb computer to differentiate between period and decimal point!)

For examples,

Block: A block (or a compound statement) is a group of statements surrounded by braces { }. All the statements inside the block is treated as one unit. Blocks are used as the body in constructs like function, if-else and loop, which may contain multiple statements but are treated as one unit. There is no need to put a semi-colon after the closing brace to end a complex statement. Empty block (without any statement) is permitted. For examples,

White Spaces and Formatting Source Codes

White Spaces: Blank, tab and new-line are collectively called white spaces. C++, like most of the computing languages, ignores extra white spaces. That is, multiple contiguous white spaces are treated as a single white space.

You need to use a white space to separate two keywords or tokens, e.g.,

Additional white spaces and extra lines are, however, ignored, e.g.,

Formatting Source Codes: As mentioned, extra white spaces are ignored and have no computational significance. However, proper indentation (with tabs and blanks) and extra empty lines greatly improves the readability of the program, which is extremely important for others (and yourself three days later) to understand your programs. For example, the following hello-world works, but can you understand the program?

Braces: Place the beginning brace at the end of the line, and align the ending brace with the start of the statement.

Indentation: Indent the body of a block by an extra 3 (or 4 spaces), according to its level.

For example,

Most IDEs (such as CodeBlocks, Eclipse and NetBeans) have a command to re-format your source code automatically.

Note: Traditional C-style formatting places the beginning and ending braces on the same column. For example,

Preprocessor Directives

C++ source code is pre-processed before it is compiled into object code (as illustrated).

A preprocessor directive, which begins with a # sign (such as #include, #define), tells the preprocessor to perform a certain action (such as including a header file, or performing text replacement), before compiling the source code into object code. Preprocessor directives are not programming statements, and therefore should NOT be terminated with a semi-colon. For example,

In almost all of the C++ programs, we use #include <iostream> to include the input/output stream library header into our program, so as to use the IO library function to carry out input/output operations (such as cin and cout).

More on preprocessor directives later.

Variables and Types

Variables

Computer programs manipulate (or process) data. A variable is used to store a piece of data for processing. It is called variable because you can change the value stored.

More precisely, a variable is a named storage location, that stores a value of a particular data type. In other words, a variable has a name, a type and stores a value.

  • A variable has a name (or identifier), e.g., radius, area, age, height. The name is needed to uniquely identify each variable, so as to assign a value to the variable (e.g., radius=1.2), and retrieve the value stored (e.g., area = radius*radius*3.1416).
  • A variable has a type. Examples of type are,
    • int: for integers (whole numbers) such as 123 and -456;
    • double: for floating-point or real numbers such as 3.1416, -55.66, having a decimal point and fractional part.
  • A variable can store a value of that particular type. It is important to take note that a variable in most programming languages is associated with a type, and can only store value of the particular type. For example, a int variable can store an integer value such as 123, but NOT real number such as 12.34, nor texts such as 'Hello'.
  • The concept of type was introduced into the early programming languages to simplify interpretation of data made up of 0s and 1s. The type determines the size and layout of the data, the range of its values, and the set of operations that can be applied.

The following diagram illustrates two types of variables: int and double. An int variable stores an integer (whole number). A double variable stores a real number.

Identifiers

An identifier is needed to name a variable (or any other entity such as a function or a class). C++ imposes the following rules on identifiers:

  • An identifier is a sequence of characters, of up to a certain length (compiler-dependent, typically 255 characters), comprising uppercase and lowercase letters (a-z, A-Z), digits (0-9), and underscore '_'.
  • White space (blank, tab, new-line) and other special characters (such as +, -, *, /, @, &, commas, etc.) are not allowed.
  • An identifier must begin with a letter or underscore. It cannot begin with a digit. Identifiers beginning with an underscore are typically reserved for system use.
  • An identifier cannot be a reserved keyword or a reserved literal (e.g.,int, double, if, else, for).
  • Identifiers are case-sensitive. A rose is NOT a Rose, and is NOT a ROSE.

Caution: Programmers don't use blank character in names. It is either not supported, or will pose you more challenges.

Variable Naming Convention

A variable name is a noun, or a noun phrase made up of several words. The first word is in lowercase, while the remaining words are initial-capitalized, with no spaces between words. For example, thefontSize, roomNumber, xMax, yMin, xTopLeft and thisIsAVeryLongVariableName. This convention is also known as camel-case.

Recommendations
  1. It is important to choose a name that is self-descriptive and closely reflects the meaning of the variable, e.g., numberOfStudents or numStudents.
  2. Do not use meaningless names like a, b, c, d, i, j, k, i1, j99.
  3. Avoid single-alphabet names, which is easier to type but often meaningless, unless they are common names like x, y, z for coordinates, i for index.
  4. It is perfectly okay to use long names of says 30 characters to make sure that the name accurately reflects its meaning!
  5. Use singular and plural nouns prudently to differentiate between singular and plural variables. For example, you may use the variable row to refer to a single row number and the variable rows to refer to many rows (such as an array of rows - to be discussed later).

Variable Declaration

To use a variable in your program, you need to first 'introduce' it by declaring its name and type, in one of the following syntaxes:

SyntaxExample

Example,

Take note that:

  • In C++, you need to declare the name of a variable before it can be used.
  • C++ is a 'strongly-type' language. A variable takes on a type. Once the type of a variable is declared, it can only store a value belonging to this particular type. For example, an int variable can hold only integer such as 123, and NOT floating-point number such as -2.17 or text string such as 'Hello'. The concept of type was introduced into the early programming languages to simplify interpretation of data made up of 0s and 1s. Knowing the type of a piece of data greatly simplifies its interpretation and processing.
  • Each variable can only be declared once.
  • In C++, you can declare a variable anywhere inside the program, as long as it is declared before used. (In C prior to C99, all the variables must be declared at the beginning of functions.) It is recommended that your declare a variable just before it is first used.
  • The type of a variable cannot be changed inside the program.
CAUTION: Uninitialized Variables

When a variable is declared, it contains garbage until you assign an initial value. It is important to take note that C/C++ does not issue any warning/error if you use a variable before initialize it - which certainly leads to some unexpected results. For example,

Constants (const)

Constants are non-modifiable variables, declared with keyword const. Their values cannot be changed during program execution. Also, const must be initialized during declaration. For examples:

Constant Naming Convention: Use uppercase words, joined with underscore. For example, MIN_VALUE, MAX_SIZE.

Expressions

An expression is a combination of operators (such as addition '+', subtraction '-', multiplication '*', division '/') and operands (variables or literal values), that can be evaluated to yield a single value of a certain type. For example,

Assignment (=)

An assignment statement:

  1. assigns a literal value (of the RHS) to a variable (of the LHS); or
  2. evaluates an expression (of the RHS) and assign the resultant value to a variable (of the LHS).

The RHS shall be a value; and the LHS shall be a variable (or memory address).

The syntax for assignment statement is:

SyntaxExample

The assignment statement should be interpreted this way: The expression on the right-hand-side (RHS) is first evaluated to produce a resultant value (called rvalue or right-value). The rvalue is then assigned to the variable on the left-hand-side (LHS) (or lvalue, which is a location that can hold a rvalue). Take note that you have to first evaluate the RHS, before assigning the resultant value to the LHS. For examples,

The symbol '=' is known as the assignment operator. The meaning of '=' in programming is different from Mathematics. It denotes assignment instead of equality. The RHS is a literal value; or an expression that evaluates to a value; while the LHS must be a variable. Note that x = x + 1 is valid (and often used) in programming. It evaluates x + 1 and assign the resultant value to the variable x. x = x + 1 illegal in Mathematics. While x + y = 1 is allowed in Mathematics, it is invalid in programming (because the LHS of an assignment statement must be a variable). Some programming languages use symbol ':=', '←', '->', or '→' as the assignment operator to avoid confusion with equality.

Fundamental Types

Integers: C++ supports these integer types: char, short, int, long, long long (in C++11) in a non-decreasing order of size. The actual size depends on the implementation. The integers (except char) are signed number (which can hold zero, positive and negative numbers). You could use the keyword unsigned [char short int long long long] to declare an unsigned integers (which can hold zero and positive numbers). There are a total 10 types of integers - signed unsigned combined with char short int long long long.

Characters: Characters (e.g., 'a', 'Z', '0', '9') are encoded in ASCII into integers, and kept in type char. For example, character '0' is 48 (decimal) or 30H (hexadecimal); character 'A' is 65 (decimal) or 41H (hexadecimal); character 'a' is 97 (decimal) or 61H (hexadecimal). Take note that the type char can be interpreted as character in ASCII code, or an 8-bit integer. Unlike int or long, which is signed, char could be signed or unsigned, depending on the implementation. You can use signed char or unsigned char to explicitly declare signed or unsigned char.

Floating-point Numbers: There are 3 floating point types: float, double and long double, for single, double and long double precision floating point numbers. float and double are represented as specified by IEEE 754 standard. A float can represent a number between ±1.40239846×10^-45 and ±3.40282347×10^38, approximated. A double can represented a number between ±4.94065645841246544×10^-324 and ±1.79769313486231570×10^308, approximated. Take note that not all real numbers can be represented by float and double, because there are infinite real numbers. Most of the values are approximated.

Boolean Numbers: A special type called bool (for boolean), which takes a value of either true or false.

The table below shows the typical size, minimum, maximum for the primitive types. Again, take note that the sizes are implementation dependent.

CategoryTypeDescriptionBytes
(Typical)
Minimum
(Typical)
Maximum
(Typical)
Integersint
(or signed int)
Signed integer (of at least 16 bits)4 (2)-21474836482147483647
unsigned intUnsigned integer (of at least 16 bits)4 (2)04294967295
charCharacter
(can be either signed or unsigned depends on implementation)
1
signed charCharacter or signed tiny integer
(guarantee to be signed)
1-128127
unsigned charCharacter or unsigned tiny integer
(guarantee to be unsigned)
10255
short
(or short int)
(or signed short)
(or signed short int)
Short signed integer (of at least 16 bits)2-3276832767
unsigned short
(or unsigned shot int)
Unsigned short integer (of at least 16 bits)2065535
long
(or long int)
(or signed long)
(or signed long int)
Long signed integer (of at least 32 bits)4 (8)-21474836482147483647
unsigned long
(or unsigned long int)
Unsigned long integer (of at least 32 bits)4 (8)0same as above
long long
(or long long int)
(or signed long long)
(or signed long long int) (C++11)
Very long signed integer (of at least 64 bits)8-263263-1
unsigned long long
(or unsigned long long int) (C++11)
Unsigned very long integer (of at least 64 bits)80264-1
Real NumbersfloatFloating-point number, ≈7 digits
(IEEE 754 single-precision floating point format)
43.4e383.4e-38
doubleDouble precision floating-point number, ≈15 digits
(IEEE 754 double-precision floating point format)
81.7e3081.7e-308
long doubleLong double precision floating-point number, ≈19 digits
(IEEE 754 quadruple-precision floating point format)
12 (8)
Boolean
Numbers
boolBoolean value of either true or false1false (0)true (1 or non-zero)
Wide
Characters
wchar_t
char16_t (C++11)
char32_t (C++11)
Wide (double-byte) character2 (4)

In addition, many C++ library functions use a type called size_t, which is equivalent (typedef) to a unsigned int, meant for counting, size or length, with 0 and positive integers.

*The sizeof Operator

C/C++ provides an unary sizeof operator to get the size of the operand (in bytes). The following program uses sizeof operator to print the size of the fundamental types.

The results may vary among different systems.

*Header <climits>

The climits header (ported to C++ from C's limits.h) contains information about limits of integer type. For example,

Again, the outputs depend on the system.

The minimum of unsigned integer is always 0. The other constants are SHRT_MAX, SHRT_MIN, USHRT_MAX, LONG_MIN, LONG_MAX, ULONG_MAX. Try inspecting this header (search for climits under your compiler).

*Header <cfloat>

Similarly, the cfloat header (ported from C's float.h) contain information on limits for floating point numbers, such as minimum number of significant digits (FLT_DIG, DBL_DIG, LDBL_DIG for float, double and long double), number of bits for mantissa (FLT_MANT_DIG, DBL_MANT_DIG, LDBL_MANT_DIG), maximum and minimum exponent values, etc. Try inspecting this header (search for cfloat under your compiler).

*Header <limits>

The climits and cfloat headers are ported over from C's limit.h and float.h. C++ added a new header called limits.

[TODO]

Choosing Types

As a programmer, you need to choose variables and decide on the type of the variables to be used in your programs. Most of the times, the decision is intuitive. For example, use an integer type for counting and whole number; a floating-point type for number with fractional part, char for a single character, and boolean for binary outcome.

Rule of Thumb
  • Use int for integer and double for floating point numbers. Use byte, short, long and float only if you have a good reason to choose that specific precision.
  • Use int (or unsigned int) for counting and indexing, NOT floating-point type (float or double). This is because integer type are precise and more efficient in operations.
  • Use an integer type if possible. Use a floating-point type only if the number contains a fractional part.

Read my article on 'Data Representation' if you wish to understand how the numbers and characters are represented inside the computer memory. In brief, It is important to take note that char '1' is different from int 1, short 1, float 1.0, double 1.0, and String '1'. They are represented differently in the computer memory, with different precision and interpretation. For example, short 1 is '00000000 00000001', int 1 is '00000000 00000000 00000000 00000001', long long 1 is '00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001', float 1.0 is '0 01111111 0000000 00000000 00000000', double 1.0 is '0 01111111111 0000 00000000 00000000 00000000 00000000 00000000 00000000', char '1' is '00110001'.

There is a subtle difference between int 0 and double 0.0.

Furthermore, you MUST know the type of a value before you can interpret a value. For example, this value '00000000 00000000 00000000 00000001' cannot be interpreted unless you know the type.

*The typedef Statement

Typing 'unsigned int' many time can get annoying. The typedef statement can be used to create a new name for an existing type. For example, you can create a new type called 'uint' for 'unsigned int' as follow. You should place the typedef immediately after #include. Use typedef with care because it makes the program hard to read and understand.

Many C/C++ compilers define a type called size_t, which is a typedef of unsigned int.

Literals for Fundamental Types and String

A literal is a specific constant value, such as 123, -456, 3.14, 'a', 'Hello', that can be assigned directly to a variable; or used as part of an expression. They are called literals because they literally and explicitly identify their values.

Integer Literals

A whole number, such as 123 and -456, is treated as an int, by default. For example,

An int literal may precede with a plus (+) or minus (-) sign, followed by digits. No commas or special symbols (e.g., $ or space) is allowed (e.g., 1,234 and $123 are invalid). No preceding 0 is allowed too (e.g., 007 is invalid).

Besides the default base 10 integers, you can use a prefix '0' (zero) to denote a value in octal, prefix '0x' for a value in hexadecimal, and prefix '0b' for binary value (in some compilers), e.g.,

A long literal is identified by a suffix 'L' or 'l' (avoid lowercase, which can be confused with the number one). A long long int is identified by a suffix 'LL'. You can also use suffix 'U' for unsigned int, 'UL' for unsigned long, and 'ULL' for unsigned long long int. For example,

No suffix is needed for short literals. But you can only use integer values in the permitted range. For example,

Floating-point Literals

A number with a decimal point, such as 55.66 and -33.44, is treated as a double, by default. You can also express them in scientific notation, e.g., 1.2e3, -5.5E-6, where e or E denotes the exponent in power of 10. You could precede the fractional part or exponent with a plus (+) or minus (-) sign. Exponent shall be an integer. There should be no space or other characters (e.g., space) in the number.

You MUST use a suffix of 'f' or 'F' for float literals, e.g., -1.2345F. For example,

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Use suffix 'L' (or 'l') for long double.

Character Literals and Escape Sequences

A printable char literal is written by enclosing the character with a pair of single quotes, e.g., 'z', '$', and '9'. In C++, characters are represented using 8-bit ASCII code, and can be treated as a 8-bit signed integers in arithmetic operations. In other words, char and 8-bit signed integer are interchangeable. You can also assign an integer in the range of [-128, 127] to a char variable; and [0, 255] to an unsigned char.

You can find the ASCII code table HERE.

For example,

Non-printable and control characters can be represented by a so-called escape sequence, which begins with a back-slash (). The commonly-used escape sequences are:

Escape SequenceDescriptionHex (Decimal)
nNew-line (or Line-feed)0AH (10D)
rCarriage-return0DH (13D)
tTab09H (9D)
'Double-quote (needed to include ' in double-quoted string)22H (34D)
'Single-quote27H (39D)
Back-slash (to resolve ambiguity)5CH (92D)

Notes:

  • New-line (0AH) and carriage return (0dH), represented by n, and r respectively, are used as line delimiter (or end-of-line, or EOL). However, take note that Unixes/Mac use n as EOL, Windows use rn.
  • Horizontal Tab (09H) is represented as t.
  • To resolve ambiguity, characters back-slash (), single-quote (') and double-quote (') are represented using escape sequences , ' and ', respectively. This is because a single back-slash begins an escape sequence, while single-quotes and double-quotes are used to enclose character and string.
  • Other less commonly-used escape sequences are: ? or ?, a for alert or bell, b for backspace, f for form-feed, v for vertical tab. These may not be supported in some consoles.
The <cctype> Header

The cctype header (ported from C's ctype.h) provides functions such as isalpha(), isdigit(), isspace(), ispunct(), isalnum(), isupper(), islower() to determine the type of character; and toupper(), tolower() for case conversion.

String Literals

A String literal is composed of zero of more characters surrounded by a pair of double quotes, e.g., 'Hello, world!', 'The sum is ', '. For example,

String literals may contains escape sequences. Inside a String, you need to use ' for double-quote to distinguish it from the ending double-quote, e.g. 'quoted'. Single quote inside a String does not require escape sequence. For example,

TRY: Write a program to print the following picture. Take note that you need to use escape sequences to print special characters.

bool Literals

There are only two bool literals, i.e., true and false. For example,

In an expression, bool values and literals are converted to int 0 for false and 1 (or a non-zero value) for true.

Example (Literals)

Operations

Arithmetic Operators

C++ supports the following arithmetic operators for numbers: short, int, long, long long, char (treated as 8-bit signed integer), unsigned short, unsigned int, unsigned long, unsigned long long, unsigned char, float, double and long double.

OperatorDescriptionUsageExamples
*Multiplicationexpr1 * expr22 * 3 → 6; 3.3 * 1.0 → 3.3
/Divisionexpr1 / expr21 / 2 → 0; 1.0 / 2.0 → 0.5
%Remainder (Modulus)expr1 % expr25 % 2 → 1; -5 % 2 → -1
+Additionexpr1 + expr21 + 2 → 3; 1.1 + 2.2 → 3.3
-Subtractionexpr1 - expr21 - 2 → -1; 1.1 - 2.2 → -1.1

All the above operators are binary operators, i.e., they take two operands. The multiplication, division and remainder take precedence over addition and subtraction. Within the same precedence level (e.g., addition and subtraction), the expression is evaluated from left to right. For example, 1+2+3-4 is evaluated as ((1+2)+3)-4.

It is important to take note that int/int produces an int, with the result truncated, e.g., 1/2 → 0 (instead of 0.5).

Take note that C/C++ does not have an exponent (power) operator ('^' is exclusive-or, not exponent).

Arithmetic Expressions

In programming, the following arithmetic expression:

must be written as (1+2*a)/3 + (4*(b+c)*(5-d-e))/f - 6*(7/g+h). You cannot omit the multiplication symbol '*' (as in Mathematics).

Like Mathematics, the multiplication '*' and division '/' take precedence over addition '+' and subtraction '-'. Parentheses () have higher precedence. The operators '+', '-', '*', and '/' are left-associative. That is, 1 + 2 + 3 + 4 is treated as (((1+2) + 3) + 4).

Mixed-Type Operations

If both the operands of an arithmetic operation belong to the same type, the operation is carried out in that type, and the result belongs to that type. For example, int/int → int; double/double → double.

However, if the two operands belong to different types, the compiler promotes the value of the smaller type to the larger type (known as implicit type-casting). The operation is then carried out in the larger type. For example, int/double → double/double → double. Hence, 1/2 → 0, 1.0/2.0 → 0.5, 1.0/2 → 0.5, 1/2.0 → 0.5.

For example,

TypeExampleOperation
int2 + 3int 2 + int 3 → int 5
double2.2 + 3.3double 2.2 + double 3.3 → double 5.5
mix2 + 3.3int 2 + double 3.3 → double 2.0 + double 3.3 → double 5.3
int1 / 2int 1 / int 2 → int 0
double1.0 / 2.0double 1.0 / double 2.0 → double 0.5
mix1 / 2.0int 1 / double 2.0 → double 1.0 + double 2.0 → double 0.5
Example

Overflow/UnderFlow

Study the output of the following program:

In arithmetic operations, the resultant value wraps around if it exceeds its range (i.e., overflow or underflow). C++ runtime does not issue an error/warning message but produces incorrect result.

It is important to take note that checking of overflow/underflow is the programmer's responsibility, i.e., your job!

This feature is an legacy design, where processors were slow. Checking for overflow/underflow consumes computation power and reduces performance.

To check for arithmetic overflow (known as secure coding) is tedious. Google for 'INT32-C. Ensure that operations on signed integers do not result in overflow' @ www.securecoding.cert.org.

Compound Assignment Operators

Besides the usual simple assignment operator '=' described earlier, C++ also provides the so-called compound assignment operators as listed:

OperatorUsageDescriptionExample
=var = exprAssign the value of the LHS to the variable at the RHSx = 5;
+=var += exprsame as var = var + exprx += 5; same as x = x + 5
-=var -= exprsame as var = var - exprx -= 5; same as x = x - 5
*=var *= exprsame as var = var * exprx *= 5; same as x = x * 5
/=var /= exprsame as var = var / exprx /= 5; same as x = x / 5
%=var %= exprsame as var = var % exprx %= 5; same as x = x % 5

Increment/Decrement Operators

C++ supports these unary arithmetic operators: increment '++' and decrement '--'.

OperatorExampleResult
++x++; ++xIncrement by 1, same as x += 1
--x--; --xDecrement by 1, same as x -= 1

For example,

The increment/decrement unary operator can be placed before the operand (prefix operator), or after the operands (postfix operator). They takes on different meaning in operations.

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OperatorDescriptionExampleResult
++varPre-Increment
Increment var, then use the new value of var
y = ++x;same as x=x+1; y=x;
var++Post-Increment
Use the old value of var, then increment var
y = x++;same as oldX=x; x=x+1; y=oldX;
--varPre-Decrementy = --x;same as x=x-1; y=x;
var--Post-Decrementy = x--;same as oldX=x; x=x-1; y=oldX;

If '++' or '--' involves another operation, then pre- or post-order is important to specify the order of the two operations. For examples,

Prefix operator (e.g, ++i) could be more efficient than postfix operator (e.g., i++) in some situations.

Implicit Type-Conversion vs. Explicit Type-Casting

Converting a value from one type to another type is called type casting (or type conversion). There are two kinds of type casting:

  1. Implicit type-conversion performed by the compiler automatically, and
  2. Explicit type-casting via an unary type-casting operator in the form of (new-type)operand or new-type(operand).
Implicit (Automatic) Type Conversion

When you assign a value of a fundamental (built-in) type to a variable of another fundamental type, C++ automatically converts the value to the receiving type, if the two types are compatible. For examples,

  • If you assign an int value to a double variable, the compiler automatically casts the int value to a double double (e.g., from 1 to 1.0) and assigns it to the double variable.
  • if you assign a double value of to an int variable, the compiler automatically casts the double value to an int value (e.g., from 1.2 to 1) and assigns it to the int variable. The fractional part would be truncated and lost. Some compilers issue a warning/error 'possible loss in precision'; others do not.

C++ will not perform automatic type conversion, if the two types are not compatible.

Explicit Type-Casting

You can explicitly perform type-casting via the so-called unary type-casting operator in the form of (new-type)operand or new-type(operand). The type-casting operator takes one operand in the particular type, and returns an equivalent value in the new type. Take note that it is an operation that yields a resultant value, similar to an addition operation although addition involves two operands. For example,

Example: Suppose that you want to find the average (in double) of the integers between 1 and 100. Study the following codes:

You don't get the fractional part although the average is a double. This is because both the sum and 100 are int. The result of division is an int, which is then implicitly casted to double and assign to the double variable average. To get the correct answer, you can do either:

Example:

Example:

*Operator static-cast<type>

C++ introduces a new operator called static_cast<type> to perform type conversion (because the regular cast mentioned earlier is too lax and could produce expected results). static_cast signal an error if conversion fails. For example,

Relational and Logical Operators

Very often, you need to compare two values before deciding on the action to be taken, e.g., if mark is more than or equal to 50, print 'PASS'.

C++ provides six comparison operators (or relational operators):

OperatorDescriptionUsageExample (x=5, y=8)
Equal toexpr1expr2(xy) → false
!=Not Equal toexpr1 != expr2(x != y) → true
>Greater thanexpr1 > expr2(x > y) → false
>=Greater than or equal toexpr1 >= expr2(x >= 5) → true
<Less thanexpr1 < expr2(y < 8) → false
<=Less than or equal toexpr1 >= expr2(y <= 8) → true

In C++, these comparison operations returns a bool value of either false (0) or true (1 or a non-zero value).

Each comparison operation involves two operands, e.g., x <= 100. It is invalid to write 1 < x < 100 in programming. Instead, you need to break out the two comparison operations x > 1, x < 100, and join with with a logical AND operator, i.e., (x > 1) && (x < 100), where && denotes AND operator.

C++ provides four logical operators (which operate on boolean operands only):

OperatorDescriptionUsage
&&Logical ANDexpr1 && expr2
Logical ORexpr1expr2
!Logical NOT!expr
^Logical XORexpr1 ^ expr2

The truth tables are as follows:

AND (&&)truefalse
truetruefalse
falsefalsefalse
OR ( )truefalse
truetruetrue
falsetruefalse
NOT (!)truefalse
falsetrue
XOR (^)truefalse
truefalsetrue
falsetruefalse

Example:

Exercise: Given the year, month (1-12), and day (1-31), write a boolean expression which returns true for dates before October 15, 1582 (Gregorian calendar cut over date).

Ans: (year < 1582) (year1582 && month < 10) (year1582 && month10 && day < 15)

Flow Control

There are three basic flow control constructs - sequential, conditional (or decision), and loop (or iteration), as illustrated below.

Sequential Flow Control

A program is a sequence of instructions. Sequential flow is the most common and straight-forward, where programming statements are executed in the order that they are written - from top to bottom in a sequential manner.

Conditional (Decision) Flow Control

There are a few types of conditionals, if-then, if-then-else, nested-if (if-elseif-elseif-...-else), switch-case, and conditional expression.

SyntaxExampleFlowchart

'switch-case' is an alternative to the 'nested-if'. In a switch-case statement, a break statement is needed for each of the cases. If break is missing, execution will flow through the following case. You can use either an int or char variable as the case-selector.

Conditional Operator: A conditional operator is a ternary (3-operand) operator, in the form of booleanExpr ? trueExpr : falseExpr. Depending on the booleanExpr, it evaluates and returns the value of trueExpr or falseExpr.

SyntaxExample

Braces: You could omit the braces { }, if there is only one statement inside the block. For example,

However, I recommend that you keep the braces, even though there is only one statement in the block, to improve the readability of your program.

Exercises

[TODO]

Loop Flow Control

Again, there are a few types of loops: for-loop, while-do, and do-while.

SyntaxExampleFlowchart

The difference between while-do and do-while lies in the order of the body and condition. In while-do, the condition is tested first. The body will be executed if the condition is true and the process repeats. In do-while, the body is executed and then the condition is tested. Take note that the body of do-while will be executed at least once (vs. possibly zero for while-do).

Suppose that your program prompts user for a number between 1 to 10, and checks for valid input, do-while with a boolean flag could be more appropriate.

Below is an example of using while-do:

Example (Counter-Controlled Loop): Prompt user for an upperbound. Sum the integers from 1 to a given upperbound and compute its average.

Example (Sentinel-Controlled Loop): Prompt user for positive integers, and display the count, maximum, minimum and average. Terminate when user enters -1.

Program Notes

  • In computing, a sentinel value is a special value that indicates the end of data (e.g., a negative value to end a sequence of positive value, end-of-file, null character in the null-terminated string). In this example, we use -1 as the sentinel value to indicate the end of inputs, which is a sequence of positive integers. Instead of hardcoding the value of -1, we use a variable called sentinel for flexibility and ease-of-maintenance.
  • Take note of the while-loop pattern in reading the inputs. In this pattern, you need to repeat the prompting statement.
  • To control the precision of floating point numbers, use: where n is the number of decimal places (after the decimal point). You need to include <iomanip> header. The setprecision() is sticky. That is, it will remain in effect until another value is set.
Exercises

[TODO]

Interrupting Loop Flow - 'break' and 'continue'

The break statement breaks out and exits the current (innermost) loop.

The continue statement aborts the current iteration and continue to the next iteration of the current (innermost) loop.

break and continue are poor structures as they are hard to read and hard to follow. Use them only if absolutely necessary. You can always write the same program without using break and continue.

Example (break): The following program lists the non-prime numbers between 2 and an upperbound.

Let's rewrite the above program to list all the primes instead. A boolean flag called isPrime is used to indicate whether the current number is a prime. It is then used to control the printing.

Let's rewrite the above program without using break statement. A while loop is used (which is controlled by the boolean flag) instead of for loop with break.

Example (continue):

Example (break and continue): Study the following program.

Terminating Program

There are a few ways that you can terminate your program, before reaching the end of the programming statements.

exit(): You could invoke the function exit(int exitCode), in <cstdlib> (ported from C's 'stdlib.h'), to terminate the program and return the control to the Operating System. By convention, return code of zero indicates normal termination; while a non-zero exitCode (-1) indicates abnormal termination. For example,

abort(): The header <cstdlib> also provide a function called abort(), which can be used to terminate the program abnormally.

The 'return' Statement: You could also use a 'return returnValue' statement in the main() function to terminate the program and return control back to the Operating System. For example,

Nested Loops

The following diagram illustrates a nested for-loop, i.e., an inner for-loop within an outer for-loop.

Try out the following program, which prints a 8-by-8 checker box pattern using nested loops, as follows:

This program contains two nested for-loops. The inner loop is used to print a row of eight '# ', which is followed by printing a newline. The outer loop repeats the inner loop to print all the rows.

Suppose that you want to print this pattern instead (in program called PrintCheckerPattern.cpp):

You need to print an additional space for even-number rows. You could do so by adding the following statement before Line 8.

Exercises
  1. Print these patterns using nested loop (in a program called PrintPattern1x). Use a variable called size for the size of the pattern and try out various sizes. You should use as few printing statements as possible. Hints:
    The equations for major and opposite diagonals are row = col and row + col = size + 1. Decide on what to print above and below the diagonal.
  2. Print the timetable of 1 to 9, as follows, using nested loop. (Hints: you need to use an if-else statement to check whether the product is single-digit or double-digit, and print an additional space if needed.)
  3. Print these patterns using nested loop.

Some Issues in Flow Control

Dangling else: The 'dangling else' problem can be illustrated as follows:

The else clause in the above codes is syntactically applicable to both the outer-if and the inner-if. The C++ compiler always associate the else clause with the innermost if (i.e., the nearest if). Dangling else can be resolved by applying explicit parentheses. The above codes are logically incorrect and require explicit parentheses as shown below.

Endless Loop: The following constructs:

is commonly used. It seems to be an endless loop (or infinite loop), but it is usually terminated via a break or return statement inside the loop body. This kind of code is hard to read - avoid if possible by re-writing the condition.

Exercises

[TODO]

Writing Correct and Good Programs

It is important to write programs that produce the correct results. It is also important to write programs that others (and you yourself three days later) can understand, so that the programs can be maintained - I call these programs good programs.

Here are the suggestions:

  • Follow established convention so that everyone has the same basis of understanding.
  • Format and layout of the source code with appropriate indents, white spaces and white lines. Use 3 or 4 spaces for indent, and blank lines to separate sections of codes.
  • Choose good names that are self-descriptive and meaningful, e.g., row, col, size, xMax, numStudents. Do not use meaningless names, such as a, b, c, d. Avoid single-alphabet names (easier to type but often meaningless), except common names likes x, y, z for co-ordinates and i for index.
  • Provide comments to explain the important as well as salient concepts. Comment your codes liberally.
  • Write your program documentation while writing your programs.
  • Avoid un-structured constructs, such as break and continue, which are hard to follow.
  • Use 'mono-space' fonts (such as Consola, Courier New, Courier) for writing/displaying your program.
Programming Errors

There are generally three classes of programming errors:

  1. Compilation Error (or Syntax Error): can be fixed easily.
  2. Runtime Error: program halts pre-maturely without producing the results - can also be fixed easily.
  3. Logical Error: program completes but produces incorrect results. It is easy to detect if the program always produces wrong result. It is extremely hard to fix if the program produces the correct result most of the times, but incorrect result sometimes. For example, This kind of errors is very serious if it is not caught before production. Writing good programs helps in minimizing and detecting these errors. A good testing strategy is needed to ascertain the correctness of the program. Software testing is an advanced topics which is beyond our current scope.
Debugging Programs

Here are the common debugging techniques:

  1. Stare at the screen! Unfortunately, errors usually won't pop-up even if you stare at it extremely hard.
  2. Study the error messages! Do not close the console when error occurs and pretending that everything is fine. This helps most of the times.
  3. Insert print statements at appropriate locations to display the intermediate results. It works for simple toy program, but it is neither effective nor efficient for complex program.
  4. Use a graphic debugger. This is the most effective means. Trace program execution step-by-step and watch the value of variables and outputs.
  5. Advanced tools such as profiler (needed for checking memory leak and function usage).
  6. Proper program testing to wipe out the logical errors.
Testing Your Program for Correctness

How to ensure that your program always produces correct result, 100% of the times? It is impossible to try out all the possible outcomes, even for a simple program. Program testing usually involves a set of representative test cases, which are designed to catch the major classes of errors. Program testing is beyond the scope of this writing.

Strings

C++ supports two types of strings:

  1. the original C-style string: A string is a char array, terminated with a NULL character '0' (Hex 0). It is also called Character-String or C-style string. C-string will be discussed later.
  2. the new string class introduced in C++98.

The 'high-level' string class is recommended, because it is much easier to use and understood. However, many legacy programs used C-strings; many programmers also use 'low-level' C-strings for full control and efficiency; furthermore, in some situation such as command-line arguments, only C-strings are supported. Hence, you may have to understand both sets of strings. However, avoid C-string unless it is absolutely necessary.

We shall describe string class here, and C-string later.

String Declaration and Initialization

To use the string class, include the <string> header and 'using namespace std'.

You can declare and (a) initialize a string with a string literal, (b) initialize to an empty string, or (c) initialize with another string object. For example,

String Input/Output

For example,

NOTES:

  • We need to '#include <string>' to use the string class, and 'using namespace std' as string is defined under std namespace.
  • 'cin >> aStr' reads a word (delimited by space) from cin (keyboard), and assigns to string variable aStr.
  • getline(cin, aStr) reads the entire line (up to 'n') from cin, and assigns to aStr. The 'n' character is discarded.
  • To flush cin, you could use ignore(numeric_limits<streamsize>::max(), 'n') function to discard all the characters up to 'n'. numeric_limits is in the <limits> header.

String Operations

  • Checking the length of a string:
  • Check for empty string:
  • Copying from another string: Simply use the assignment (=) operator.
  • Concatenated with another string: Use the plus (+) operator, or compound plus (+=) operator.
  • Read/Write individual character of a string:
  • Extracting sub-string:
  • Comparing with another string:
  • Search/Replacing characters: You can use the functions available in the <algorithm> such as replace(). For example,
  • Many others.
Example 1:
Example 2:

[TODO]

Exercises

[TODO]

Formatting Input/Output using IO Manipulators (Header <iomanip>)

The <iomanip> header provides so-called I/O manipulators for formatting input and output:

  • setw(int field-widht): set the field width for the next IO operation. setw() is non-sticky and must be issued prior to each IO operation. The field width is reset to the default after each operation (with just enough width to accommodate the field).
  • setfill(char fill-char): set the filled character for padding to the field width.
  • left right internal: set the alignment
  • fixed/scientific (for floating-point numbers): use fixed-point notation (e.g, 12.34) or scientific notation (e.g., 1.23e+006).
  • setprecision(int numDecimalDigits) (for floating-point numbers): specify the number of digits after the decimal point.
  • boolalpha/noboolalpha (for bool): display bool values as alphabetic string (true/false) or 1/0.

Output Formatting

Example

Input Formatting

Example

Exercises

[TODO]

Arrays

Array Declaration and Usage

Suppose that you want to find the average of the marks for a class of 30 students, you certainly do not want to create 30 variables: mark1, mark2, ..., mark30. Instead, You could use a single variable, called an array, with 30 elements.

An array is a list of elements of the same type, identified by a pair of square brackets [ ]. To use an array, you need to declare the array with 3 things: a name, a type and a dimension (or size, or length). The syntax is:

I recommend using a plural name for array, e.g., marks, rows, numbers. For example,

Take note that, in C++, the value of the elements are undefined after declaration.

You can also initialize the array during declaration with a comma-separated list of values, as follows:

You can refer to an element of an array via an index (or subscript) enclosed within the square bracket [ ]. C++'s array index begins with zero. For example, suppose that marks is an int array of 5 elements, then the 5 elements are: marks[0], marks[1], marks[2], marks[3], and marks[4].

To create an array, you need to known the length (or size) of the array in advance, and allocate accordingly. Once an array is created, its length is fixed and cannot be changed. At times, it is hard to ascertain the length of an array (e.g., how many students in a class?). Nonetheless, you need to estimate the length and allocate an upper bound. This is probably the major drawback of using an array. C++ has a vector template class (and C++11 added an array template class), which supports dynamic resizable array.

You can find the array length using expression sizeof(arrayName)/sizeof(arrayName[0]), where sizeof(arrayName) returns the total bytes of the array and sizeof(arrayName[0]) returns the bytes of first element.

C/C++ does not perform array index-bound check. In other words, if the index is beyond the array's bounds, it does not issue a warning/error. For example,

This is another pitfall of C/C++. Checking the index bound consumes computation power and depicts the performance. However, it is better to be safe than fast. Newer programming languages such as Java/C# performs array index bound check.

Array and Loop

Arrays works hand-in-hand with loops. You can process all the elements of an array via a loop, for example,

Exercises

[TODO]

Range-based for loop (C++11)

C++11 introduces a range-based for loop (or for-each loop) to iterate thru an array, as illustrated in the following example:

To compile the program under GNU GCC (g++), you may need to specify option -std=c++0x or -std=c++11:

Multi-Dimensional Array

For example,

For 2D array (table), the first index is the row number, second index is the column number. The elements are stored in a so-called row-major manner, where the column index runs out first.

Example

Array of Characters - C-String

In C, a string is a char array terminated by a NULL character '0' (ASCII code of Hex 0). C++ provides a new string class under header <string>. The original string in C is known as C-String (or C-style String or Character String). You could allocate a C-string via:

For novices, avoid C-string. Use C++ string (in header <string>) discussed earlier.

Example

You can use cin and cout to handle C-strings.

  • cin << reads a string delimited by whitespace;
  • cin.getline(var, size) reads a string of into var till newline of length up to size-1, discarding the newline (replaced by '0'). The size typically corresponds to the length of the C-string array.
  • cin.get(var, size) reads a string till newline, but leaves the newline in the input buffer.
  • cin.get(), without argument, reads the next character.

Exercises

[TODO]

Functions

Why Functions?

At times, a certain portion of codes has to be used many times. Instead of re-writing the codes many times, it is better to put them into a 'subroutine', and 'call' this 'subroutine' many time - for ease of maintenance and understanding. Subroutine is called method (in Java) or function (in C/C++).

The benefits of using functions are:

  1. Divide and conquer: construct the program from simple, small pieces or components. Modularize the program into self-contained tasks.
  2. Avoid repeating codes: It is easy to copy and paste, but hard to maintain and synchronize all the copies.
  3. Software Reuse: you can reuse the functions in other programs, by packaging them into library codes.

Two parties are involved in using a function: a caller who calls the function, and the function called. The caller passes argument(s) to the function. The function receives these argument(s), performs the programmed operations within the function's body, and returns a piece of result back to the caller.

Using Functions

Get Started with an Example

Suppose that we need to evaluate the area of a circle many times, it is better to write a function called getArea(), and re-use it when needed.

In the above example, a reusable function called getArea() is defined, which receives a parameter (in double) from the caller, performs the calculation, and return a piece of result (in double) to the caller. In the main(), we invoke getArea() functions thrice, each time with a different parameter.

In C++, you need to declare a function prototype (before the function is used), and provide a function definition, with a body containing the programmed operations.

Function Definition

The syntax for function definition is as follows:

  • The parameterList consists of comma-separated parameter-type and parameter-name, i.e., param-1-type param-1-name, param-2-type param-2-name,...
  • The returnValueType specifies the type of the return value, such as int or double. An special return type called void can be used to denote that the function returns no value. In C++, a function is allowed to return one value or no value (void). It cannot return multiple values. [C++ does not allow you to return an array!]

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The 'return' Statement

Inside the function's body, you could use a return statement to return a value (of the returnValueType declared in the function's header) and pass the control back to the caller. The syntax is:

Take note that invoking a function (by the caller) transfers the control to the function. The return statement in the function transfers the control back to the caller.

Function Naming Convention

A function's name shall be a verb or verb phrase (action), comprising one or more words. The first word is in lowercase, while the rest are initial-capitalized (known as camel-case). For example, getArea(), setRadius(), moveDown(), isPrime(), etc.

Function Prototype

In C++, a function must be declared before it can be called. It can be achieved by either placing the function definition before it is being used, or declare a so-called function prototype.

A function prototype tells the compiler the function's interface, i.e., the return-type, function name, and the parameter type list (the number and type of parameters). The function can now be defined anywhere in the file. For example,

You could optionally include the parameter names in the function prototype. The names will be ignored by the compiler, but serve as documentation. For example,

Function prototypes are usually grouped together and placed in a so-called header file. The header file can be included in many programs. We will discuss header file later.

Another Example

We have a function called max(int, int), which takes two int and return their maximum. We invoke the max() function from the main().

The 'void' Return Type

Suppose that you need a function to perform certain actions (e.g., printing) without a need to return a value to the caller, you can declare its return-value type as void. In the function's body, you could use a 'return;' statement without a return value to return control to the caller. In this case, the return statement is optional. If there is no return statement, the entire body will be executed, and control returns to the caller at the end of the body.

Actual Parameters vs. Formal Parameters

Recall that a function receives arguments from its caller, performs the actions defined in the function's body, and return a value (or nothing) to the caller.

In the above example, the variable (double radius) declared in the signature of getArea(double radius) is known as formal parameter. Its scope is within the function's body. When the function is invoked by a caller, the caller must supply so-called actual parameters (or arguments), whose value is then used for the actual computation. For example, when the function is invoked via 'area1 = getArea(radius1)', radius1 is the actual parameter, with a value of 1.1.

Scope of Function's Local Variables and Parameters

All variables, including function's parameters, declared inside a function are available only to the function. They are created when the function is called, and freed (destroyed) after the function returns. They are called local variables because they are local to the function and not available outside the function. They are also called automatic variables, because they are created and destroyed automatically - no programmer's explicit action needed to allocate and deallocate them.

Boolean Functions

A boolean function returns a bool value (of either true or false) to the caller.

Suppose that we wish to write a function called isOdd() to check if a given number is odd.

This seemingly correct codes produces false for -5, because -5%2 is -1 instead of 1. You may rewrite the condition:

The above code produces the correct answer, but is poor. For boolean function, you should simply return the resultant bool value of the comparison, instead of using a conditional statement, as follow:

Default Arguments

C++ introduces so-called default arguments for functions. These default values would be used if the caller omits the corresponding actual argument in calling the function. Default arguments are specified in the function prototype, and cannot be repeated in the function definition. The default arguments are resolved based on their positions. Hence, they can only be used to substitute the trailing arguments to avoid ambiguity. For example,

You should specify the default arguments in the function prototype (declaration). They can only be defined once (one-definition rule), and cannot be repeated in the function definition.

Default argument is not absolutely necessary. The codes could be hard to maintain.

Function Overloading

C++ introduces function overloading (or function polymorphism, which means many forms), which allows you to have multiple versions of the same function name, differentiated by the parameter list (number, type or order of parameters). The version matches the caller's argument list will be selected for execution. For example,

Overloaded functions cannot be differentiated by the return-type (compilation error).

*Name Mangling

To differentiate between different versions of an overloaded function, many compilers (such as GNU GCC) adopt a name mangling or name decoration scheme for naming functions.

Each of the function is identified via a prefix __Z, followed by an integer containing the number of characters of the function name (3 in this case for 'fun'), followed by the parameter type list (where i for int and d for double). For example, di means a double followed by an int; id for an int followed by a double; iii for 3 ints.

You can choose to use the C's naming protocol by appending the keyword extern 'C' to the function prototype. C does not support function overloading. Thus, it does not need name mangling. It simply append an underscore in front of the function name. For example,

Functions and Arrays

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You can also pass arrays into function. However, you also need to pass the size of the array into the function. This is because there is no way to tell the size of the array from the array argument inside the called function.

For example,

Example: Computing the Sum of an Array and Print Array's Contents

Pass-by-Value vs. Pass-by-Reference

There are two ways that a parameter can be passed into a function: pass by value vs. pass by reference.

Pass-by-Value

In pass-by-value, a 'copy' of argument is created and passed into the function. The invoked function works on the 'clone', and cannot modify the original copy. In C/C++, fundamental types (such as int and double) are passed by value. That is, you cannot modify caller's value inside the function - there is no side effect.

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Example (Fundamental Types are Passed by Value)
Pass-by-Reference

On the other hand, in pass-by-reference, a reference of the caller's variable is passed into the function. In other words, the invoked function works on the same data. If the invoked function modifies the parameter, the same caller's copy will be modified as well.

In C/C++, arrays are passed by reference. That is, you can modify the contents of the caller's array inside the invoked function - there could be side effect in passing arrays into function.

C/C++ does not allow functions to return an array. Hence, if you wish to write a function that modifies the contents of an array (e.g., sorting the elements of an array), you need to rely on pass-by-reference to work on the same copy inside and outside the function. Recall that in pass-by-value, the invoked function works on a clone copy and has no way to modify the original copy.

Example (Array is passed by Reference): Increment Each Element of an Array

Array is passed into function by reference. That is, the invoked function works on the same copy of the array as the caller. Hence, changes of array inside the function is reflected outside the function (i.e., side effect).

Why Arrays are Pass-by-Reference?

Array is designed to be passed by reference, instead of by value using a cloned copy. This is because passing huge array by value is inefficient - the huge array needs to be cloned.

const Function Parameters

Pass-by-reference risks corrupting the original data. If you do not have the intention of modifying the arrays inside the function, you could use the const keyword in the function parameter. A const function argument cannot be modified inside the function.

Use const whenever possible for passing references as it prevent you from inadvertently modifying the parameters and protects you against many programming errors.

Example: Search an Array using Linear Search

In a linear search, the search key is compared with each element of the array linearly. If there is a match, it returns the index of the array between [0, size-1]; otherwise, it returns -1 or the size of of the array (some implementations deal with only positive indexes). Linear search has complexity of O(n).

Program Notes:

  • [TODO]
Example: Sorting an Array using Bubble Sort

Wiki 'Bubble Sort' for the detailed algorithm and illustration. In brief, we pass thru the list, compare two adjacent items and swap them if they are in the wrong order. Repeat the pass until no swaps are needed. For example,

Bubble sort is not efficient, with complexity of O(n2).

Program Notes:

  • [TODO]
Example: Sorting an Array using Insertion Sort

Wiki 'Insertion Sort' for the algorithm and illustration. In brief, pass thru the list. For each element, compare with all previous elements and insert it at the correct position by shifting the other elements. For example,

Insertion sort is also not efficient, with complexity of O(n2).

Program Notes:

  • [TODO]
Example: Sorting an Array using Selection Sort

Wiki 'Selection Sort' for the algorithm and illustration. In brief, Pass thru the list. Select the smallest element and swap with the head of the list. For example,

Selection sort is also not efficient, with complexity of O(n2).

Program Notes:

  • [TODO]
'const' Fundamental-Type Function Parameters?

You could also use const for fundamental-type function parameters (such as int, double) to prevent the parameters from being modified inside the function. However, as fundamental-type parameters are passed by value (with a cloned copy), there will never be side effect on the caller. We typically do not use the const keyword for fundamental types. In other words, const is used to indicate that there shall NOT be side-effect.

Pass-by-Reference via 'Reference' Parameters

As mentioned, parameters of fundamental types (such as int, double) are passed-by-value. That is, a clone copy is used inside the function. Change to the cloned copy inside the function has no side-effect to the caller's copy.

Nonetheless, you can pass a fundamental type parameter by reference via the so-called reference parameter denoted by &. For example,

In function squareByReference(), we declare the parameter number is passed by reference by declaring its type as int & (reference of int). In this way, the caller's copy is used inside the function (instead of a cloned copy in pass-by-value). Changes inside the function has side-effect.

Pass-by-reference is NOT commonly used for fundamental types (such as int, double) - the above example is purely meant for academic illustration. But it is used extensively for compound types (such as arrays and objects) to avoid cloning huge data for better performance. We shall revisit pass-by-reference in Object-Oriented Programming (OOP) chapters.

Mathematical Functions (Header <cmath>)

C++ provides many common-used Mathematical functions in library <cmath> (ported over from C's 'math.h'). The signatures of some of these functions are:

Generating Random Numbers

The cstdlib header (ported from C's stdlib.h) provides a function rand(), which generates a pseudo-random integral number between 0 and RAND_MAX (inclusive). RAND_MAX is a constant defined in cstdlib (typically the maximum value of 16-/32-bit signed integer, such as 32767). You can generate a random number between [0,n) via rand() % n.

rand() generates the same squence of pseudo-random numbers on different invocations. The cstblib also provides a srand() function to seed or initialize the random number generator. We typically seed it with the current time obtained via time(0) function (in <ctime> header), which returns the number of seconds since January 1st, 1970.

Example 1: Test rand() and srand(time(0))
Example 2: Test rand()'s Distribution

We shall test the rand()'s distribution by repeatedly throwing a 6-sided die and count the occurrences.

As seen from the output, rand() is fairly uniformly-distributed over [0, RAND_MAX].

Exercises

[TODO]

File Input/Output (Header <fstream>)

The <fstream> header provides ifstream (input file stream) and ofstream (output file stream) for file input and output. The steps for file input/output are:

  1. Create a ifstream for input, or ofstream for output.
  2. Connect the stream to an input or output file via open(filename).
  3. Perform formatted output via stream insertion operator <<, or input via stream extraction operator >>, similar to cout << and cin >>.
  4. Close the file and free the stream.

Example: File IO

Input File: in.txt
Output File: out.txt

Program Notes:

  • Once the file is opened, you can use >> and << for input and output, similar to cin >> and cout <<. (Advanced note: ifstream is a subclass of istream, where cin belongs. ofstream is a subclass of ostream, where cout belongs.)
  • Similarly, IO manipulators, such as fixed, setprecision() and setw(), work on the file streams.

Exercises

[TODO]

Namespace

When you use different library modules, there is always a potential for name crashes, as different library may use the same name for different purposes. This problem can be resolved via the use of namespace in C++. A namespace is a collection for identifiers under the same naming scope. (It is known as package in UML and Java.) The entity name under a namespace is qualified by the namespace name, followed by :: (known as scope resolution operator), in the form of namespace::entityName.

To place an entity under a namespace, use keyword namespace as follow:

A namespace can contain variables, functions, arrays, and compound types such as classes and structures.

Namespace Example

Namespaces are opened. In other words, you can add more names into an existing namespace using additional namespace definition.

Using Namespace

For example, all the identifiers in the C++ standard libraries (such as cout, endl and string) are placed under the namespace called std. To reference an identifier under a namespace, you have three options:

  1. Use the fully qualified names, such as std::cout, std::endl, std::setw() and std::string. For example, Missing the 'std::' results in 'error: 'xxx' was not declared in this scope'.
  2. Use a using declaration to declare the particular identifiers. For example, You can omit the 'std::' for cout and endl, but you still have to use 'std::' for setw.
  3. Use a using namespace directive. For example, The using namespace directive effectively brings all the identifiers from the specified namespace to the global scope, as if they are available globally. You can reference them without the scope resolution operator. Take note that the using namespace directive may result in name crashes with identifier in the global scope.
  4. For long namespace name, you could define a shorthand (or alias) to the namespace, as follows: You can now refer to your class as shorthand::entityName.

As mentioned, all the standard C++ library components are packaged inside a namespace called std. They include cin, cout, endl, setw(), setprecision(), and string. Hence, we always included a 'using namespace std;' directive after the '#include <iostream>' and '#include <string>'. You could also use the qualified name such as std::cin, std::cout, std::endl, std::string, instead of the using directive. You could also use selective using such as 'using std::cin;', 'using std::cout;', 'using std::endl;' and 'using std::string;'.

Compare the following two code samples:

Note: you could use the C-style header 'iostream.h' which does not use namespace to replace the first two lines in the first sample.

Global Namespace

In C++, an entity (variable, function, or class) belongs to the global namespace (identified by :: with no namespace name), if it is not enclose within a namespace declaration. For example, main() can be identified as ::main().

Link to 'C++ References and Resources'