Identifiers

Identifiers, File Nomenclature, Keywords, Constants, Comments

The following topics are covered in this section:

Identifiers
File Nomenclature
Keywords
Constants
Comments

Identifiers:

Identifiers are very important in C++ and are used in all programs. What is an identifier? In the previous program ‘check’ was declared as a character variable. The identifier in this case is ‘check’. Basically identifiers are names that are given to variables, constants or functions. In the previous program there are two identifiers: ‘check’ and ‘i’.

C++ permits the use of a huge variety of names for identifiers like:

test
Test
int_test
var1
var123
c_b_f
_var

Almost any name you can think of can be used as an identifier but there are some restrictions. An identifier should not start with a number. The first character of an identifier should be an alphabet or an underscore ( _ ). After the first character, numbers can be used in the identifier. Remember the following:

Never start an identifier with anything other than a letter or an underscore.
It is better to use a letter than starting with underscores.
Do not use keywords as identifiers (ex: do not name an identifier as int).
Uppercase and Lowercase identifiers are different (Check is different from check).
Be careful when using characters that look similar to each other. For example ‘1’ (the number one) and ‘l’ (the lowercase alphabet L) look alike.
Use names that are easy to understand (if you are storing the salary of a person, name the variable as ‘salary’ instead of declaring it as ‘x’.
Do not use very long names.
Do not name many variables with similar names (avoid using identifiers like: count, counter, counting etc.).

Keywords:

Keywords are reserved words in C++ programming. They should not be used as identifiers and all keywords should be in lower case. The 63 keywords are tabulated below.

Asm	Auto	bool	break	case	Catch	char
Class	Const	const_cast	continue	default	Delete	do
Double	Dynamic_cast	else	enum	explicit	Export	extern
False	Float	For	friend	goto	If	inline
Int	Long	mutable	namespace	new	Operator	private
Protected	Public	register	reinterpret_cast	return	Short	signed
Sizeof	Static	static_cast	struct	switch	Template	this
Throw	True	Try	typedef	typeid	typename	union
Unsigned	Using	virtual	void	volatile	wchar_t	while

File Naming Convention:

A few points regarding file nomenclature are:

In MSDOS a filename cannot have more than 8 characters whereas in Windows there is no such restriction.
Use meaningful names (depending on what the program performs).
Avoid using underscores in the file names.
Be careful when using characters that look similar to each other. For example ‘1’ (the number one) and ‘l’ (the lowercase alphabet L) look alike.

File Type	Extension used
C++ source file	*.cpp
C++/C header file	*.h
C source file	*.c
Executable file	*.exe
Object Code	*.obj
Text file	*.txt
Word document	*.doc
Image/graphics file	.jpg/ .gif/ *.bmp

Constants:

Constants are used in expressions and their values cannot be changed. They are used in the program code without using any variable name to refer to them. For example consider the statements:

int x = 5;

float y = 3.33;

In this, 5 is an integer constant and 3.33 is a floating type constant. You cannot change 5 or 3.33 but you can assign these values to variables (the variables in the above code fragment are ‘x’ and ‘y’).

Constants can be classified based on their data type as follows:

Numeric type constants (includes integers, floating-point etc.)
Character Constants
String Constants

Numeric Constants:

Numeric constants consist of a series of digits. Integer constants can be written in different number systems: hexadecimal (base 16), octal (base 8) or in decimal (base 10).

A decimal integer constant is any normal whole number (can consist of digits from 0 to 9):

2,34, 100, 900, 1456 etc.

An octal integer constant can contain digits from 0 to 7 only and it should start with a zero (so that the computer will know it is an octal number). For example:

023, 0567, 0214 etc.

A hexadecimal integer constant should start with 0x or 0X and can consist of digits from 0 to 9 and A to F (uppercase and lowercase letters are allowed). For example:

0x10, 0x1FF etc.

Floating point constants will have the decimal point in the number.

Character and String Constants:

Character constants are single characters enclosed within two single quotes (or between two apostrophes). For example:

‘a’

‘b’

‘x’

‘1’

‘A’

‘*’

A single character enclosed within single quotes is a character constant. All character constants will have an integer value (determined from the ASCII table). A C++ statement:

char ch = ‘B’;

will assign the character constant ‘B’ to the character variable ch.

String constants consist of a series of characters enclosed within double quotes. For example:

"hello"

"This is a string"

"x"

Even "x" is a string because it is enclosed in double quotes. "x" is different from ‘x’ (this is a character constant). The reason is because "x" actually consists of ‘x’ and ‘\0’ (the null character which will be explained later).

Constants are also called as ‘literal’. You might come across the terms character literal, integer literal, string literal etc.

Beware: The constants described in this section are different from variables using the ‘const’ keyword (this is discussed in Unit 6).

Comments:

Comments are not meant for the compiler to read. They are meant to make the program easier to understand for humans who might read the coding later. It is a good practice to write comments (even for small programs). What should be written in the comments? First of all, a couple of lines about the aim of the program should be mentioned. You should also mention the name of the programmer and the date when the program has been modified. Comments should be updated every time you modify the code. As the program gets larger, it is advisable to write comments for the various functions that are used in the program. For instance, if a function is used to calculate the sum of numbers, then this could be mentioned in the comments. Basically you are permitted to write anything within comments because it will not affect the program.

There are two methods used to denote comments:

Single line commenting (//)
Multiple line commenting (/*…..*/)

For single line comments use double slashes (//) followed by the comments (but it should be on the same line as the double slash). Example:

//this is a single line comment

Multiple line commenting permits you to write line after line of comments. It will be as follows:

/* Multiple line commenting…

…this is also part of the comment…

……comment continued……

*/

Everything in between /* and */ will be commented. By using multiple line commenting the programmer needn’t use double slashes for each and every line.

Recap:

The file iostream.h provides the general basic input and output functions.
‘cout’ is used to display data on the screen while ‘cin’ is used to obtain inputs from the user.
Every variable/function in C++ has to have a name. This name is called an identifier.
Keywords are specially reserved words in C++. They cannot be used as identifiers.
There are two data types in C++: fundamental and user-defined.
Variables are named memory locations (their name is provided by the programmer) where values can be stored and changed frequently in the program.
Constants are usually used to assign values to the variables.
All data types have different sizes and are used for specific purposes. The type of a variable determines what it can store.
Comments are written for easy understanding of the program (they are not processed by the compiler).

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