Difference between Keyword and Identifier in C

Last Updated : 23 Dec, 2024

In C, keywords and identifiers are basically the fundamental parts of the language used. Identifiers are the names that can be given to a variable, function or other entity while keywords are the reserved words that have predefined meaning in the language.

The below table illustrates the primary differences between the keywords and identifiers:

Parameters	Keywords	Identifiers
Definition	Keywords are predefined word that gets reserved for working program that have special meaning and cannot get used anywhere else.	Identifiers are the values used to define different programming items such as variables, integers, structures, unions and others and mostly have an alphabetic character.
Use	Specify the type/kind of entity.	Identify the name of a particular entity.
	It always starts with a lowercase letter.	First character can be a uppercase, lowercase letter or underscore.
Rules of Definition	A keyword should be in lower case and can only contains alphabetical characters.	An identifier can be in upper case or lower case and can consist of alphabetical characters, digits and underscores.
Purpose	They help to identify a specific property that exists within a computer language.	They help to locate the name of the entity that gets defined along with a keyword.
Examples	int, char, if, while, do, class etc.	Test, count1, high_speed, etc.

Keywords

Keywords are specific reserved words in C each of which has a specific feature associated with it. Almost all of the words which help us use the functionality of the C language are included in the list of keywords. So, you can imagine that the list of keywords is not going to be a small one! There are a total of 32 keywords in C:

auto	break	case	char	const	continue	default	do
double	else	enum	extern	float	for	goto	if
int	long	register	return	short	signed	sizeof	static
struct	switch	typedef	union	unsigned	void	volatile	while

Identifiers

Identifiers are used as the general terminology for naming of variables, functions and arrays. These are user defined names consisting of arbitrarily long sequence of letters and digits with either a letter or the underscore(_) as a first character. Identifier names must differ in spelling and case from any keywords. You cannot use keywords as identifiers; they are reserved for special use. Once declared, you can use the identifier in later program statements to refer to the associated value. A special kind of identifier, called a statement label, can be used in goto statements.

Code Illustration

The below example illustrate the different purpose of the keywords and identifers in C:

#include <stdio.h>

// 'main' is also an identifier
// although it is predefined
int main() {
  
    // Example of a keyword: int
  	// Here, age is identifier
    int age = 25;

    printf("Age: %d\n", age);

    // return is a keyword that exits function
    return 0; 
}

Output

Age: 25

Explanation: In the above code, age is an identifier used to name a variable. The type of variable is integer that is specified using int keyword as it already is defined in C to indicate that the variable will store integer data.

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Difference between Keyword and Identifier in C

Keywords

Identifiers

Code Illustration

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