Keywords and Identifiers in C programming

In previous post, we executed our first C program. That was interesting and a stepping-stone towards complex C programs.

A C program consists of various programming elements. These programming elements are the building blocks of any simple or complex application. Programming elements such as keyword, identifier, operator, expression, constant etc.

In this section and subsections, I am will cover all fundamental programming elements of a C program.

C character set

C character set defines set of valid characters used to write a C program. The set consists of alphabets (both upper case and lower case), digits (0-9), various special characters symbols and white space characters.

  • Alphabets: A-Z a-z
  • Digits: 0-9
  • Special character: ` ~ ! @ # $ % ^ & * ( ) _ – + = { } [ ] | \ : ; “ ‘ < , > . ? /
  • White space characters: Blank space, new line, horizontal tab, vertical tab, formfeed and carriage return.

Tokens in C programming

Tokens are the smallest individual unit of a program. They are the building blocks of a program. For example, lets us consider an English paragraph:

“I love programming in C. I love Codeforwin.”

The above sentence is made with various basic elements such as – Alphabets (a-z, A-Z), punctuation marks and blank spaces. These are tokens of the given paragraph.
Similarly, all basic elements of a program collectively known as Tokens. There are five types of tokens.

  1. Keywords
  2. Identifiers
  3. Operators
  4. Separators
  5. Literals

Consider the below C statement –
int num = 5 + 4;


  • int is keyword.
  • num is identifier.
  • = and + are operators.
  • 5 and 4 are literals.
  • ; and white spaces are separators.

What are Keywords?

Keywords are the reserved words whose meaning is predefined by the programming language specification. They convey some special meaning in programming and we must not use them for other purposes.

For example – int is a C keyword that defines integer data type. You must not use int for other purposes.

There are total 32 keywords in C as per ANSI standards.


Note: Apart from the 32 standard C keyword, different compilers can have more number of keywords.

What are Identifiers?

Identifiers are the names given to various programming elements. Such as name given to a variable, function, user defined type etc.

In real life, you may think name given to a person or object as an identifier. In programming, we use identifiers to identify a programming element uniquely.

Rules for naming an identifier

  1. Identifier must not be a keyword. For example – you cannot use int, return etc. as an identifier.
  2. Identifiers must begin with an alphabet a-z A-Z or underscore _ symbol.
  3. Identifiers can contain digits 0-9.
  4. Identifiers must not contain special character other than alphabets, digits and underscore symbol.
  5. C is a case sensitive language hence you must be careful while naming an identifier. For example – num, NUM, Num all are different.

Examples of some valid identifiers

num _num num1 _num1 _1num1 NUM _NUM_

Examples of some invalid identifiers

int int is a keyword and must not be used as an identifier.
1num Identifier should begin with an alphabet or underscore symbol.
num$ It must not contain any special symbol $ in this case.
first num It must not contain spaces.
date-of-birth It must not contain any special symbol in this case.

Note: We must give a meaningful name to an identifier. However, it is not a naming convention but considered as a good programming practice. A meaningful identifier increases program readability and clarity.

In C, every word is either a keyword or an identifier.