Keywords, Constants, Variables: C

Computers do not understand English language. It can only understand machine code, a binary stream of 1s and 0s.

Learning The Building Blocks of C programming Language

To learn any language we need to first learn alphabets, then we learn to write sentences and then to write paragraphs. Similarly, in learning C programming language, we first learn about the Character Set(Alphabets, Digits, Special Symbols), next we learn words(keywords, variables, constants), and then we learn statements(C programming instructions), and then we write C programs.

Keywords, Identifiers And Literals: C



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Keywords

Every word in a C program is classified as either a keyword or an identifier. All keywords have fixed meanings predefined in the language and these meanings can not be changed.

Identifier / Literals

In programming languages, constants are usually called as literals and variables are called as identifiers.

Constants / Variables

As name suggests, a constant is an entity whose value doesn’t change during the course of program execution. A variable, on the other hand, is an entity whose value may change during the course of program execution.



C Constants

C Constants can be divided into 2 categories.
1. Primary Constants.
2. Secondary Constants.

Primary Constants are further divided into Integer Constants, Real Constants and Character constants.

Keyword for Integer is int and the format specifier is %d.
Keyword for Real is float or double and the format specifier is %f.
Keyword for Character is char and the format specifier is %c.

  1.    
  2. #include < stdio.h >  
  3. int main()  
  4. {  
  5.     const int a = 5;  
  6.   
  7.     a = a + 1;  
  8.   
  9.     printf("Value of a is %d", a);  
  10. }  

Output:
error: Assignment of read-only variable ‘a’.

a is a constant variable and thus its value can not be changed through the course of program execution.

  1.    
  2. #include < stdio.h >  
  3. int main()  
  4. {  
  5.     int a = 5;  
  6.   
  7.     a = a + 1;  
  8.   
  9.     printf("Value of a is %d", a);  
  10. }  

Output:
Value of a is 6

Note: Literals are constant values and not constant variables.