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

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.

 
#include < stdio.h >
int main()
{
    const int a = 5;

    a = a + 1;

    printf("Value of a is %d", a);
}

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.

 
#include < stdio.h >
int main()
{
    int a = 5;

    a = a + 1;

    printf("Value of a is %d", a);
}

Output:
Value of a is 6

Note: Literals are constant values and not constant variables.