Constants, Variables and Keywords in C

A constant, Variable or a keyword can be formed from any combination of Alphabets, Digits and Special Symbols. A constant is an entity whose value does not change throughout the program execution.

A variable is an entity whose value keeps on changing throughout the program execution. However, it’s not a rule that the value of the variable will change. A variable value might remain same throughout the program execution. However the main difference between variable and constant is that we can’t change the value of constant in between the program, but we can change the value of the variable during program execution.
Let us take an example:
We define a variable named x in a program. Let us say x=3.
After some time during execution we change the value of x to 6. Now, x=6.
Here value of x is getting changed. But the value of 3 and 6 will never change. Hence, x is called a variable and 3 is called a constant.

Keywords are the words whose meaning has already been explained to the C compiler. The keyword cannot be used as the variable name. If we try to do so we are trying to assign new meaning to the keyword. The keywords are also known as ‘Reserved words’.

E.g.: for, if, static, while, do, break etc.