Increment and Decrement operator in C language

Increment and Decrement operator are used to increment or decrement value by 1.

There are two variants of increment/decrement operator.

• Prefix (pre-increment and pre-decrement)
• Postfix (post-increment and post-decrement)

Syntax of increment/decrement operator

Important note: ++ and -- operators are used with variables. Using ++ or -- with constant will result in error. Such as expressions like 10++, (a + b)++ etc. are invalid and causes compilation error.

Let us consider an integer variable int a = 10;. To increment a by 1, you can use either
a = a + 1 (Simple assignment)
a += 1 (Shorthand assignment)
a++ (Post increment)
++a (Pre increment)

Result of all the above code are similar.

Prefix vs Postfix

Both prefix and postfix does same task of incrementing/decrementing the value by 1. However, there is a slight difference in order of evaluation.

Prefix first increment/decrements its value then returns the result. Whereas postfix first returns the result then increment/decrement the value.

To understand this efficiently let’s consider an example program

#include <stdio.h>

int main()
{
    int a, b, c;

    a = 10;   // a = 10
    b = ++a;  // a=11, b=11
    c = a++;  // a=12, c=11

    printf("a=%d, b=%d, c=%d", a, b, c);

    return 0;
}

Output of above program is a=12, b=11, c=11. Let us understand the code.

a = 10 Assigns 10 to variable a
b = ++a Since we have used prefix notation. Hence, first it increments the value of a to 11, then assigns the incremented value of a to b.
c = a++ Here we have used postfix notation. Hence, first it assigns the current value of a i.e. 11 to c, then increments the value of a to 12.

Important note: Never use postfix and prefix operator at once otherwise it will result in error. Such as ++a++, ++a-- both results in compilation error.