Prefix form

The operator is placed before the variable: ++x or --x. The expression gives the updated value. Starting with x = 5, y = ++x; leaves both x and y equal to 6. Starting with x = 5, y = --x; leaves both equal to 4.

Postfix form

The operator is placed after the variable: x++ or x--. The expression gives the previous value while the variable is updated. Starting with x = 5, y = x++; leaves y equal to 5 and x equal to 6. Starting with x = 5, y = x--; leaves y equal to 5 and x equal to 4.

When used alone

As separate statements, ++x; and x++; both increase x by one. Their difference matters when the expression value is used, as in an assignment. The same rule applies to decrement.

Important points

1. Writable variable: The operand must be something C permits to be changed. A constant such as 5 cannot be incremented.
2. Valid range: The updated value must fit the type; signed overflow has no defined result.
3. Separate changes: Do not use i++ + ++i. It changes i without the required ordering and has undefined behaviour, so there is no guaranteed answer.
4. Clear code: Use separate statements when several updates would be confusing.

Reference table

C program and output

/* CodeBhavya | PPS AR25 | Learning example */
#include <stdio.h>
int main(void)
{
    int x = 5;
    int before = x++;
    int after = ++x;
    printf("before = %d\n", before);
    printf("after = %d\n", after);
    printf("x = %d\n", x);
    return 0;
}

before = 5
after = 7
x = 7

Key points

• Prefix and postfix differ when their expression value is used.
• Use separate statements when sequencing matters.