Increment and Decrement Operators in C

The increment operator ++ increases the value of an object by one.

int number = 10;

number++;

printf("%d", number);

Output:

11

The decrement operator -- decreases the value of an object by one.

int number = 10;

number--;

printf("%d", number);

Output:

9

The ++ operator is used when you want to increase a variable by one.

int count = 5;

count++;

printf("%d", count);

Output:

6

The -- operator decreases a variable by one.

int count = 5;

count--;

printf("%d", count);

Output:

4

For an appropriate modifiable arithmetic object, incrementing by one can be written as number += 1.

int number = 10;

number += 1;

printf("%d", number);

Output:

11

The ++ operator provides a shorter way to increment by one.

For an appropriate modifiable arithmetic object, decrementing by one can be written as number -= 1.

int number = 10;

number -= 1;

printf("%d", number);

Output:

9

In prefix increment, the ++ operator is placed before the variable.

int number = 10;

++number;

printf("%d", number);

Output:

11

The variable is incremented before the statement continues.

In postfix increment, the ++ operator is placed after the variable.

int number = 10;

number++;

printf("%d", number);

Output:

11

When used as a standalone statement, prefix and postfix increment both increase the variable by one.

In prefix decrement, the -- operator is placed before the variable.

int number = 10;

--number;

printf("%d", number);

Output:

9

In postfix decrement, the -- operator is placed after the variable.

int number = 10;

number--;

printf("%d", number);

Output:

9

Prefix and postfix forms differ when the value of the expression itself is used.

int a = 5;
int b = ++a;

printf("a = %d\n", a);
printf("b = %d", b);

Output:

a = 6
b = 6

The prefix increment changes a before its value is assigned to b.

int a = 5;
int b = a++;

printf("a = %d\n", a);
printf("b = %d", b);

Output:

a = 6
b = 5

The old value of a is used for the expression, and then a is incremented.

int a = 5;
int b = --a;

printf("a = %d\n", a);
printf("b = %d", b);

Output:

a = 4
b = 4

The decrement happens before the value is used in the expression.

int a = 5;
int b = a--;

printf("a = %d\n", a);
printf("b = %d", b);

Output:

a = 4
b = 5

The old value is used for the expression, and then a is decremented.

#include <stdio.h>

int main()
{
    for (int i = 1; i <= 5; i++)
    {
        printf("%d ", i);
    }

    return 0;
}

Output:

1 2 3 4 5

The increment operator increases i after each loop iteration.

#include <stdio.h>

int main()
{
    for (int i = 5; i >= 1; i--)
    {
        printf("%d ", i);
    }

    return 0;
}

Output:

5 4 3 2 1

The decrement operator decreases i after each iteration.

int i = 1;

while (i <= 5)
{
    printf("%d ", i);
    i++;
}

Output:

1 2 3 4 5

The variable is incremented inside the loop body.

int i = 5;

while (i >= 1)
{
    printf("%d ", i);
    i--;
}

Output:

5 4 3 2 1

int count = 0;

count++;
count++;
count++;

printf("Count = %d", count);

Output:

Count = 3

Increment is commonly used for counters.

int count = 5;

count--;
count--;

printf("Count = %d", count);

Output:

Count = 3

Decrement is useful when a counter needs to move downward.

#include <stdio.h>

int main()
{
    int number;

    printf("Enter a number: ");
    scanf("%d", &number);

    number++;

    printf("After increment = %d", number);

    return 0;
}

If the user enters 10, the output will be After increment = 11.

#include <stdio.h>

int main()
{
    int number;

    printf("Enter a number: ");
    scanf("%d", &number);

    number--;

    printf("After decrement = %d", number);

    return 0;
}

If the user enters 10, the output will be After decrement = 9.

int number = 10;

if (number > 5)
{
    number++;
}

printf("%d", number);

Output:

11

The value is incremented only when the condition is true.

int number = 5;

printf("%d", ++number);

Output:

6

With prefix increment, the variable is incremented before its value is used by the expression.

int number = 5;

printf("%d", number++);

printf("\n%d", number);

Output:

5
6

The expression uses the old value first, and then the variable is incremented.

int number = 5;

printf("%d", --number);

Output:

4

The variable is decremented before its value is used.

int number = 5;

printf("%d", number--);

printf("\n%d", number);

Output:

5
4

The old value is used first, and then the variable is decremented.

• Confusing prefix and postfix behavior when the expression value is used.
• Trying to increment a constant or other non-modifiable object.
• Forgetting to update a loop counter.
• Using decrement when an increment is required.
• Writing complicated expressions with multiple modifications of the same variable.
• Using ++ or -- on expressions that are not modifiable objects.

#include <stdio.h>

int main()
{
    int number = 10;

    printf("Original = %d\n", number);

    number++;
    printf("After increment = %d\n", number);

    number++;
    printf("After second increment = %d\n", number);

    number--;
    printf("After decrement = %d\n", number);

    number--;
    printf("After second decrement = %d\n", number);

    return 0;
}

Output:

Original = 10
After increment = 11
After second increment = 12
After decrement = 11
After second decrement = 10

Practice the following programs:

1. Increment a number by one.
2. Decrement a number by one.
3. Use prefix increment in an expression.
4. Use postfix increment in an expression.
5. Use prefix decrement in an expression.
6. Use postfix decrement in an expression.
7. Print numbers from 1 to 10 using increment.
8. Print numbers from 10 to 1 using decrement.
9. Create a counter using ++.
10. Create a countdown using --.

Practice both prefix and postfix forms so that you understand when the variable is modified relative to the value produced by the expression.