C Program to Find Absolute Value of a Number

To find the absolute value in C, use the code: if (num < 0) num = -num; which changes negative numbers to positive, leaving positive numbers unchanged.

Examples

Input-5

OutputAbsolute value of -5 is 5

Input10

OutputAbsolute value of 10 is 10

Input0

OutputAbsolute value of 0 is 0

How to Think About It

To find the absolute value, check if the number is less than zero. If yes, multiply it by -1 to make it positive. If not, keep it as is because it's already positive or zero.

Algorithm

1

Get the input number from the user

2

Check if the number is less than zero

3

If yes, multiply the number by -1 to make it positive

4

Print the resulting number as the absolute value

Code

c

#include <stdio.h>

int main() {
    int num;
    printf("Enter an integer: ");
    scanf("%d", &num);
    if (num < 0) {
        num = -num;
    }
    printf("Absolute value of the number is %d\n", num);
    return 0;
}

Output

Enter an integer: -7
Absolute value of the number is 7

Dry Run

Let's trace the input -7 through the code

1

Input

User enters -7, so num = -7

2

Check if num < 0

Since -7 < 0 is true, enter the if block

3

Make num positive

num = -(-7) = 7

4

Print result

Print 'Absolute value of the number is 7'

Step	num	Condition (num < 0)	Action
1	-7	True	num = -num => 7
2	7	False	Print 7

Why This Works

Step 1: Check if number is negative

The code uses if (num < 0) to find if the number is less than zero.

Step 2: Convert negative to positive

If the number is negative, multiplying by -1 makes it positive.

Step 3: Print absolute value

The final value of num is printed as the absolute value.

Alternative Approaches

Using ternary operator

c

#include <stdio.h>

int main() {
    int num;
    printf("Enter an integer: ");
    scanf("%d", &num);
    num = (num < 0) ? -num : num;
    printf("Absolute value of the number is %d\n", num);
    return 0;
}

This method is shorter and uses a single line to assign the absolute value.

Using built-in function abs()

c

#include <stdio.h>
#include <stdlib.h>

int main() {
    int num;
    printf("Enter an integer: ");
    scanf("%d", &num);
    printf("Absolute value of the number is %d\n", abs(num));
    return 0;
}

This uses the standard library function <code>abs()</code> which is simple and reliable.

Complexity: O(1) time, O(1) space

Time Complexity

The program performs a constant number of operations regardless of input size, so it runs in constant time.

Space Complexity

Only a fixed amount of memory is used for variables, so space complexity is constant.

Which Approach is Fastest?

All approaches run in constant time; using abs() is simplest and less error-prone.

Approach	Time	Space	Best For
If-else check	O(1)	O(1)	Understanding logic
Ternary operator	O(1)	O(1)	Concise code
Built-in abs()	O(1)	O(1)	Simplicity and reliability