C Program to Check Positive, Negative or Zero

In C, you can check if a number is positive, negative, or zero using if statements like if (num > 0) for positive, else if (num < 0) for negative, and else for zero.

Examples

Input5

OutputPositive number

Input-3

OutputNegative number

Input0

OutputZero

How to Think About It

To decide if a number is positive, negative, or zero, first compare it with zero using > and < operators. If it is greater than zero, it is positive; if less, it is negative; otherwise, it is zero.

Algorithm

1

Get input number from the user

2

Check if the number is greater than zero

3

If yes, print 'Positive number'

4

Else check if the number is less than zero

5

If yes, print 'Negative number'

6

Otherwise, print 'Zero'

Code

c

#include <stdio.h>

int main() {
    int num;
    printf("Enter a number: ");
    scanf("%d", &num);

    if (num > 0) {
        printf("Positive number\n");
    } else if (num < 0) {
        printf("Negative number\n");
    } else {
        printf("Zero\n");
    }
    return 0;
}

Output

Enter a number: 5
Positive number

Dry Run

Let's trace the input 5 through the code

1

Input

User enters num = 5

2

Check if num > 0

5 > 0 is true

3

Print result

Print 'Positive number'

Step	Condition	Result
Check if num > 0	5 > 0	true
Check if num < 0	5 < 0	false
Else	-	Print 'Positive number'

Why This Works

Step 1: Input reading

The program reads an integer from the user using scanf.

Step 2: Positive check

It uses if (num > 0) to check if the number is positive.

Step 3: Negative and zero check

If not positive, it checks else if (num < 0) for negative, else it must be zero.

Alternative Approaches

Using switch with sign function

c

#include <stdio.h>

int sign(int n) {
    return (n > 0) - (n < 0);
}

int main() {
    int num;
    printf("Enter a number: ");
    scanf("%d", &num);

    switch(sign(num)) {
        case 1:
            printf("Positive number\n");
            break;
        case -1:
            printf("Negative number\n");
            break;
        default:
            printf("Zero\n");
    }
    return 0;
}

This uses a helper function and switch-case for clarity but is slightly more complex.

Using ternary operator

c

#include <stdio.h>

int main() {
    int num;
    printf("Enter a number: ");
    scanf("%d", &num);

    (num > 0) ? printf("Positive number\n") : (num < 0) ? printf("Negative number\n") : printf("Zero\n");
    return 0;
}

This is a compact form using ternary operators but can be harder to read for beginners.

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

Time Complexity

The program performs a fixed number of comparisons regardless of input size, so it runs in constant time O(1).

Space Complexity

It uses a fixed amount of memory for variables and no extra data structures, so space complexity is O(1).

Which Approach is Fastest?

All approaches run in constant time; the simple if-else is easiest to read and fastest to write.

Approach	Time	Space	Best For
If-else statements	O(1)	O(1)	Simplicity and clarity
Switch with sign function	O(1)	O(1)	Structured code with switch-case
Ternary operator	O(1)	O(1)	Compact code, less readable