if, else if and else

if

if runs a block of code only when its condition is true:

#include <stdio.h>

int main() {
    int temperature = 32;
    if (temperature > 30) {
        printf("It's hot today!\n");
    }
    printf("Have a nice day.\n");
    return 0;
}

Output

It's hot today!
Have a nice day.

The condition goes in round brackets ( ). The code to run goes in curly braces { }. The last line runs every time, because it is outside the braces.

else

else gives the other path: what to do when the condition is false.

#include <stdio.h>

int main() {
    int age;
    scanf("%d", &age);
    if (age >= 18) {
        printf("You can vote.\n");
    } else {
        printf("You can vote in %d years.\n", 18 - age);
    }
    return 0;
}

Input (what the user types)

15

Output

You can vote in 3 years.

else if: more than two choices

Chain several conditions with else if. C checks them from top to bottom and runs only the first one that is true:

#include <stdio.h>

int main() {
    int marks;
    scanf("%d", &marks);
    if (marks >= 90) {
        printf("Grade A\n");
    } else if (marks >= 75) {
        printf("Grade B\n");
    } else if (marks >= 50) {
        printf("Grade C\n");
    } else {
        printf("Try again\n");
    }
    return 0;
}

Input (what the user types)

82

Output

Grade B

82 is not ≥ 90, but it is ≥ 75, so C prints Grade B and skips the rest.

Think of it like this: An if / else if chain is like a sorting machine for letters: each letter falls into the first slot it fits, and never into two.

Even or odd?

A classic example using %:

#include <stdio.h>

int main() {
    int n;
    scanf("%d", &n);
    if (n % 2 == 0) {
        printf("%d is even\n", n);
    } else {
        printf("%d is odd\n", n);
    }
    return 0;
}

Input (what the user types)

7

Output

7 is odd

Common mistake: Putting a semicolon after the condition: if (x > 5);. The ; ends the if right there, so the block after it always runs.

Tip: Always use curly braces, even for one line. It keeps your code clear and safe when you add more lines later.