C If ... Else

A program becomes useful when it can choose what to do. The if statement runs a block of code only when a condition is true.

if

#include <stdio.h>

int main(void) {
    int temperature = 31;
    if (temperature > 30) {
        printf("It is hot today.\n");
    }
    printf("Done.\n");
    return 0;
}

It is hot today.
Done.

The condition goes in brackets; the code to run goes in curly braces. Braces are optional for a single statement, but always writing them avoids a classic bug when a second line is added later.

else

else runs when the condition is false, so exactly one of the two blocks runs.

#include <stdio.h>

int main(void) {
    int number = 7;
    if (number % 2 == 0) {
        printf("%d is even\n", number);
    } else {
        printf("%d is odd\n", number);
    }
    return 0;
}

7 is odd

else if

To choose between more than two cases, chain tests with else if. They are checked from the top, and the first true one wins; the rest are skipped.

#include <stdio.h>

int main(void) {
    int marks;
    printf("Marks (0-100): ");
    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("Grade D\n");
    }
    return 0;
}

78

Marks (0-100): Grade B

The order matters: if the test for marks >= 50 came first, a mark of 95 would get a C. Run the example and type different marks to see each branch.

Combining conditions

Use && (and), || (or) and ! (not) to combine conditions: if (age >= 13 && age <= 19) is true for teenagers. Use brackets when you mix && and || so the meaning is clear.

A common mistake: = instead of ==

if (x = 5) does not compare: it stores 5 in x, and since 5 is not zero, the condition is always true. Always use == to compare. With -Wall, the compiler warns about this mistake.

The conditional operator

For a choice between two values, there is a short form: condition ? valueIfTrue : valueIfFalse.

#include <stdio.h>

int main(void) {
    int a = 12, b = 30;
    int bigger = (a > b) ? a : b;
    printf("The bigger number is %d\n", bigger);
    printf("%s\n", bigger > 20 ? "large" : "small");
    return 0;
}

The bigger number is 30
large

Use it for short choices only; for anything longer, if and else are easier to read.

Practice exercises

1. Read a year and print whether it is a leap year: divisible by 4, but not by 100 unless also by 400. Test 2000, 1900 and 2024.
2. Read three numbers and print the largest.
3. Read a temperature and print "cold" below 15, "pleasant" from 15 to 30 and "hot" above 30.