2.2 Conditional Branches

Conditional branches let a program choose different code paths for different situations. C uses if, else if, and else for branching, and groups the statements that belong to a branch into a block with braces {}.

if

An if statement checks a condition. If the condition is true, C runs the block inside the braces. If the condition is false, C skips that block.

int age;
scanf("%d", &age);

if (age > 0 && age < 18) {
    printf("Minor\n");
}

One possible run:

17
Minor

In C, the statements that belong to an if go inside braces {}. Even for a single line, adding braces is recommended to avoid mistakes when the code grows later.

if-else

if-else is used for a two-way choice: run the if block when the condition is true; otherwise run the else block.

Here is a leap-year example. A year is a leap year if it satisfies one of these conditions:

• It is divisible by 4, and not divisible by 100.
• Or it is divisible by 400.

int year;
scanf("%d", &year);

int is_leap_year = (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0);

if (is_leap_year) {
    printf("Leap year\n");
} else {
    printf("Common year\n");
}

One possible run:

2020
Leap year

Saving the complex condition into is_leap_year makes the if line easier to read. In real programs, this is often more maintainable than placing a long condition directly inside if.

if-else if-else

When there are more than two cases, use if-else if-else. C checks conditions from top to bottom. It runs the first branch whose condition is true, then skips the remaining branches.

char c;
scanf(" %c", &c);

if (c >= 'a' && c <= 'z') {
    printf("Lowercase\n");
} else if (c >= 'A' && c <= 'Z') {
    printf("Uppercase\n");
} else if (c >= '0' && c <= '9') {
    printf("Digit\n");
} else {
    printf("Special character\n");
}

One possible run:

T
Uppercase

The order of else if branches matters. Put more specific or higher-priority conditions earlier, and use else as the fallback case.