C switch Statement

Some decisions are really one question with many fixed answers: which button was pressed, which menu option was chosen, which day of the week it is. The C switch statement handles exactly this kind of choice. In this lesson, you'll learn how switch, case, break and default work together, and how to avoid the "fall-through" bug that surprises almost everyone the first time.

What is a switch statement in C?

A switch statement picks one of several blocks of code based on the value of a single integer expression. Here's the general shape:

switch (expression) {
    case VALUE_1:
        // statements for VALUE_1
        break;
    case VALUE_2:
        // statements for VALUE_2
        break;
    default:
        // statements when nothing matched
        break;
}

• switch (expression): the value being tested, such as a variable.
• case VALUE:: a label. If the expression equals this value, execution starts here.
• break;: jumps out of the switch, to the first line after its closing brace.
• default:: optional; runs when no case matches, like the final else in an if chain.

A first example: days of the week

#include <stdio.h>

int main(void) {
    int day = 3;

    switch (day) {
        case 1:
            printf("Monday\n");
            break;
        case 2:
            printf("Tuesday\n");
            break;
        case 3:
            printf("Wednesday\n");
            break;
        case 4:
            printf("Thursday\n");
            break;
        case 5:
            printf("Friday\n");
            break;
        case 6:
            printf("Saturday\n");
            break;
        case 7:
            printf("Sunday\n");
            break;
        default:
            printf("Not a valid day\n");
            break;
    }

    return 0;
}

Wednesday

C evaluates day once, jumps to case 3:, prints Wednesday, and the break sends it out of the switch. Try day = 9 and you'll see Not a valid day.

What can a case label be?

Case labels have strict rules:

• They must be integer constants: numbers like 3, characters like 'B' (a char is a small integer), or names created with #define, which you met in C constants.
• They can't be variables, ranges (case score >= 90: is not allowed), decimal numbers or strings.
• Two cases in the same switch can't have the same value. GCC reports duplicate case value.

Because characters count as integers, switch is perfect for a vending machine keypad:

#include <stdio.h>

int main(void) {
    char button = 'B';

    switch (button) {
        case 'A':
            printf("Water - $1.25\n");
            break;
        case 'B':
            printf("Orange juice - $2.50\n");
            break;
        case 'C':
            printf("Chips - $1.75\n");
            break;
        default:
            printf("Unknown button, please try again\n");
            break;
    }

    return 0;
}

Orange juice - $2.50

What happens if you forget break? (fall-through)

A case label only marks where execution starts. It doesn't mark where it stops. Without a break, C keeps going straight into the next case's code, and the next, until it reaches a break or the end of the switch. This is called fall-through.

Here's a bank menu where every break was forgotten. The user chose option 2:

#include <stdio.h>

int main(void) {
    int choice = 2;

    switch (choice) {
        case 1:
            printf("Checking balance...\n");
        case 2:
            printf("Depositing money...\n");
        case 3:
            printf("Withdrawing money...\n");
        default:
            printf("Goodbye!\n");
    }

    return 0;
}

Depositing money...
Withdrawing money...
Goodbye!

The program jumped to case 2 correctly, then fell through cases 3 and default as well. The customer wanted to deposit, but the program also "withdrew" money. GCC catches this with -Wextra: warning: this statement may fall through.

Grouping cases that share code

Fall-through has a useful side. When several values need the same code, stack their labels on top of each other. Empty labels fall through to the shared code, and GCC doesn't warn about this pattern:

#include <stdio.h>

int main(void) {
    int day = 6;

    switch (day) {
        case 1:
        case 2:
        case 3:
        case 4:
        case 5:
            printf("Weekday: the gym opens at 6 AM.\n");
            break;
        case 6:
        case 7:
            printf("Weekend: the gym opens at 8 AM.\n");
            break;
        default:
            printf("Invalid day number.\n");
            break;
    }

    return 0;
}

Weekend: the gym opens at 8 AM.

Using fall-through on purpose

Sometimes falling through is exactly what you want. In an airline loyalty program, a gold member gets every silver perk plus more, and a silver member gets every bronze perk plus more. Start at the member's tier and let execution fall down through the lower tiers:

#include <stdio.h>

int main(void) {
    int tier = 2;   /* 1 = bronze, 2 = silver, 3 = gold */

    printf("Your perks:\n");

    switch (tier) {
        case 3:
            printf("- Free airport lounge access\n");
            /* fall through */
        case 2:
            printf("- Free checked bag\n");
            /* fall through */
        case 1:
            printf("- Priority boarding\n");
            break;
        default:
            printf("- No perks yet\n");
            break;
    }

    return 0;
}

Your perks:
- Free checked bag
- Priority boarding

The /* fall through */ comment tells other programmers (and GCC, which recognizes it and stays quiet) that the missing break is intentional.

Real-world example: a simple calculator

A switch on an operator character is a classic way to build a calculator. Notice that a case can contain any code, including an if statement:

#include <stdio.h>

int main(void) {
    double a = 12.0;
    double b = 4.0;
    char op = '/';

    switch (op) {
        case '+':
            printf("%.1f + %.1f = %.1f\n", a, b, a + b);
            break;
        case '-':
            printf("%.1f - %.1f = %.1f\n", a, b, a - b);
            break;
        case '*':
            printf("%.1f * %.1f = %.1f\n", a, b, a * b);
            break;
        case '/':
            if (b != 0.0) {
                printf("%.1f / %.1f = %.1f\n", a, b, a / b);
            } else {
                printf("Cannot divide by zero.\n");
            }
            break;
        default:
            printf("Unknown operator: %c\n", op);
            break;
    }

    return 0;
}

12.0 / 4.0 = 3.0

You'll turn this into a full interactive program in the calculator project.

switch vs if else: which should you use?

Anything a switch does, an if else chain can do too. A switch is simply cleaner when you're matching one value against a menu of options, and compilers can often turn it into a fast jump table.

Common mistakes with switch

1. Forgetting break

The most common switch bug by far. Unless you're grouping cases or falling through on purpose (with a comment), every case should end with break.

2. Using a range or a variable as a label

switch (score) {
    case score >= 90:   // error: not a constant
        printf("A\n");
        break;
}

Case labels must be fixed integer values known when the program is compiled. Use an if else chain for ranges.

3. Switching on a string or a double

switch (name) where name holds text, or switch (price) where price is a double, won't compile. GCC says switch quantity not an integer.

4. Declaring a variable right after a case label

Before C23, a label had to be followed by a statement, not a declaration, so case 1: int total = 0; is rejected or warned about by many compilers. It also creates a variable that every later case can see. Wrap the case's code in braces instead: case 1: { int total = 0; ... break; }.

Practice exercise

#include <stdio.h>

int main(void) {
    char vehicle = 'c';

    switch (vehicle) {
        case 'M':
        case 'm':
            printf("Motorcycle: $3\n");
            break;
        case 'C':
        case 'c':
            printf("Car: $8\n");
            break;
        case 'T':
        case 't':
            printf("Truck: $15\n");
            break;
        default:
            printf("Unknown vehicle type\n");
            break;
    }

    return 0;
}

Car: $8

What's next?

For a quick choice between just two values, there's an even shorter tool than if else. Next, learn the C ternary operator.