C - Switch with multiple case numbers

C Switch Statement with Multiple Case Numbers

Learn how to effectively use the switch statement in C to handle multiple case values for a single block of code, improving readability and efficiency.

The switch statement in C is a powerful control flow construct that allows a program to execute different blocks of code based on the value of an expression. While typically each case label corresponds to a unique value, there are common scenarios where you want the same code to execute for several different input values. This article explores various techniques to achieve this, focusing on readability and best practices.

The Fall-Through Mechanism

The most common and idiomatic way to handle multiple case numbers for a single block of code in C's switch statement is by utilizing the 'fall-through' mechanism. In C, if a case block does not end with a break statement, execution will 'fall through' to the next case label. By intentionally omitting break statements between multiple case labels, you can group them to execute the same code.

#include <stdio.h>

int main() {
    int day = 6;

    switch (day) {
        case 1:
        case 2:
        case 3:
        case 4:
        case 5:
            printf("It's a weekday.\n");
            break;
        case 6:
        case 7:
            printf("It's the weekend!\n");
            break;
        default:
            printf("Invalid day.\n");
            break;
    }

    return 0;
}

Using fall-through for multiple case labels

flowchart TD
    A[Start] --> B{Evaluate 'day'};
    B --> |day = 1,2,3,4,5| C[Print "It's a weekday."];
    B --> |day = 6,7| D[Print "It's the weekend!"];
    B --> |default| E[Print "Invalid day."];
    C --> F[End];
    D --> F;
    E --> F;

Flowchart illustrating the switch statement with grouped cases

Alternative: Using Logical OR (if-else if)

Although the switch statement is often preferred for its clarity with discrete values, an if-else if ladder using logical OR (||) can also achieve the same outcome. This approach might be more suitable when the conditions are not simple equality checks or when the values are not contiguous.

#include <stdio.h>

int main() {
    int day = 6;

    if (day == 1 || day == 2 || day == 3 || day == 4 || day == 5) {
        printf("It's a weekday.\n");
    } else if (day == 6 || day == 7) {
        printf("It's the weekend!\n");
    } else {
        printf("Invalid day.\n");
    }

    return 0;
}

Achieving multiple case logic with if-else if and logical OR

Best Practices and Considerations

When dealing with multiple case numbers, consider the following to write robust and maintainable code:

1. Readability: The fall-through method is highly readable for grouping cases. Ensure your case labels are ordered logically.
2. default Case: Always include a default case to handle unexpected or invalid input values. This makes your code more robust.
3. break Statements: Remember that break statements are crucial to prevent unintended fall-through. Only omit them when you explicitly want execution to continue to the next case.
4. Ranges: For checking if a value falls within a range (e.g., case 1 ... 5 in GCC extensions), standard C does not support this directly within switch. You would typically use an if-else if structure or a series of fall-through cases for discrete values within the range.