Dive Into Systems

2.9.1. switch Statements:

The C switch statement can be used in place of some, but not all, chaining if-else if code sequences. While switch doesn’t provide any additional expressive power to the C programming language, it often yields more concise code branching sequences. It may also allow the compiler to produce branching code that executes more efficiently than equivalent chaining if-else if code.

The C syntax for a switch statement looks like:

switch (<expression>) {

   case <literal value 1>:
        <statements>;
        break;         // breaks out of switch statement body
   case <literal value 2>:
        <statements>;
        break;         // breaks out of switch statement body
   ...
   default:            // default label is optional
        <statements>;
}

A switch statement is executed as follows:

The expression evaluates first.
Next, the switch searches for a case literal value that matches the value of the expression.
Upon finding a matching case literal, it begins executing the statements that immediately follow it.
If no matching case is found, it will begin executing the statements in the default label if one is present.
Otherwise, no statements in the body of the switch statement get executed.

A few rules about switch statements:

The value associated with each case must be a literal value — it cannot be an expression. The original expression gets matched for equality only with the literal values associated with each case.
Reaching a break statement stops the execution of all remaining statements inside the body of the switch statement. That is, break breaks out of the body of the switch statement and continues execution with the next statement after the entire switch block.
The case statement with a matching value marks the starting point into the sequence of C statements that will be executed — execution jumps to a location inside the switch body to start executing code. Thus, if there is no break statement at the end of a particular case, then the statements under the subsequent case statements execute in order until either a break statement is executed or the end of the body of the switch statement is reached.
The default label is optional. If present, it must be at the end.

Here’s an example program with a switch statement:

#include <stdio.h>

int main() {
    int num, new_num = 0;

    printf("enter a number between 6 and 9: ");
    scanf("%d", &num);

    switch(num) {
        case 6:
            new_num = num + 1;
            break;
        case 7:
            new_num = num;
            break;
        case 8:
            new_num = num - 1;
            break;
        case 9:
            new_num = num + 2;
            break;
        default:
            printf("Hey, %d is not between 6 and 9\n", num);
    }
    printf("num %d  new_num %d\n", num, new_num);
    return 0;
}

Here are some example runs of this code:

./a.out
enter a number between 6 and 9: 9
num 9  new_num 11

./a.out
enter a number between 6 and 9: 6
num 6  new_num 7

./a.out
enter a number between 6 and 9: 12
Hey, 12 is not between 6 and 9
num 12  new_num 0