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Created by Zed A. Shaw Updated 2024-07-21 04:57:00

Exercise 10: Switch Statements

In other languages like Ruby, you have a switch-statement that can take any expression. Some languages like Python don't have a switch-statement because an if-statement with Boolean expressions is about the same thing. For these languages, switch-statements are more like alternatives to if-statements and work the same internally.

In C, the switch-statement is actually quite different and is really a jump table. Instead of random Boolean expressions, you can only put expressions that result in integers. These integers are used to calculate jumps from the top of the switch to the part that matches that value. Here's some code to help you understand this concept of jump tables:

View Source file ex10.c Only

#include <stdio.h>

int main(int argc, char *argv[])
{
    if (argc != 2) {
        printf("ERROR: You need one argument.\n");
        // this is how you abort a program
        return 1;
    }

    int i = 0;
    for (i = 0; argv[1][i] != '\0'; i++) {
        char letter = argv[1][i];

        switch (letter) {
            case 'a':
            case 'A':
                printf("%d: 'A'\n", i);
                break;

            case 'e':
            case 'E':
                printf("%d: 'E'\n", i);
                break;

            case 'i':
            case 'I':
                printf("%d: 'I'\n", i);
                break;

            case 'o':
            case 'O':
                printf("%d: 'O'\n", i);
                break;

            case 'u':
            case 'U':
                printf("%d: 'U'\n", i);
                break;

            case 'y':
            case 'Y':
                // why i > 2? is this a bug?
                if (i > 2) {
                    // it's only sometimes Y
                    printf("%d: 'Y'\n", i);
                }
                break;

            default:
                printf("%d: %c is not a vowel\n", i, letter);
        }
    }

    return 0;
}

In this program, we take a single command line argument and print out all vowels in an incredibly tedious way to demonstrate a switch-statement. Here's how the switch-statement works:

This is a deep dive into how the switch-statement works, but in practice, you just have to remember a few simple rules:

What You Should See

Here's an example of me playing with this, and also demonstrating various ways to pass in the argument:

View Source file ex10.sh-session Only

$ make ex10
cc -Wall -g    ex10.c   -o ex10
$ ./ex10
ERROR: You need one argument.
$
$ ./ex10 Zed
0: Z is not a vowel
1: 'E'
2: d is not a vowel
$
$ ./ex10 Zed Shaw
ERROR: You need one argument.
$
$ ./ex10 "Zed Shaw"
0: Z is not a vowel
1: 'E'
2: d is not a vowel
3:   is not a vowel
4: S is not a vowel
5: h is not a vowel
6: 'A'
7: w is not a vowel
$

Remember that there's an if-statement at the top that exits with a return 1; when you don't provide enough arguments. A return that's not 0 indicates to the OS that the program had an error. You can test for any value that's greater than 0 in scripts and other programs to figure out what happened.

How to Break It

It's incredibly easy to break a switch-statement. Here are just a few ways you can mess one of these up:

You can also break this program in a few other ways. See if you can bust it yourself.

Extra Credit

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