Module 15 · Interactive lesson

switch Statement

Choosing by value

When one value decides between many fixed choices — a menu number, a grade letter, a day of the week — switch is clearer than a long else-if ladder. In the lab below you watch switch jump to the matching case, see what break does, what happens without it, and which case labels the compiler refuses.

switchjumps straight to the matching case
breakleaves the switch
defaultruns when nothing matches

choice Remove the breaks

main.c

Ready

Statements run 0 Lines printed 0

Ready

Keyboard: ← → step · Space play / pause · R reset · Esc exit classroom mode

How it works

The ideas behind the lab

Exactly what you watched happen, in plain steps.

The shape of a switch

switch takes ONE whole-number value (an int, a char …) and jumps straight to the case label with the same value. From there the statements run until a break leaves the switch. If no label matches, it jumps to default — or skips the whole switch if there is no default.

switch (expression) {
    case 1:
        statements;
        break;          // leave the switch
    case 2:
    case 3:             // 2 and 3 share this block
        statements;
        break;
    default:            // no case matched
        statements;
}

switch or else-if ladder?
	switch	else-if ladder
Tests	one value against fixed constants	any conditions
Types	whole numbers and characters only	anything (double, ranges, several variables)
Ranges like x > 5	no	yes
How it decides	jumps straight to the matching case	checks the conditions from the top
Best for	menus, commands, codes, letters	ranges, mixed conditions

switch and case

The expression in switch ( ) is worked out once; it must be a whole number (int, char, long …).
case labels are constants: case 1:, case 'a':, case MAX: (a #define). Note the colon.
switch JUMPS to the matching label — the cases above it are not run.
Every label must be different.
Use a switch when one value chooses among many fixed options.

break and fall-through

break leaves the switch immediately.
Without break, execution FALLS THROUGH into the next case, ignoring its label.
Forgetting break is the most common switch bug (gcc -Wextra: -Wimplicit-fallthrough).
Stacked labels (case 'a': case 'e': …) share one block — fall-through on purpose.
Mark intentional fall-through with a comment: /* fall through */.

default and the rules

default runs when no case matches; it is optional but recommended.
Without default and without a match, the switch silently does nothing.
default can be anywhere, but it is usually last (then it needs no break).
No doubles, no strings, no variables, no ranges as case labels.
To declare a variable inside a case, put the case's statements in { }.

Walkthrough

A tiny calculator

Every line, explained.

1 #include <stdio.h>

3 int main(void)

4 {

5 double a = 12, b = 4;

6 char op = '/'; The operator is a char — a small whole number, so it can be switched on.

7 switch (op) { Jump to the case for '/'.

8 case '+': printf("%.2f\n", a + b); break;

9 case '-': printf("%.2f\n", a - b); break;

10 case '*': printf("%.2f\n", a * b); break;

11 case '/': This is the matching case.

12 if (b != 0) A case can contain any statements — even an if.

13 printf("%.2f\n", a / b); Prints 3.00

14 else

15 printf("Cannot divide by zero\n");

16 break; Leave the switch.

17 default: Any other character lands here.

18 printf("Unknown operator %c\n", op);

19 }

20 return 0;

21 }

Common mistakes

Spot the bug

Mistakes almost every beginner makes — and how to fix them.

A forgotten break

✗ Wrong

case 1: puts("Tea");
case 2: puts("Coffee"); break;

✓ Right

case 1: puts("Tea"); break;
case 2: puts("Coffee"); break;

Without break, choice 1 prints Tea AND Coffee: execution falls through into case 2.

A condition as a case label

✗ Wrong

switch (marks) {
    case marks >= 90: puts("A");
}

✓ Right

if (marks >= 90)
    puts("A");

Labels must be constants. Ranges and conditions need if / else if.

A semicolon instead of a colon

✗ Wrong

case 1; puts("one"); break;

✓ Right

case 1: puts("one"); break;

A case label ends with a colon. The semicolon gives a compiler error: expected ':' before ';'.

Declaring a variable right after a label

✗ Wrong

case 1:
    int total = 5;
    break;

✓ Right

case 1: {
    int total = 5;
    break;
}

Before C23, a label cannot be followed by a declaration: "a label can only be part of a statement". Braces make a block.

switch on a string

✗ Wrong

switch (answer) {
    case "yes": …
}

✓ Right

switch (answer[0]) {
    case 'y': …
}

switch works only with whole numbers. Compare strings with strcmp (in the Strings part), or switch on one character.

Practice

Check your understanding

Answer each question — you will see the explanation straight away.

0 / 5 correct

Question 1

int x = 2; switch (x) { case 1: puts("A"); break; case 2: puts("B"); break; default: puts("C"); } — output?

Question 2

The same switch WITHOUT any break and x = 2 — output?

Question 3

Which type can NOT be used in switch ( )?

Question 4

When does default run?

Question 5

Which is a valid case label?

Assessment

switch Statement quiz

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