C switch Statement | case, break, fallthrough

📖 What to learn on this page

✅ Must-know essentials

switch (x) { case 1: ... break; default: ...; } form
Always break; at the end of each case
default runs when no case matches

⭐ Read if you have time

Intentional fall-through (omitting break)
case labels must be constant expressions
switch works only on integer types (no strings)

switch statement — branching by value

A handy construct for dispatching on the value of a variable.

int day = 3;
switch (day) {
  case 1: printf("Monday\n"); break;
  case 2: printf("Tuesday\n"); break;
  case 3: printf("Wednesday\n"); break;
  default: printf("Unknown\n");
}

switch (expr)

The value to branch on.

case value:

Runs when the expression matches this value.

break;

Exits the switch (don't forget this).

default:

Runs when no case matches.

switch works on integer types only (int, char, and so on). You can't switch on doubles or strings.

Why break matters — fallthrough

Without break;, execution "falls through" into the next case and keeps running — a classic source of bugs.

switch (n) {
  case 1: printf("A\n");   // no break!
  case 2: printf("B\n");   // no break!
  case 3: printf("C\n"); break;
}
// When n=1: prints A, B, and C!

🔴 What actually happens when you forget break

Say you want to print the day of the week for day 1–3, but you forgot every single break:

int day = 1;
switch(day){
  case 1: printf("Mon\n");   // no break
  case 2: printf("Tue\n");   // no break
  case 3: printf("Wed\n");   // no break
  default: printf("Unknown\n");
}

Output (day=1):

Mon
Tue
Wed
Unknown

→ After matching case 1, execution keeps falling through cases 2, 3, and default.

The scary part: no compile error. You only notice at runtime, when "everything prints."

🔴 What if you write case 1; instead of case 1:?

The colon : and semicolon ; sit next to each other on the keyboard, so this typo comes up all the time.

switch(day){
  case 1; // ← typed ; instead of :
    printf("Mon\n");
    break;
}

Result: compile error (gcc):
error: expected ':' or '...' before ';' token
→ The compiler expects a : after case 1 to complete the label.

🔑 Memory hook: the : after case marks a label (same syntax as goto labels). Statements end with ;; labels end with :.

⚠️ Related trap: default; instead of default:

default is a label too. default; may compile (as an empty statement), but it becomes a silent bug — the "no case matched" branch never runs.

Intentional fallthrough: when "1 or 2 should do the same thing," you can drop break on purpose. Add a comment so reviewers know it's deliberate:
case 1: case 2: printf("small\n"); break; // intentional

Try it yourself — switch

switch.c

Output

Press "Run" to execute...

💡 Try these ideas too

Add default to catch unmatched values
Drop a break on purpose to see fall-through
Use character constants (like 'a') in case labels
Share one body across several cases (case 1: case 2:)

Review Quiz

Check your understanding of this lesson!

Q1. What happens if you omit break in a switch statement?

Compile error

Execution falls through into the next case

Nothing happens

Without break, execution keeps running into every case that follows the matched one. This is called "fallthrough."

Q2. Is default required in a switch statement?

Required

Optional (can be omitted)

Required when there are three or more cases

default is optional, but it's usually worth including so you have somewhere to handle unmatched values.

Q3. Which type cannot be used in a switch condition?

int

char

float

switch only accepts integer types (int, char, and so on). float and double aren't allowed.

Lesson 10: switch

switch statement — branching by value

Why break matters — fallthrough

Try it yourself — switch

Related Lessons

Review Quiz

Q1. What happens if you omit break in a switch statement?

Q2. Is default required in a switch statement?

Q3. Which type cannot be used in a switch condition?