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 when you want to dispatch based 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)

Evaluates the value used for branching.

case value:

Runs when the expression equals this value.

break;

Leaves the switch (very important).

default:

Runs when no case matches.

switch can dispatch on int or char values only. It cannot dispatch on doubles or strings.

Why break matters — fallthrough

Without break; execution "falls through" into the next case and keeps running — a common 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

Suppose 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 run time when "everything is printed".

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

Colon : and semicolon ; sit next to each other on the keyboard, so this typo happens often.

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 form a 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 also a label. default; may compile (as an empty statement) but it becomes a silent bug: the "no case matched" branch never runs.

Intentional fallthrough: When you want "1 or 2 does the same thing", you can omit break on purpose. Mark it with a comment so reviewers know it's intentional:
case 1: case 2: printf("small\n"); break; // intentional

Try it yourself — switch

switch.c

Output

Click "Run" to execute...

💡 Try these ideas too

Add default for unmatched cases
Remove break deliberately to see fall-through
Use character constants ('a') in case
Share one body across multiple 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 continues 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 recommended because it lets you handle the case when nothing matches.

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

int

char

float

switch supports integer types (int, char, etc.) only. float and double cannot be used.

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?

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