C - The Switch Statement

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A switch statement allows a variable to be tested for equality against a list of values. Each value is called a case, and the variable being switched on is checked for each switch case.

C switch-case Statement

The switch-case statement is a decision-making statement in C. The if-else statement provides two alternative actions to be performed, whereas the switch-case construct is a multi-way branching statement. A switch statement in C simplifies multi-way choices by evaluating a single variable against multiple values, executing specific code based on the match. It allows a variable to be tested for equality against a list of values.

Syntax of switch-case Statement

The flow of the program can switch the line execution to a branch that satisfies a given case. The schematic representation of the usage of switch-case construct is as follows −

switch (Expression){

   // if expr equals Value1
   case Value1: 
      Statement1;
      Statement2;
      break;
   
   // if expr equals Value2
   case Value2: 
      Statement1;
      Statement2;
      break;
      .
      . 
   // if expr is other than the specific values above
   default:
      Statement1;
      Statement2;
}

How switch-case Statement Work?

The parenthesis in front of the switch keyword holds an expression. The expression should evaluate to an integer or a character. Inside the curly brackets after the parenthesis, different possible values of the expression form the case labels.

One or more statements after a colon(:) in front of the case label forms a block to be executed when the expression equals the value of the label.

You can literally translate a switch-case as "in case the expression equals value1, execute the block1", and so on.

C checks the expression with each label value, and executes the block in front of the first match. Each case block has a break as the last statement. The break statement takes the control out of the scope of the switch construct.

You can also define a default case as the last option in the switch construct. The default case block is executed when the expression doesn’t match with any of the earlier case values.

Flowchart of switch-case Statement

The flowchart that represents the switch-case construct in C is as follows −

Rules for Using the switch-case Statement

The following rules apply to a switch statement −

• The expression used in a switch statement must have an integral or enumerated type, or be of a class type in which the class has a single conversion function to an integral or enumerated type.

• You can have any number of case statements within a switch. Each case is followed by the value to be compared to and a colon.

• The constant-expression for a case must be the same data type as the variable in the switch, and it must be a constant or a literal.

• When the variable being switched on is equal to a case, the statements following that case will execute until a break statement is reached.

• When a break statement is reached, the switch terminates, and the flow of control jumps to the next line following the switch statement.

• Not every case needs to contain a break. If no break appears, the flow of control will fall through to subsequent cases until a break is reached.

• A switch statement can have an optional default case, which must appear at the end of the switch. The default case can be used for performing a task when none of the cases is true. No break is needed in the default case.

The switch-case statement acts as a compact alternative to cascade of if-else statements, particularly when the Boolean expression in "if" is based on the "=" operator.

If there are more than one statements in an if (or else) block, you must put them inside curly brackets. Hence, if your code has many if and else statements, the code with that many opening and closing curly brackets appears clumsy. The switch-case alternative is a compact and clutter-free solution.

C switch-case Statement Examples

Example 1

In the following code, a series of if-else statements print three different greeting messages based on the value of a "ch" variable ("m", "a" or "e" for morning, afternoon or evening).

#include <stdio.h>

int main(){

   /* local variable definition */
   char ch = 'e';
   printf("Time code: %c\n\n", ch);

   if (ch == 'm')
      printf("Good Morning");
   
   else if (ch == 'a')
      printf("Good Afternoon");
   else
      printf("Good Evening");

   return 0;
}

The if-else logic in the above code is replaced by the switch-case construct in the code below −

#include <stdio.h>
 
int main (){

   // local variable definition
   char ch = 'm';
   printf("Time code: %c\n\n", ch);
   
   switch (ch){
      
      case 'a':
         printf("Good Afternoon\n");
         break;
      
      case 'e':
         printf("Good Evening\n");
         break;
      
      case 'm':
         printf("Good Morning\n");
   }
   return 0;
}

Output

Change the value of "ch" variable and check the output. For ch = 'm', we get the following output −

Time code: m

Good Morning

The use of break is very important here. The block of statements corresponding to each case ends with a break statement. What if the break statement is not used?

The switch-case works like this: As the program enters the switch construct, it starts comparing the value of switching expression with the cases, and executes the block of its first match. The break causes the control to go out of the switch scope. If break is not found, the subsequent block also gets executed, leading to incorrect result.

Example 2

Let us comment out all the break statements in the above code.

#include <stdio.h>

int main (){

   /* local variable definition */
   char ch = 'a';
   printf("Time code: %c\n\n", ch);
   
   switch (ch){
      case 'a':
         printf("Good Afternoon\n");
         // break;
      
      case 'e':
         printf("Good Evening\n");
         // break;
      
      case 'm':
         printf("Good Morning\n");
   }
   return 0;
}

Output

You expect the "Good Morning" message to be printed, but you find all the three messages printed!

Time code: a

Good Afternoon
Good Evening
Good Morning

This is because C falls through the subsequent case blocks in the absence of break statements at the end of the blocks.

Example 3

In the following program, "grade" is the switching variable. For different cases of grades, the corresponding result messages will be printed.

#include <stdio.h>
 
int main(){

   /* local variable definition */
   char grade = 'B';

   switch(grade){
      case 'A' :
         printf("Outstanding!\n" );
         break;
      case 'B':
        printf("Excellent!\n");
        break;
      case 'C':
         printf("Well Done\n" );
         break;
      case 'D':
         printf("You passed\n" );
         break;
      case 'F':
         printf("Better try again\n" );
         break;
      default :
         printf("Invalid grade\n" );
   }
   printf("Your grade is  %c\n", grade);
 
   return 0;
}

Output

Run the code and check its output −

Excellent!
Your grade is  B

Now change the value of "grade" (it is a "char" variable) and check the outputs.

Example 4

The following example displays a menu for arithmetic operations. Based on the value of the operator code (1, 2, 3, or 4), addition, subtraction, multiplication or division of two values is done. If the operation code is something else, the default case is executed.

#include <stdio.h>
 
int main (){

   // local variable definition
   int a = 10, b = 5;
   
   // Run the program with other values 2, 3, 4, 5
   int op = 1; 
   float result;
  
   printf("1: addition\n");
   printf("2: subtraction\n");
   printf("3: multiplication\n");
   printf("4: division\n");
   
   printf("\na: %d b: %d : op: %d\n", a, b, op);
   switch (op){
      case 1:
         result = a + b;
         break;
      case 2:
         result = a - b;
         break;
      case 3:
         result = a * b;
         break;
      case 4:
         result = a / b;
         break;
      default:
         printf("Invalid operation\n");
   }
   if (op >= 1 && op <= 4)
      printf("Result: %6.2f", result);
   
   return 0;
}

Output

1: addition
2: subtraction
3: multiplication
4: division

a: 10 b: 5 : op: 1
Result:  15.00

For other values of "op" (2, 3, and 4), you will get the following outputs −

a: 10 b: 5 : op: 2
Result:   5.00

a: 10 b: 5 : op: 3
Result:  50.00

a: 10 b: 5 : op: 4
Result:   2.00

a: 10 b: 5 : op: 5
Invalid operation

Combining Multiple Cases

If you have a situation where the same code block is to be executed for more than one case labels of an expression, you can combine them by putting the two cases one below the other, as shown below −

switch (exp) {
   case 1:
   case 2:
      statements;
      break;
   case 3:
      statements;
      break;
   default:
      printf("%c is a non-alphanumeric character\n", ch);
}

You can also use the ellipsis (…) to combine a range of values for an expression. For example, to match the value of switching variable with any number between 1 to 10, you can use "case 1 … 10"

Example 1

#include <stdio.h>

int main (){

   // local variable definition
   int number = 5;

   switch (number){

      case 1 ... 10:
         printf("The number is between 1 and 10\n");
         break;

      default:
         printf("The number is not between 1 and 10\n");
   }
   return 0;
}

Output

Run the code and check its output. For "number = 5", we get the following output −

The number is between 1 and 10

Example 2

The following program checks whether the value of the given char variable stores a lowercase alphabet, an uppercase alphabet, a digit, or any other key.

#include <stdio.h>

int main (){

   char ch = 'g';

   switch (ch){
      case 'a' ... 'z':
         printf("%c is a lowercase alphabet\n", ch);
         break;
      
      case 'A' ... 'Z':
         printf("%c is an uppercase alphabet\n", ch);
         break;
      
      case 48 ... 57:
         printf("%c is a digit\n", ch);
         break;
      
      default:
         printf("%c is a non-alphanumeric character\n", ch);
   }
   return 0;
}

Output

For ch = 'g', we get the following output −

g is a lowercase alphabet

Test the code output with different values of "ch".