Switch statement in C

When you want to solve multiple option type problems, for example: Menu like program, where one value is associated with each option and you need to choose only one at a time, then, switch statement is used.

Switch statement is a control statement that allows us to choose only one choice among the many given choices. The expression in switch evaluates to return an integral value, which is then compared to the values present in different cases. It executes that block of code which matches the case value. If there is no match, then default block is executed(if present). The general form of switch statement is,

switch(expression)
{
    case value-1:
    	block-1;
    	break;
    case value-2:
    	block-2;
    	break;
    case value-3:
    	block-3;
    	break;
    case value-4:
    	block-4;
   	    break;
    default:
   	    default-block;
    	break;
}

Rules for using switch statement

  1. The expression (after switch keyword) must yield an integer value i.e the expression should be an integer or a variable or an expression that evaluates to an integer.
  2. The case label values must be unique.
  3. The case label must end with a colon(:)
  4. The next line, after the case statement, can be any valid C statement.

Points to Remember

  1. We don't use those expressions to evaluate switch case, which may return floating point values or strings or characters.
  2. break statements are used to exit the switch block. It isn't necessary to use break after each block, but if you do not use it, then all the consecutive blocks of code will get executed after the matching block.
    int i = 1;
    switch(i)
    {
        case 1:    
            printf("A");        // No break
        case 2:
            printf("B");        // No break
        case 3:
            printf("C");
            break;
    }

    A B C

    The output was supposed to be only A because only the first case matches, but as there is no break statement after that block, the next blocks are executed too, until it a break statement in encountered or the execution reaches the end of the switch block.

  3. default case is executed when none of the mentioned case matches the switch expression. The default case can be placed anywhere in the switch case. Even if we don't include the default case, switch statement works.
  4. Nesting of switch statements are allowed, which means you can have switch statements inside another switch block. However, nested switch statements should be avoided as it makes the program more complex and less readable.

Example of switch statement

#include<stdio.h>
void main( )
{
    int a, b, c, choice;
    while(choice != 3)
    {
        /* Printing the available options */
        printf("\n 1. Press 1 for addition");
        printf("\n 2. Press 2 for subtraction");
        printf("\n Enter your choice");
        /* Taking users input */
        scanf("%d", &choice);
        
        switch(choice)
        {
            case 1:
                printf("Enter 2 numbers");
                scanf("%d%d", &a, &b);
                c = a + b;
                printf("%d", c);
                break;
            case 2:
                printf("Enter 2 numbers");
                scanf("%d%d", &a, &b);
                c = a - b;
                printf("%d", c);
                break;
            default:
                printf("you have passed a wrong key");
                printf("\n press any key to continue");
        }
    } 
} 

Difference between switch and if

  • if statements can evaluate float conditions. switch statements cannot evaluate float conditions.
  • if statement can evaluate relational operators. switch statement cannot evaluate relational operators i.e they are not allowed in switch statement.