C Flow Control Structures – II

In the previous lesson, we learned about the flow control structure. We learned about the if-else structures and nested if structures. In this lesson, we will learn about another flow control feature – switch-case.

If-else Construct

Consider a C program for calculator, the user is given a menu with options to choose from.

1. Add
2. Subtract
3. Multiply
4. Divide

We can write a program using if-else construct for the above problem.

#include <stdio.h>
int main()
{
    int choice;
    int a,b,c;
    a = 10;
    b = 20;
    printf ("Enter your choice:");
    if (choice == 1)
    c = a+b;
    if (choice == 2)
    c = a – b;
    if (choice == 3)
    c = a * b;
    else if(choice == 4)
    c = a/b;
    else
    {
    printf("Wrong input, Try again!");
    }
    getch();
    return 0;
}

The user enters a number, and it is matched with if-else blocks and whenever there is a match that block is executed.

The problem with these kinds of the construct is that when the menu is large, then the if-else construct will be difficult to manage. Also, there are many comparisons – each if a block is tested.

Switch-Case

The switch-case is suitable for menu driven C programs. Whenever the program is executed, a menu with a number of choices like the example below is presented to the user.

/* Calculator Application in C */
Enter your choice:
1. Add
2. Subtract
3. Multiply
4. Divide

The switch block look like the following in program source code. The switch block accepts a number as user choice.

When the user enters a number next to an option and the switch accepts the choice(number).

int choice;
switch (choice)
{
  statements;
}

The switch () matches the user choice with a list of cases. Each of the cases has a number associated with them followed by a colon.

case 2:
    Statement1;
    break;
case 3:
    Statement2;
    Statement3;
    break;

When the user choice and the case number match, the statement from the case is executed and terminated by a break, statement.

The entire switch-case block look like following

int choice;
scanf("%d",&choice);
Switch(choice)
{
Case 1:
    Do something;
    break;
Case 2:
    Do something;
    break;
default:
    Do something;
    break;
}

Let’s create an example program using the switch-case. We shall write the Calculator program using the switch-case.

Flowchart – C calculator using switch-case block

Program Code – C calculator using switch-case

/* C Program for a Calculator using switch-case */
#include <stdio.h>
int main()
{
    int choice;
    int a, b, c;
    a = 10;
    b = 20;
    printf ("Enter your choice:");
 
    scanf ("%d", &choice);
    Switch (choice)
    {
    case 1:
        c = a + b;
        break;
    case 2:
        c = a - b;
        break;
    case 3:
        c = a * b;
        break;
    case 4:
        c = a / b;
        break;
    default:
        printf ("Wrong choice, try again!");
        break;
    }
    printf ("Result = %d\n", c);
    getch();
    return 0;
}

Output – calculator program using switch-case block

The output of the program is given below.

Enter your choice :1
Result = 30

References

• Balagurusamy, E. 2000. Programming in ANSI C. Tata McGraw-Hill Education,.
• Kanetkar, Yashavant. 20 November 2002. Let us C. Bpb Publications.