C Programs

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Decision Making Programs in C

Simple calculator using switch-case

Introduction

A calculator is one of the most common beginner programs in C. It helps you understand decision-making and control flow.

Unlike if-else, the switch-case structure allows us to handle multiple conditions more cleanly when we know the exact values to check (e.g., operators like +, -, *, /).

In this program:

• The user enters two numbers and chooses an operator.
• A switch statement checks the operator and performs the respective operation.
• Division is handled carefully to avoid division by zero.

C Program: Simple calculator using switch-case

Method 1:

#include <stdio.h>
 
int main() {
    float num1, num2, result;
    char op;
 
    // Input numbers
    printf("Enter first number: ");
    scanf("%f", &num1);
 
    printf("Enter second number: ");
    scanf("%f", &num2);
 
    // Input operator
    printf("Enter operator (+, -, *, /): ");
    scanf(" %c", &op);  // Note the space before %c to ignore newline
 
    // Switch-case for calculation
    switch (op) {
        case '+':
            result = num1 + num2;
            printf("Result: %.2f + %.2f = %.2f\n", num1, num2, result);
            break;
 
        case '-':
            result = num1 - num2;
            printf("Result: %.2f - %.2f = %.2f\n", num1, num2, result);
            break;
 
        case '*':
            result = num1 * num2;
            printf("Result: %.2f * %.2f = %.2f\n", num1, num2, result);
            break;
 
        case '/':
            if (num2 != 0) {
                result = num1 / num2;
                printf("Result: %.2f / %.2f = %.2f\n", num1, num2, result);
            } else {
                printf("Error: Division by zero is not allowed.\n");
            }
            break;
 
        default:
            printf("Invalid operator!\n");
    }
 
    return 0;
}
    

 
OUTPUT 1 :
Enter first number: 12
Enter second number: 4
Enter operator (+, -, *, /): *

Result: 12.00 * 4.00 = 48.00

OUTPUT 2 :

Enter first number: 5
Enter second number: 2
Enter operator (+, -, *, /): ^

Invalid operator!


OUTPUT 3 :
Enter first number: 10
Enter second number: 0
Enter operator (+, -, *, /): /

Error: Division by zero is not allowed.

C Program: Simple calculator using switch-case

Method 2:

#include <stdio.h>
 
int main() {
    char operator;
    double num1, num2;
 
    // Prompt user for operator
    printf("Enter an operator (+, -, *, /): ");
    scanf("%c", &operator);
 
    // Prompt user for two numbers
    printf("Enter two numbers: ");
    scanf("%lf %lf", &num1, &num2);
 
    // Use switch-case to perform the operation
    switch (operator) {
        case '+':
            printf("%.2lf + %.2lf = %.2lf\n", num1, num2, num1 + num2);
            break; // Exit the switch statement after a case is matched
        case '-':
            printf("%.2lf - %.2lf = %.2lf\n", num1, num2, num1 - num2);
            break;
        case '*':
            printf("%.2lf * %.2lf = %.2lf\n", num1, num2, num1 * num2);
            break;
        case '/':
            if (num2 != 0) { // Handle division by zero
                printf("%.2lf / %.2lf = %.2lf\n", num1, num2, num1 / num2);
            } else {
                printf("Error: Division by zero is not allowed.\n");
            }
            break;
        default: // Executed if no case matches
            printf("Error: Invalid operator.\n");
    }
 
    return 0;
}
    

 
OUTPUT 1 :
Enter an operator (+, -, *, /): +
Enter two numbers: 5
10
5.00 + 10.00 = 15.00

OUTPUT 2 :

Enter an operator (+, -, *, /): -
Enter two numbers: 5 10
5.00 - 10.00 = -5.00


OUTPUT 3 :
Enter an operator (+, -, *, /): ^
Enter two numbers: 5
10
ERROR!
Error: Invalid operator.


OUTPUT 4 :

Enter an operator (+, -, *, /): /
Enter two numbers: 10 0
ERROR!
Error: Division by zero is not allowed.

Explanation:

• #include <stdio.h>: This line includes the standard input/output library, providing functions like printffor printing to the console and scanf for reading user input.
• int main() { ... }: This is the main function where the program execution begins.
• Variable Declaration:
  • char operator;: Declares a character variable operatorto store the arithmetic operator entered by the user.
  • double num1, num2;: Declares two double-precision floating-point variables num1and num2 to store the numbers for calculation, allowing for decimal values.
• User Input:
  • printf("Enter an operator (+, -, *, /): ");: Displays a message prompting the user to enter an operator.
  • scanf("%c", &operator);: Reads a single character from the user and stores it in the operator
  • printf("Enter two numbers: ");: Prompts the user to enter two numbers.
  • scanf("%lf %lf", &num1, &num2);: Reads two double-precision floating-point numbers from the user and stores them in num1and num2.
• switch (operator) { ... }: This switchstatement evaluates the value of the operator
  • case '+':, case '-':, case '*':, case '/':: These caselabels represent the different possible values of operator. If the operator matches a case value, the code block associated with that case is executed.
    • Inside each arithmetic case, the corresponding operation is performed, and the result is printed using printf. %.2lfformats the output to two decimal places.
  • if (num2 != 0) { ... } else { ... }: For division, a check is included to prevent division by zero, which would cause a runtime error.
  • break;: The breakstatement is crucial. After a case is executed, break terminates the switch statement, preventing "fall-through" to subsequent case
  • default:: If the operatordoes not match any of the case labels, the code block under default is executed, indicating an invalid operator.
• return 0;: Indicates successful program execution.