C Program to Create Simple Calculator with Basic Operations

A simple calculator in C can be created by using switch to select operations and scanf to get user input, like: switch(choice) { case '+': result = a + b; break; }.

Examples

InputEnter first number: 5 Enter second number: 3 Enter operator (+, -, *, /): +

OutputResult: 8

InputEnter first number: 10 Enter second number: 2 Enter operator (+, -, *, /): /

OutputResult: 5

InputEnter first number: 7 Enter second number: 0 Enter operator (+, -, *, /): /

OutputError! Division by zero.

How to Think About It

To build a simple calculator, first get two numbers and an operator from the user. Then, use a switch or if statements to decide which operation to perform based on the operator. Finally, print the result or an error if the operation is invalid, like division by zero.

Algorithm

1

Get the first number from the user

2

Get the second number from the user

3

Get the operator (+, -, *, /) from the user

4

Use a switch or if-else to perform the operation based on the operator

5

Check for division by zero if operator is division

6

Print the result or error message

Code

c

#include <stdio.h>

int main() {
    double num1, num2, result;
    char op;

    printf("Enter first number: ");
    scanf("%lf", &num1);
    printf("Enter second number: ");
    scanf("%lf", &num2);
    printf("Enter operator (+, -, *, /): ");
    scanf(" %c", &op);

    switch(op) {
        case '+': result = num1 + num2; break;
        case '-': result = num1 - num2; break;
        case '*': result = num1 * num2; break;
        case '/': 
            if(num2 != 0) result = num1 / num2;
            else {
                printf("Error! Division by zero.\n");
                return 1;
            }
            break;
        default:
            printf("Invalid operator!\n");
            return 1;
    }

    printf("Result: %.2lf\n", result);
    return 0;
}

Output

Enter first number: 5
Enter second number: 3
Enter operator (+, -, *, /): +
Result: 8.00

Dry Run

Let's trace the input 5, 3, and '+' through the code

1

Input numbers and operator

num1 = 5, num2 = 3, op = '+'

2

Switch on operator

case '+': result = 5 + 3 = 8

3

Print result

Output: Result: 8.00

Step	Operation	Values
1	Input	num1=5, num2=3, op='+'
2	Calculate	result=8
3	Output	Result: 8.00

Why This Works

Step 1: Getting user input

The program uses scanf to read two numbers and an operator from the user, storing them in variables.

Step 2: Choosing operation

A switch statement checks the operator and performs the matching arithmetic operation.

Step 3: Handling division by zero

Before dividing, the program checks if the second number is zero to avoid errors and prints an error message if so.

Step 4: Displaying the result

The result is printed with two decimal places using printf.

Alternative Approaches

Using if-else instead of switch

c

#include <stdio.h>

int main() {
    double a, b, res;
    char op;
    printf("Enter first number: ");
    scanf("%lf", &a);
    printf("Enter second number: ");
    scanf("%lf", &b);
    printf("Enter operator (+, -, *, /): ");
    scanf(" %c", &op);

    if(op == '+') res = a + b;
    else if(op == '-') res = a - b;
    else if(op == '*') res = a * b;
    else if(op == '/') {
        if(b != 0) res = a / b;
        else {
            printf("Error! Division by zero.\n");
            return 1;
        }
    } else {
        printf("Invalid operator!\n");
        return 1;
    }

    printf("Result: %.2lf\n", res);
    return 0;
}

This approach uses if-else statements which some find easier to read but can be longer for many cases.

Using functions for each operation

c

#include <stdio.h>

double add(double x, double y) { return x + y; }
double sub(double x, double y) { return x - y; }
double mul(double x, double y) { return x * y; }
double divi(double x, double y) { return y != 0 ? x / y : 0; }

int main() {
    double num1, num2, result;
    char op;
    printf("Enter first number: ");
    scanf("%lf", &num1);
    printf("Enter second number: ");
    scanf("%lf", &num2);
    printf("Enter operator (+, -, *, /): ");
    scanf(" %c", &op);

    switch(op) {
        case '+': result = add(num1, num2); break;
        case '-': result = sub(num1, num2); break;
        case '*': result = mul(num1, num2); break;
        case '/': 
            if(num2 == 0) {
                printf("Error! Division by zero.\n");
                return 1;
            }
            result = divi(num1, num2);
            break;
        default:
            printf("Invalid operator!\n");
            return 1;
    }
    printf("Result: %.2lf\n", result);
    return 0;
}

This method organizes code better by separating operations into functions, improving readability and reuse.

Complexity: O(1) time, O(1) space

Time Complexity

The program performs a fixed number of operations regardless of input size, so it runs in constant time O(1).

Space Complexity

It uses a few variables for input and output, so space usage is constant O(1).

Which Approach is Fastest?

All approaches run in constant time; using functions adds slight overhead but improves code clarity.

Approach	Time	Space	Best For
Switch statement	O(1)	O(1)	Simple and clear operation selection
If-else statements	O(1)	O(1)	Easier for beginners to understand
Functions for operations	O(1)	O(1)	Better code organization and reuse