Simple Calculator in C

Algorithm

1. Read the left operand, the operator and the right operand in one scanf call.
2. If scanf does not return 3, report invalid input and stop.
3. Use a switch on the operator and compute the matching result.
4. For division, check the divisor is not zero before dividing.
5. Print the result with two decimal places.

C program

#include <stdio.h>

int main(void) {
    double left, right, result;
    char op;

    printf("Enter expression (number operator number):\n");
    if (scanf("%lf %c %lf", &left, &op, &right) != 3) {
        printf("Error: invalid input\n");
        return 1;
    }

    switch (op) {
        case '+':
            result = left + right;
            break;
        case '-':
            result = left - right;
            break;
        case '*':
            result = left * right;
            break;
        case '/':
            if (right == 0) {
                printf("Error: division by zero\n");
                return 1;
            }
            result = left / right;
            break;
        default:
            printf("Error: unknown operator %c\n", op);
            return 1;
    }

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

Sample input and output

7 / 2

Enter expression (number operator number):
Result: 3.50

9 / 0

Enter expression (number operator number):
Error: division by zero

How the program works

The format string "%lf %c %lf" reads a decimal number, one operator character and another decimal number. Each space in the format skips zero or more whitespace characters, so the input can be written as 7 / 2 or 7/2 and both parse the same way.

The division case is the one that needs a guard. Dividing a double by zero does not stop the program in C; it produces infinity, which is almost never what a calculator should show. Checking the divisor first lets the program report the problem clearly.

Viva questions

Common mistakes

• Declaring the operands as int, which silently truncates results such as 7 / 2.
• Forgetting the zero-divisor check, so the program prints inf instead of a message.
• Using %f instead of %lf in scanf for a double; printf accepts %f but scanf needs %lf.