calculator.c

#include <stdio.h>

int main(void)
{
    int num1 = -1, num2 = -1;
    int result;
    char key;

    printf("Enter two numbers: ");
    scanf("%d,%d", &num1, &num2);
    /** Validate input */
    if(num1 < 0 || num2 < 0)
    {
       printf("Error: enter positive two numbers only, eg. 3,4 \n"); 
       return 1;
    }
    printf("Enter an operator: ");
    /**
     * the space before %c will help you to eat up \n char left over by first scanf
     */
    scanf(" %c", &key); 

    switch(key) 
    {
        case '*':
            result = num1 * num2;
            printf("%d %c %d = %d \n", num1, key, num2, result);
            break;
        case '-':
            result = num1 - num2;
            printf("%d %c %d = %d \n", num1, key, num2, result);
            break;
        case '+':
            result = num1 + num2;
            printf("%d %c %d = %d \n", num1, key, num2, result);
            break;
        case '/':
            result = num1 / num2;
            printf("%d %c %d = %d \n", num1, key, num2, result);
            break;
        default:
            printf("Invalid Operator. use one of *, +, - or / \n");
    }
    return 0;
}