Command line calculator in C. What is wrong?

Question

I have this code as a simple command line calculator in C.

#include <stdio.h>
#include <math.h>

int main(int argc, char* argv[]){

int num1, num2, result;
char op1;

/* convert 1st and 3st of the arguments into int */
/* assign the argv[1] to num1, assign the argv[2] to op1,
 assign the argv[3] to num2 */

if (argc == 4){
    sscanf(argv[1],"%d",&num1);
    scanf(argv[2],"%c",&op1);
    sscanf(argv[3],"%d",&num2);
}

else{   // if the input is wrong
    printf("Input Error\n");
}

switch (op1){

    case'+':
        result = num1 + num2;
        break;
    case'-':
        result = num1 - num2;
        break;
    case'x':
        result = num1 * num2;
        break;
    case'/':
        result = num1 / num2;
        break;
    case'%':
        result = num1 % num2;
        break;
    default: printf("Input Error!!!\n");
}
printf("%d\n", result);
}

After I complied the code and ran it. Some error happens.

For example, I use command: ./main 1 + 2 (Which should be 3)

The output is: Input Error!!! 0

And if I use command: ./main

The output is: Input Error Input Error!!! 0

I don't where is wrong. Please help!! Thank you so much!! I'm a completely beginner to C.

Answer 1

Some problems:

1. You used scanf instead of sscanf for parsing a character.
2. You don't check the return value of those sscanf s.
3. The switch and the final printf should be inside the if , not outside it.
4. You don't check for division by zero issues.

Here is the corrected code with some more minor modifications (untested) :

#include <stdio.h>
/* #include <math.h> Unused header */

int main(int argc, char* argv[])
{
    int num1, num2, result;
    char op1;

    if (argc != 4)
    {
        fprintf(stderr, "Program requires exactly 3 arguments (excluding program name), Too %s given; Exiting...\n", argc < 4 ? "few" : "many");
        return -1;
    }

    if(sscanf(argv[1], "%d", &num1) == 1 &&
       sscanf(argv[2], "%c", &op1 ) == 1 &&
       sscanf(argv[3], "%d", &num2) == 1)
    {
        switch (op1)
        {
            case '+':
                result = num1 + num2;
                break;

            case '-':
                result = num1 - num2;
                break;

            case 'x':
                result = num1 * num2;
                break;

            case '/':
                if(num2 == 0)
                {
                    fputs("Division by zero is not possible; Exiting...", stderr);
                    return -2;
                }
                result = num1 / num2;
                break;

            case '%':
                if(num2 == 0)
                {
                    fputs("Division by zero is not possible; Exiting...", stderr);
                    return -3;
                }
                result = num1 % num2;
                break;

            default:
                fprintf(stderr, "Invalid operator '%c'; Exiting...\n", op1);
                return -4;
        }

        printf("Result = %d\n", result);
    }
    else
    {
        fputs("Failed to parse arguments successfully; Exiting...", stderr);
        return -5;
    }

    return 0;
}

Oh, and the shell has some special characters that do stuff. To avoid issues, wrap each argument with " like:

./main "1" "+" "2"

Answer 2

There is no need to sscanf() given every part of the expression is in a separate string already:

if (argc == 4){
    num1 = atoi(argv[1]);
    op1 = argv[2][0];
    num2 = atoi(argv[3]);
}