calc.c

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>

int main() {
	// your code goes here
	double sum = 0;
	const char *input = "300 - 400";
	// const char *input = "1 + 3 / 5 * 8";
	// const char *input = "10 + 34 / 3 + 8";
	
	double second_num = 0;
	
	// read the first num
    sum = atof(input);
    
    while(isdigit(*input) || isspace(*input))
    {
        ++input;
    }
    
    while(*input)
    {
    	switch(*input)
    	{
            case '/':
            {
                second_num = atof(++input);
                sum /= second_num;
                break;
            }
            case '+':
            {
                second_num = atof(++input);
                sum += second_num;
                break;
            }
            case '-':
            {
                second_num = atof(++input);
                sum -= second_num;
                break;
            }
            case '*':
            {
                second_num = atof(++input);
                sum *= second_num;
                break;
            }
            
    	}

        while(isdigit(*input) || isspace(*input))
        {
            ++input;
        }
    }
    
    printf("%g\n", sum);
    
	return 0;
}