simple calculator v4.c

// Simple calculator v3, works with real numbers,
// without how it works prints and comments

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

int main()
{
    char c, str[20], snum1[20], snum2[20];
    int i=0, i1=0, i2=0, sign_num1=1, sign_num2=1;
    float num1,num2;

    printf("\n CALCULATOR (for real numbers) \n");
    printf("\n Enter first number and operator (+,-,*,/) and second number: \n");
    printf("\n Example: -123+-456 \n\n");

    scanf("%s",str);

    if(str[0]=='-'){
        sign_num1 = -1;
        i++;
    }
    else if(str[0]=='+'){
        i++;
    }

    while(isdigit(str[i]) && str[i]!='\0'){
        snum1[i1]=str[i];
        i++;
        i1++;
    }
    snum1[i1]='\0';

    num1=sign_num1*atof(snum1);

    c=str[i];

    i++;

    if(str[i]=='-'){
        sign_num2 = -1;
        i++;
    }
    else if(str[i]=='+'){
        i++;
    }

    while(isdigit(str[i]) && str[i]!='\0'){
        snum2[i2]=str[i];
        i++;
        i2++;
    }

    snum2[i2]='\0';

    num2=sign_num2*atof(snum2);

    printf("\n (%f) %c (%f) = ",num1,c,num2);

    switch(c)
    {
        case '+':
            printf("(%f) \n",num1+num2);
            break;

        case '-':
            printf("(%f) \n",num1-num2);
            break;

        case '*':
            printf("(%f) \n",num1*num2);
            break;

        case '/':
            printf("(%f) \n",num1/num2);
            break;
        default:
            printf("Error! Invalid Operator.");
    }

    printf("\n\n");
    return 0;
}