simple calculator v1.c

// Simple calculator v1 with how it works prints and comments

#include<stdio.h>

int main()
{
// c is operator, snum1 and snum2 are entered strings of numbers num1 and num2
    char c, str[20], snum1[20],snum2[20];
// i is counter, snum2length is length of snum2, num1 and num2 are entered numbers
    int i,snum2length=0,num1,num2;

    printf("\n  1. CALCULATOR \n\n");
    printf("\n  2. Enter first number, operator (+,-,*,/) and second number \n\n");
    scanf("%s",str);    // no need for &str because str is address of str[0]

    printf("\n  3. Entered string: %s \n", str);

// print str[i]
    while(str[i]!='\0'){    // from beginning to end of str
        printf("\n  4. str[%d] = %c \n",i,str[i]);
        i++;    // next character of str
    }

    i=0;    // reset counter

// while str[i] is digit and it is not the end of str
// read snum1[i] from str[i] until str[i] is operator (not digit)
    while(isdigit(str[i]) && str[i]!='\0'){ // from beginning to end of str
        snum1[i]=str[i];    // copy str[i] to snum1[i]
        printf("\n  5. str[%d] = %c \t snum1[%d] = %c \n",i,str[i],i,snum1[i]);
        i++;    // next character of str and snum1
    }
    snum1[i]='\0';  // finish snum1

    num1=atoi(snum1);   // convert string snum1 to integer num1

    c=str[i];   // read operator c from str[i]

    printf("\n  6. snum1 = %s \n", snum1);

    printf("\n  7. num1 = %d \n", num1);

    printf("\n  8. i = %d \t c = %c \n",i,c);

    i++;    // next character of str, this is first character of snum2

    printf("\n  9. i = %d \n",i);

// while str[i] is digit and it is not the end of str
// read snum2[i] from str[i] until the end of str
    while(isdigit(str[i]) && str[i]!='\0'){ // from first character of snum2 in str, to end of str
        snum2[snum2length]=str[i];  // copy str[i] to snum2[snum2length]
        printf("\n 10. str[%d] = %c \t snum2[%d] = %c \n",i,str[i],snum2length,snum2[snum2length]);
        i++;    // next character of str
        snum2length++;  // next character of snum2, increase the lenghth of string snum2
    }

    printf("\n 11. i = %d \t snum2length = %d \n",i,snum2length);

    snum2[snum2length]='\0';    // finish snum2

    i=0;    // reset counter
    while(snum2[i]!='\0'){
        printf("\n 12. snum2[%d] = %c \n",i,snum2[i]);
        i++;    // next character of snum2
    }

    printf("\n 13. snum2 = %s \n", snum2);

    num2=atoi(snum2);   // convert string snum2 to integer num2

    printf("\n 14. num2 = %d \n", num2);


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

    switch(c)   // analyze operator c
    {
        case '+':
            printf("%d \n",num1+num2);
            break;

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

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

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

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