simple calculator v3.c

// Simple calculator v3, works with real numbers,
// with how it works prints and comments

#include<stdio.h>
#include <stdlib.h> // for atof() function

int main()
{
// c is operator, str is entered string, snum1 and snum2 are entered strings of numbers num1 and num2
    char c, str[20], snum1[20], snum2[20];
// i is counter for str, i1 is counter for snum1, i2 is counter for snum2
    int i=0, i1=0, i2=0, sign_num1=1, sign_num2=1;
// num1 and num2 are entered real numbers
    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);    // no need for &str because str is address of str[0]

    printf("\n  1. Entered string: %s \n", str);

// print str[i]
    while(str[i]!='\0'){    // from beginning to end of str
        printf("\n  2. str[%d] = %c \n",i,str[i]);
        i++;    // next character of str
    }

    i=0;    // reset counter

/*
   read sign_num1 (default is +1) of first number num1
   if first character of str:
   -, sign_num1 is -1
   +, sign_num1 is +1 (default is +1)
*/

    if(str[0]=='-'){
        sign_num1 = -1;
        i++;    // next character of str, this is first character of snum1
    }
    else if(str[0]=='+'){
        //(sign_num1 default is +1)
        i++;    // next character of str, this is first character of snum1
    }

    printf("\n  3. i = %d \t sign_num1 = %d \n",i,sign_num1);

// read snum1[i] from str[i] until str[i] is operator (not digit)
// while str[i] is digit and it is not the end of str
    while(isdigit(str[i]) && str[i]!='\0'){ // from beginning to end of str
        snum1[i1]=str[i];   // copy str[i] to snum1[i1]
        printf("\n  4. str[%d] = %c \t snum1[%d] = %c \n",i,str[i],i1,snum1[i1]);
        i++;    // next character of str
        i1++;   // next character of snum1
    }
    snum1[i1]='\0'; // finish snum1
    printf("\n  5. snum1 = %s \n", snum1);

    num1=sign_num1*atof(snum1); // convert string snum1 to float num1
    printf("\n  6. num1 = %f \n", num1);

    c=str[i];   // read operator c from str[i]
    printf("\n  7. i = %d \t operator c = %c \n",i,c);

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

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

/*
   read sign_num2 (default is +1) of second number num2
   if str[i]:
   -, sign_num2 is -1
   +, sign_num2 is +1 (default is +1)
*/

    if(str[i]=='-'){
        sign_num2 = -1;
        i++;    // next character of str, this is first character of snum2
    }
    else if(str[i]=='+'){
        //(sign_num2 default is +1)
        i++;    // next character of str, this is first character of snum2
    }

    printf("\n  9. i = %d \t sign_num2 = %d \n",i,sign_num2);

// read snum2[i] from str[i] until the end of str
// while str[i] is digit and it is not the end of str
    while(isdigit(str[i]) && str[i]!='\0'){ // from first character of snum2 in str, to end of str
        snum2[i2]=str[i];   // copy str[i] to snum2[i2]
        printf("\n 10. str[%d] = %c \t snum2[%d] = %c \n",i,str[i],i2,snum2[i2]);
        i++;    // next character of str
        i2++;   // next character of snum2
    }

    printf("\n 11. i = %d \t i2 = %d \n",i,i2);

    snum2[i2]='\0'; // finish snum2

    i2=0;   // reset counter
    while(snum2[i2]!='\0'){
        printf("\n 12. snum2[%d] = %c \n",i2,snum2[i2]);
        i2++;   // next character of snum2
    }

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

    num2=sign_num2*atof(snum2); // convert string snum2 to float num2

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


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

    switch(c)   // analyze operator 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;
}