[FIXED] Why I should subtract '0' so that I can do int_arr[5] = char_string[5]?

Issue

So I was assigned to store two 50 digit integers in c language and do math equations using them. the problem for me was to store the input digit by digit in an array. I figured that I can store the input in a char string like this:

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

int main ()
{
    char string_test [50];
    scanf("%s", string_test);
    return 0;
}

But I couldn't use it as a number because it was stored as char and I couldn't copy them digit by digit into another array which was defined as int

after a whole day of searching, I found out that I need to copy my string one by one like this:

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

int main ()
{
    char string_test [50];
    scanf("%s", string_test);

    int arr[50];

    for (int i = 0; i < 50; i++)
    {
        arr[i] = string_test[i] - '0';
    }
    return 0;
}

Now my question is why do I need to subtract '0' to get a suitable result?

Solution

The ASCII values for digits 0 - 9 are:

Digit          0   1   2   3   4   5   6   7   8   9

ASCII value   48  49  50  51  52  53  54  55  56  57  

So if you have a string representation of an integer, say

char int_str[] = "123456";

and need to convert each char to its numeric value, subtracting the value for '0' (48) from each will will result in the values

int_str[0] == '1' ==>  '1' - '0' ==> 42 - 41 == 1
int_str[1] == '2' ==>  '2' - '0' ==> 43 - 41 == 2
int_str[2] == '3' ==>  '3' - '0' ==> 44 - 41 == 3
int_str[3] == '4' ==>  '4' - '0' ==> 45 - 41 == 4
int_str[4] == '5' ==>  '5' - '0' ==> 46 - 41 == 5
int_str[5] == '6' ==>  '6' - '0' ==> 47 - 41 == 6  

To get digits 1 2 3 4 5 6 into the integer 123456 requires additional steps:

This example uses the same conversions encapsulated into a function to convert discrete char digit to int digit values, then assimilate each discrete int digit value into the composite integer value:

int main(void)
{
    char str[] = "123456";
    int int_num = str2int(str);

    return 0;   
}

 int str2int(char *str)
 {
    int sum=0;

    while(*str != '\0')
    {    //qualify string
         if(*str < '0' || *str > '9')
         {
             printf("Unable to convert it into integer.\n");
             return 0;
         }
         else
         {   //assimilate digits into integer 
             sum = sum*10 + (*str - '0');
             str++;
         }
    }
    return sum;
}

Answered By - ryyker

Answer Checked By - Willingham (PHPFixing Volunteer)