Strange output of char array converted to int - magic of array[x] - '0'

Question

So I'm fooling around with char arrays conversion and testing now this simple code:

#include <stdio.h>

int main(void) {

    char arr[BUFSIZ];
    printf("Input: ");
    fgets(arr, sizeof(arr), stdin);
    int x = arr[0] - '0';
    printf("Output: %d\n", x);
}

The output should be value of variable x which is first digit of input. But, in case of passing negative integer as input I get this:

jharvard@appliance (~/CS50/pset1): ./test
Input: -123
Output: -3

The output should be -1, but is -3. Is this minus has something to do with array index? I assume this minus forces compiler to make array[-1] index the value of output?

I think that's the point, 'cause after changing the line int x = arr[0] - '0'; to int x = arr[1] - '0';, I get:

Input: -123
Output: 1

when passing the negative number.

And another example. When changing that int x line to int x = arr[BUFSIZ - 1] - '0'; I get:

jharvard@appliance (~/CS50/pset1): ./test
Input: -123
Output: -40
jharvard@appliance (~/CS50/pset1): ./test
Input: 123
Output: -40
jharvard@appliance (~/CS50/pset1): ./test
Input: 122
Output: -40
jharvard@appliance (~/CS50/pset1): ./test
Input: 890
Output: -40

No matter if various positive or negative numbers I pass, the output is always -40 in this case.

After changing index, e.g. to int x = arr[BUFSIZ - 2] - '0'; or int x = arr[BUFSIZ - 7] - '0'; I got different output - sometimes positive or negative, but always this particular number gets printed.

BUFSIZ on my PC is 8192 bytes long.

Answer 1

When you input -123, the array is being filled with the characters '-', '1', '2', and '3'. The array looks like:

arr[0] = '-'
arr[1] = '1'
arr[2] = '2'
arr[3] = '3'
arr[4...BUFSIZ] = ???

As you can see, arr[0] is the character '-', not the integer -1. The numeric value of '-' (45) is 3 less than that of '0' (48), hence the output of -3 when subtracting from arr[0], and the correct output of 1 when you change to arr[1]. To see the numerical values of all the standard characters on a keyboard, check out http://www.asciitable.com/

As for outputting from the end of the array, I think that is undefined behavior, because you are not actually assigning anything to those indices.