How does character representation in array index w

Question

I'm a beginner in C and I have code like this :

#include <stdio.h>
main()
{
    int i;
    int ndigit[10] = { [9] = 5 };
    printf("%d\n",++ndigit['9']);
}

This prints the value something like this :

-1074223011

But when I change the statement to:

++ndigit['9'-'0']

It is correctly printing the value

6

I wonder why there is a need for adding -0 in my index to make it work properly? And why just mentioning ++ndigit['9'], doesn't help me?

Thanks in advance.

Answer 1

If you want to access the 10th element in an array, you do:

array[9]

If you want to access the element at the index which has the value of the character constant for the number 9 + 1, you do:

array['9']

Due to the way ASCII (and all other character encoding schemes used by C, see Wiz's comment) is defined, the expression '9' - '0' actually equals 9, which might confuse you in this case.