c - Syntax and different meanings of '<letter>'

Question

I am learning C from the K&R book and I came across the code to count the no. of occurrence of white space characters (blank, tab, newline) and of all other characters.
The code is like this:

#include <stdio.h>
/* count digits, white space, others */
main()
{
    int c, i, nwhite, nother;
    int ndigit[10];  
    nwhite = nother = 0;
    for (i = 0; i < 10; ++i)
        ndigit[i] = 0;
    while ((c = getchar()) != EOF)
        if (c >= '0' && c <= '9')
            ++ndigit[c-'0'];
        else if (c == ' ' || c == '
' || c == '	')
            ++nwhite;
        else
            ++nother;
    printf("digits =");
    for (i = 0; i < 10; ++i)
        printf(" %d", ndigit[i]);
    printf(", white space = %d, other = %d
",
    nwhite, nother);
}

I need to ask 2 questions..
1st question:

if (c >= '0' && c <= '9')
++ndigit[c-'0']; 

I very well know that '0' and '9'represents the ASCII value of 0 & 9 respectively. But what I don't seem to understand is why we even need to use the ASCII vale and not the integer itself. Like why can't we simply use

if (c >= 0 && c <= 9)

to find if c lies between 0 and 9?

2nd question:

++ndigit[c-'0']

What does the above statement do?
Why aren't we taking the ASCII value of c here?
Because if we did, it should have been written as ['c'-'0'].

Answer 1

1. C is a character, not an integer. Thus we need to compare them to their ASCII values. The integers 0 and 9 correspond to Nul and Tab, not something we are looking for.

2. By subtracting off the ASCII value the index corresponding to the integer is increased. For example if our number is '1'. Then '1' - '0' = 1 so the index at one is increased, its a convenient way to keep track of characters. We dont put ['c' - '0'] because we care about the variable c not the character 'c'

This table shows how characters are represented, they are different from integers. The main take away is '1' != 1 http://www.asciitable.com/