Question

I’m trying to understand how this exactly works (I know what it does, I just don’t understand how).
As far as I know, this reads a char until EOF is reached, and if its a digit, put it in the array:

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

I understand I can index an array like this:

some_array[1] = 12;

which will put 12 in the second element.

What does the c-'0' do ?

(I got this from the book The C programming language 2nd edition from K&R)

Answer 1

‘0’ is a char which has a decimal value of 48 when coerced to an integer. It works because char is a built-in ordinal type.

If you look at the disassembly, you can see this in action:

      if (c >= '0' && c <= '9')
004135E3  movsx       eax,byte ptr [ebp-9] 
004135E7  cmp         eax,30h 
004135EA  jl          wmain+6Eh (41360Eh) 
004135EC  movsx       eax,byte ptr [ebp-9] 
004135F0  cmp         eax,39h 
004135F3  jg          wmain+6Eh (41360Eh) 

Notice we’re comparing against 30 hex and 39 hex, not ‘0’ and ‘9’.