((c = getchar()) != EOF)
if (c >= '0' && c <= '9')
++ndigit[c-'0'];
c gives you the current character. With the range checking you confirm it is a number by using its ASCII value
Check chr column for 0, it says 48 and for 8 it says 56.So, '8'- '0' gives 56 - 48 = 8
ndigit is used to keep track of how many times a number occurs in with each element of the array representing the number of times its subscript has occured.
ndigit[0] will ggive you number of times 0 has occurred and so on.. ndigit[x] gives number of times x appeared
[c - '0']
suppose your c i.e current character is 8 then '8' - '0' will give you 8. so you get ndigit[8] and you ++ that value [you initialised it to 0 at start]
