Counting Upper and lower Case letters

[bfelts]

I have a struct

struct charCount
{
   char letter;
   int count;
};

How would I read from a file and determine the frequencies of A-Z and a-z?
I must store it in an array of structs

[DSLq5Di1]

Does it help if I mention,

'a' - 'a' = 0
'b' - 'a' = 1
'c' - 'a' = 2

and so on..