Count number of Instances of Char in a given String Haskell

Question

Simple problem.

Given a String, I want to be able to enter a letter and then have my function count the number of times that letter appears in a string.

countLetters :: String -> Char -> Int

How can this be done?

Answer 1

With

countLetters :: String -> Char -> Int
countLetters str c = length $ filter (== c) str

This gets only the characters in str that equal c, then computes the length of that.

Answer 2

That depends on the number of calls you'd like to make to one String. Simple solution would be to just read every character in the string, if it matches the one you're looking for, increase the counter.

Now, if you're mapping over something with a counter, you can obviously use fold:

countLetters xs x = foldl (\count char -> if char == x then (count + 1) else count) 0 xs

However, if you want to do many queries, it makes more sense to build the lookup table (i.e. sort) first. Then you can get the number of repetitions of arbitrary character in O(1) (the whole algorithm is still O(n)).