Finding The Index of Element in an Array - Haskell

fst <$> find ((== 'a') . snd) $ assocs lowerCase

To get all the indices a certain element appears in your Data.Array the following list comprehension can be used:

results = [fst x | x <- (assocs lowerCase), snd x == 'a']

assocs has the following prototype:

assocs :: Ix i => Array i e -> [(i, e)]

It basically flattens a Data.Array in a List containing (i, e) pairs. For:

a = listArray ((0,0),(2,2)) ['a'..]

assocs a will output

[((0,0),'a'),((0,1),'b'),((0,2),'c'),((1,0),'d'),((1,1),'e'),((1,2),'f'),((2,0),'g'),((2,1),'h'),((2,2),'i')]

Now, in our list comprehension, we have x <- (assocs a), so x is generated by the list assocs a.

The list outputed by the list comprehension will contain only fst x where

snd x == theElementWeAreLookingFor

Every x generated by assocs a is checked and if the condition snd x == 'a' is met then fst a (the index) will be inserted in the output list.

Once the list is generated, it can be checked whether there are none, one or more outputs.

getElementIndex :: Array (Int, Int) Char -> Char -> Maybe (Int, Int)
getElementIndex a e
   | null results = Nothing
   | othwerwise   = Just $ head results
   where results = [fst x | x <- (assocs a), snd x == e]

A imperative pseduocode could look like:

results = []

for_each x in a.toList():
   if x.second == 'a':
      results.append(x.first)