CODEWITHSUNDEEP
listhaskellmap-function
Boris Kurova
Boris Kurova

Reputation: 1

Implementing the map function with a list of indices

I want to implement the map function for specific indices only, like this:

mapFor :: (a -> a) -> [Int] -> [a] -> [a]

An example would look like this.

mapFor (+10) [0,2] [1,2,3,4] == [11,2,13,4]

Upvotes: 0

Views: 209

Answers (2)

Boris Kurova
Boris Kurova

Reputation: 1

Thanks to @cornuz I figured it out.

mapFor f is xs = [if (i `elem` is) then f v else v | (i,v) <- zip [0..] xs]

If you see any issues, please let me know so I can fix them.

Upvotes: 0

Will Ness
Will Ness

Reputation: 71075

Play with the data to find the pattern, to fulfill the given requirements:

mapFor (+1) [0,2] [100,200,300,400] == [101,200,301,400] ==

  =   {   [ 100,    200,    300,    400 ]    -- values
          [ 0,        1,      2,      3 ]    -- indices
          [ 0, 2 ]                       }   -- places
     --------------------------------------
  =    (100+1) : { [200,    300,    400 ]    -- values
                   [  1,      2,      3 ]    -- indices
                   [  2 ]                }   -- places
     --------------------------------------
  =    (100+1) : 200 :   { [300,    400 ]    -- values
                           [  2,      3 ]    -- indices
                           [  2 ]        }   -- places
     --------------------------------------
  =    (100+1) : 200 : (300+1) : { [400 ]    -- values
                                   [  3 ]    -- indices
                                   []    }   -- places
     --------------------------------------
   =
       ........

where { ... } signifies our transformation.

Then, generalize! -- by replacing concrete data with symbolic variables, and you've got yourself a working -- recursive -- code.

mapFor op places values = g [0..] places values
  where
  g (i:is) (p:ps) (v:vs)         -- most general case
   | i == p      =  ....
   | otherwise   =  ....
  g  _     []      _   =   ....  -- base case
  ......                         -- more corner cases, if any

Make sure you exhaust all possibilities with your patterns.

Upvotes: 2

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