Making a new list inside a function and adding an element into it

Question

I am trying to create a function, that takes a co-ordinate, and then given a set of co-ordinates, it extracts all the coordinates that are within one unit of the given co-ordinates, and then makes a list out of that. I already know how to make this function, I just need clarification as to how I make it RETURN the new list.

For example if I give the function (2,1) [(1,3),(1,2),(3,0),(4,2),(2,2),(3,1)], it would return a list [(1,2),(3,0),(2,2),(3,1)].

I already know how to implement a function that can find if a list is within one unit, I just need to know how take the functions I found matching my pattern, and returning it into a fresh list

local_elements :: Coordinate -> List_with_coordinates -> List_with_coordinates
local_elements (x_c,y_c) list = case list of
  (x_l,y_l) :xs
    | abs (x_l - x_c) <= 1 && abs (y_l - y_c) <=1 -> "what would go here?" local_elements xs
    | otherwise -> local_elements xs        
  [] -> []

Although it's a bit nitpicking, in functional program one in general does not add elements to lists. Once a variable gets a value. It normally cannot change that value anymore... Functional programming makes use of persistent datastructures. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Apr 2, 2015 at 12:24

bitmask · Accepted Answer · 2015-04-02 11:53:03Z

4

You make a function

dist :: Int -> Coordinate -> Coordinate -> Bool
dist k (x_c,y_c) (x_1,y_1) = ((abs (x_1 - x_c)) <= k) && ((abs (y_1 - y_c)) <= k)

It checks if a coordinate is fine. Theny ou say:

local_lements c = filter (dist 1 c)

This is sufficient to do exactly what you want. If two coordinates are only 1 apart, dist 1 will return True, and the builtin function filter will take all elements x from list that cause dist 1 c x to return True.

answered Apr 2, 2015 at 11:53

bitmask

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2 Comments

seedkey Over a year ago

I see, thanks for that :) I knew filter had something to do with it. So the filter function will return a new list with all the matched elements in it?

bitmask Over a year ago

@seedkey: Well, not exactly. The expression filter (dist 1 c) returns a function that has the following property: It takes a list of coordinates and returns a list of coordinates. The returned list will only contain those coordinates with a distance at most 1 from c. You may type :t filter and :t filter (dist 1 c) into your ghci/hugs prompt to see what type they have.

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