Erlang list vs tuple comparison

erlang, functional-programming

Solution

Your filter function needs some help:

lists:filter(fun([1, _, 3]) -> false; (_) -> true end,
             [[1,2,3],[4,5,6],[1,8,3],[1,9,3]]).

You need the match in the function head to make this work.

As for having multiple filters, you can use a map of funs (not tested):

composed_filter(Tests, Input) ->
  lists:filter(fun(X) -> lists:all([F(X) || F <- Tests]) end,
               Input).

For using multiple filters, you can do:

filter(fun([1,_,3]) -> true;
          ([5,_,7]) -> true;
          ([9,_,13]) -> true;
          (_) -> false end, Input)
% or
composed_filter([fun([1,_,3]) -> true; (_) -> false end,
                 fun([5,_,7]) -> true; (_) -> false end,
                 fun([9,_,13]) -> true; (_) -> false end],
                Input)

Though unless you need a programmable dynamic composability, you should probably just use the first. Another way, again is to run the composition with a function rather than having a specialized composed_filter. This is a more combinator-style approach which the Haskell crowd would love. Note that the above functions are slightly different. The first has `orelse` semantics whereas the latter has `andalso` semantics. That is in the first case, you get the element if one or more of the patterns match, whereas in the latter case, you only get a result if all the patterns/funs match. You could use `lists:any/2` to obtain a `composed_any_filter`.

Problem

Assume that there is an erlang list of lists: Note that the size of the lists are fixed. In this case 3. ``` A = [[1,2,3],[4,5,6],[1,8,3],[1,9,3]] ``` I am trying to write function which will drop all the elements which are of the form `[1,_,3]`. So my expected output would be of the form: `[[4,5,6]]` One solution which I could think about was the lists:`dropwhile()` function which can be used. I am having trouble constructing the predicate function. Is it easier to convert every element into a tuple and then do the comparison? If so, there is no proper way I could omit the middle element. Can you please help me arrive at the solution? Any other efficient approach would also be highly appreciated. Thanks in advance! Edit: Question Extenstion: Assume two erlang list of lists: A = [[1,2,3], [2,3,4], [4,5,6]] B = [[1,4,3], [4,7,6], [7,8,9], [4,9,7], ] The output F(A,B) = [[7,8,9], [4,9,7]] F is defined to be a function, which removes from list B all the elements which are matching any one of the elements in list A in the first and the third position. In this case, all those elements in list B which are matching [1, _ ,3] or [2, _ , 4] or [4 , _ , 6] are deleted from the list to give the resulting list. I am struggling to write F.

Original source