CODEWITHSUNDEEP
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Magnus Kronqvist
Magnus Kronqvist

Reputation: 1589

Erlang: is it possible to write the minimum function as a list fold?

Given a function:

min(A, B)  when A =< B -> A;
min(_A, B)             -> B.

can I use this in the function foldlin a similar fashion to this:

lists:foldl(fun min/2, 0, [1,2,3,4,5,6,7,8,9,10])

I believe it is not possible, because I have to set an initial value that will be compared to the rest of the list, e. g. there is no identity function that I can think of. Am I right?

Syntax is written in Erlang, but should be readable for non Erlang programmers, too.

Upvotes: 5

Views: 1812

Answers (3)

Lii
Lii

Reputation: 12122

Adam Lindbergs proposal to use undefined as initial value has the disadvantage that it generates weird results for lists that has atoms as members. Erlang has a global ordering of all objects, so a nice property of a min function would be to be usable for all types.

I think its more reasonable to crash on an empty list. The difference is that the client has to worry about the case, instead of having to worry about getting a undefined as the result.

Upvotes: 2

Adam Lindberg
Adam Lindberg

Reputation: 16577

min(List) ->
    Min = fun(A,  B) when A < B -> A;
             (_A, B)            -> B end,
    lists:foldl(Min, undefined, List).

Using undefined as the initial state should do the trick. Returns undefined for an empty list, which is kind of nice as an API.

If you want it to crash on an empty list, use this function header instead:

min([Head|Rest]) ->
    Min = fun(A,  B) when A < B -> A;
             (_A, B)            -> B end,
    lists:foldl(Min, Head, Rest).

Upvotes: 11

Yasir Arsanukayev
Yasir Arsanukayev

Reputation: 9676

1> List = [42,13,25,3,19,20].
[42,13,25,3,19,20]
2> lists:foldl(fun(X, Y) -> erlang:min(X,Y) end, hd(List), tl(List)).   
3

Crashes a program on an empty list, a recommended approach "let it crash" as opposed to defensive programming.

Upvotes: 5

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