CODEWITHSUNDEEP
erlang
mrjoe
mrjoe

Reputation: 43

Erlang, Function which return the lenghts of the lists?

I'm trying to implement a function 

llen(ListOfLists)

which returns a list containing the lengths of the lists in ListOfLists. Function should use lists:map/2.

Example:

1> funs:llen([[1, 2, 3], [], [4, 5]]).
[3, 0, 2]

I know how to calculate length for one list:

list([]) -> 0;
list([_|T]) -> 1+list(T).

but i don't know how to do this for multiple lists using funs and lists.

Upvotes: 2

Views: 1160

Answers (2)

7stud
7stud

Reputation: 48599

And when you get to list comprehensions:

53> L = [[1, 2, 3], [], [4, 5]].
[[1,2,3],[],[4,5]]

54> [length(X) || X <- L].
[3,0,2]

A list comprehension is like a for-loop in other languages, and this one reads like:

length(X)  for   X in L
length(X)  ||    X <- L

The outer [ ] serves to gather up all the results into a list.

Upvotes: 2

Emil Vikstr&#246;m
Emil Vikstr&#246;m

Reputation: 91942

lists:map/2 is a higher order function that applies a function for each element of a list. erlang:length/1 is a function that returns the length of a list.

Apply erlang:length/1 on each element of your list using lists:map/2:

lists:map(fun erlang:length/1, [[1, 2, 3], [], [4, 5]])

Upvotes: 5

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