e r l a n g : c o o k b o o k

%META:TOPICINFO{author="BrentAFulgham" date="1093765536" format="1.0" version="1.1"}%

Finding Elements in One Array but Not Another

Problem

You want to find elements that are in one list but not another.

Solution

You want to find elements in list A that aren't in list B. Erlang provides several ways of doing this:

Use the lists:subtract Function

The lists module provides the subtract function, which takes two list and returns a new copy of the first list, such that the first occurrence of each element of the second list is removed. For example:

1> A = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]. [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16] 2> B = [2,4,6,8,9,10,12,14,15,16]. [2,4,6,8,9,10,12,14,15,16]. 3> lists:subtract(A,B). [1,3,5,7,11,13]

Note: Erlang provides native list operators for concatentation (++) and subtraction (--). The subtraction operator provides the same functionality as the lists:subtract function:

4> A -- B. [1,3,5,7,11,13]

Iterate over the two lists

You can iterate over the two lists (using the lists:foreach function), and filter out any entries from list A that are also members of list B.

5> lists:foldl(fun(X,ACC) -> Y=lists:member(X, B), 5> if Y -> [X|ACC]; 5> true -> ACC 5> end end, [], lists:reverse(A)). [2,4,6,8,9,10,12,14,15,16]

Discussion

Comments about this recipe

Contributors

Based on work by IvanHernandez.

-- BrentAFulgham - 29 Aug 2004 %META:FORM{name="CookbookForm"}% %META:FIELD{name="TopicType" title="TopicType" value="Recipe"}% %META:FIELD{name="ParentTopic" title="ParentTopic" value="ListRecipes"}% %META:FIELD{name="TopicOrder" title="TopicOrder" value="040"}%