Finding Elements in One Array but Not Another

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(minor language; use lists:foldr/3 instead of lists:foldr/3+lists:reverse/1 (!); use lists:filter/2; remove bogus+bizarre reference to lists:foreach/2; use lists:seq/2+3; actually filter as described)
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You want to find elements in list A that aren't in list B. Erlang provides several ways of doing this: 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 +Use lists:subtract/2 , or the equivalent -- operator.
-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:  
<code> <code>
-1> A = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16].+1> A = lists:seq(1,16).
[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> B = lists:seq(2,16,2).
-[2,4,6,8,9,10,12,14,15,16].+[2,4,6,8,10,12,14,16].
3> lists:subtract(A,B). 3> lists:subtract(A,B).
-[1,3,5,7,11,13]+[1,3,5,7,9,11,13,15]
-</code>+
- +
-Note: Erlang provides native list operators for concatentation (++) and subtraction (--). The subtraction operator provides the same functionality as the lists:subtract function: +
-<code>+
4> A -- B. 4> A -- B.
-[1,3,5,7,11,13]+[1,3,5,7,9,11,13,15]
</code> </code>
Iterate over the two lists 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. +You can iterate over the two lists, and filter out any entries from list A that are also members of list B.
<code> <code>
-5> lists:foldl(fun(X,ACC) -> Y=lists:member(X, B),+5> lists:foldr(fun(X,Acc) -> Y=lists:member(X, B),
-5> if Y -> [X|ACC];+5> if Y -> [X|Acc];
5> true -> ACC 5> true -> ACC
-5> end end, [], lists:reverse(A)).+5> end end, [], A).
-[2,4,6,8,9,10,12,14,15,16]+[1,3,5,7,9,11,13,15]
 +6> lists:filter(fun (X) -> not lists:filter(X,B) end, A).
 +[1,3,5,7,9,11,13,15]
</code> </code>
[[Category:CookBook]][[Category:ListRecipes]] [[Category:CookBook]][[Category:ListRecipes]]

Revision as of 14:58, 24 September 2006

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 lists:subtract/2 , or the equivalent -- operator. 

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

Iterate over the two lists

You can iterate over the two lists, and filter out any entries from list A that are also members of list B. 

5> lists:foldr(fun(X,Acc) -> Y=lists:member(X, B),
5>    if Y -> [X|Acc];
5>    true -> ACC
5> end end, [], A).
[1,3,5,7,9,11,13,15]
6> lists:filter(fun (X) -> not lists:filter(X,B) end, A).
[1,3,5,7,9,11,13,15]
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