Haskell : Filter list for elements of second list

Question

I am currently trying to filter a list , that consist of tuples of the form (String , Double), for a List that consists of Strings. If the tuple doesn't contain a String of the second list, it should be removed from the list of tuples. So far I came up with this:

test :: [ExamScore] -> String -> [ExamScore]
test a b = filter ((== b).fst) a

My current problem is to replace the String that is filtered for by a list of Strings. Thanks for your help! Please go easy on me, I'm a first year informatics student that hasn't coded anytime before.

please show an example call and its desired result.

Will Ness
– Will Ness

2019-11-23 11:32:01 +00:00
Commented Nov 23, 2019 at 11:32 — Will Ness
– Will Ness, Commented Nov 23, 2019 at 11:32

bereal · Accepted Answer · 2019-11-23 12:21:31Z

0

It's almost the same, just another filter function using elem:

test a b :: [ExamScore] -> [String] -> [ExamScore]
test a b = filter (\(s, _) -> elem s b) a

Or, if you're more into the composition style:

test a b = filter (flip elem b . fst) a

(It's worth noting that it's not the most efficient way, since elem is O(N) for lists, so depending on your case you may want to find a better structure for storing the keys.)

edited Nov 23, 2019 at 12:21

answered Nov 23, 2019 at 11:38

bereal

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1 Comment

Robin Zigmond Over a year ago

The inner parentheses in the second definition are unneeded.

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