Filtering a list of lists in Haskell

Question

I want a function Int -> [[String]], trying to filter out the elements that have a specific value on the Inth spot. I thought I could combine filter and !! but I can't get it to work. So far I have:

filter (!! (== value)) rows

where value is a String and rows is a [[String]]. I thought it would take the Int combined with a [String] from rows and check if that particular entry equals value and then keep those rows, but it gets interpreted differently. Any help would be appreciated.

It's often best to start by using explicit lambdas to make sure every function and operator is fully applied, then, if you wish, working to remove them one at a time. — dfeuer
– dfeuer, Commented Sep 20, 2015 at 18:12

melpomene · Accepted Answer · 2015-09-20 12:37:15Z

3

(!! (== value)) is invalid because the right operand of !! must be an Int. (== value) is a function.

Your options are

\n -> filter (\xs -> xs !! n == value) rows

or without the explicit lambda

\n -> filter ((== value) . (!! n)) rows

answered Sep 20, 2015 at 12:37

melpomene

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2 Comments

tbpotn Over a year ago

This works indeed. Glad to see my idea was correct, but it was my formulation that was lacking.

AJF Over a year ago

@tbpotn I would recommend reading up on pointfree style, to avoid future problems such as this.

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