CODEWITHSUNDEEP
listhaskell
Tomáš 'Mada' Kurila
Tomáš 'Mada' Kurila

Reputation: 23

Find x in list of lists

I have a little problem with my homework to school. I have to write code, which will find any x (int or letter) in a list of lists.

I have something like this:

 find x xxs = [ [ x | x <- xs, x `elem` xs ] | xs <- xxs ]

Hugs98 accept it without any exception, but it doesn't work. 

Input: find 2 [[1,1,1,1],[4,4,4,4],[3,3,3]]
Output: [[1,1,1,1],[4,4,4,4],[3,3,3,3]]

Upvotes: 2

Views: 175

Answers (1)

MathematicalOrchid
MathematicalOrchid

Reputation: 62848

With a little renaming, I think you just wrote this:

find x yss = [ [ y | y <- ys, y `elem` ys ] | ys <- yss ]

I don't think that's what you meant to do. (In your code, you've got two variables named x, one shadowing the other.)

Your question doesn't state what tools you are or aren't allowed to use.

Clearly elem finds stuff in a list. And by "finds", I mean it returns a Bool indicating whether the target item is present or not. But how to handle a list of lists?

A list comprehension always returns a list, so if you were hoping for find to return a Bool, you don't want a list comprehension.

You can use map to apply elem to every list in the list of lists - but now you have a list of Bools. There are two ways to deal with that. One slightly kludgy way is to use elem to see if the list contains True anywhere. But a more sane way is to use the built-in or function, which takes the logical-OR of a list of Bools. (There is also a corresponding and function.)

find x yss = or (map (x `elem`) yss)

There's another way you can do this too; there's a built-in function called any, which applies a function to a list of stuff and returns True if the supplied function ever returns True. So you can do

find x yss = any (x `elem`) yss

Upvotes: 5

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