Question

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Searching through list

I need to write a function ‘once’ which, given a list of Integers and an Integer n, returns a Boolean indicating whether n occurs exactly once in the list. E.g.

Main>  once [2,3,2,4] 2
False
Main> once [1..100] 2
True

And here is my current code:

once :: Int -> [Int] -> Bool
once x [] = False
once x (y:ys) = (x==y) || (once x ys)

It checks only whether x is part of the list, but it cannot tell x appeared more than once in the list and therefore return false.
Need help with this, thanks!

Answer 1

There are many possibilities doing that. If you know that the list is finite, you could say:

once x xs = length (filter (==x) xs) == 1

(If it’s not finite, there is no solution.)

By the way, you had it almost in your solution, you just replace

|| (once x ys)

with

&& (x `notElem` ys)