CODEWITHSUNDEEP
haskell
mathandtic
mathandtic

Reputation: 159

Haskell Efficient Program

I have this code:

trimorficos :: [Integer]
trimorficos = filter (trim) [0..]

trim :: Integer -> Bool
trim x = (show x) `isSuffixOf` (show (x^3))
      where a = show x
            b = show (x^3)

densityTrimorficos :: Integer -> Double
densityTrimorficos n = fromInteger (n - (genericLength (filter (<=10) trimorficos))) / fromInteger n

Why the last function densityTrimorficos doesn't work?

Upvotes: 0

Views: 56

Answers (1)

chi
chi

Reputation: 116139

trimorficos is an infinite list.

filter (<=10) trimorficos will never produce the end-of-list [] at the very end. To do so, it should verify that, from a certain point onward, trimorficos contains only numbers >10, but that would require infinite time. Basically, filter will returns something like a:b:c:nonTerminating instead of a:b:c:[] = [a,b,c].

Consequently, genericLength fails to terminate, since it tries to evaluate nonTerminating into either d:... or [] but that requires infinite time.

As pointed out above in the comments, you probably want takeWhile (<=10) trimorficos instead, which will produce [] as soon as the first >10 number is encountered. Note that this will not check the rest of the list, unlike filter.

Upvotes: 1

Related Questions