9 Aug
2007
9 Aug
'07
5:40 p.m.
On 8/9/07, Chad Scherrer
extract :: [Int] -> [a] -> [a] extract = f 0 where f _ _ [] = [] f _ [] _ = [] f k nss@(n:ns) (x:xs) = if n == k then x:f (k+1) ns xs else f (k+1) nss xs
This behaves roughly as extract ns xs == map (xs !!) ns
except that it's a lot more efficient, and it still works if ns or xs (but not both) are infinite. Oh, and "ns" are required to be ordered and non-negative.
Nifty function there. =) And for the record, it works just fine if both lists are infinite -- it just produces an infinite output list, but it's lazy so no problem: *Main> take 10 $ extract [1,3..] [2..] [3,5,7,9,11,13,15,17,19,21] -Brent