[Haskell-cafe] How to get subset of a list?
bjpop at csse.unimelb.edu.au
Thu Nov 30 22:28:06 EST 2006
On 01/12/2006, at 12:47 PM, Huazhi (Hank) Gong wrote:
> Like given a string list s="This is the string I want to test", I
> want to get
> the substring. In ruby or other language, it's simple like s
> [2..10], but how
> to do it in Haskell?
If your indices are in ascending order, and unique, then something
like this might
do the trick:
els1 indexes list
= els' (zip [0..] list) indexes
els'  _ = 
els' _  = 
els' ((j,x):xs) indexes@(i:is)
| i == j = x : els' xs is
| otherwise = els' xs indexes
Of course this is a right fold, so you ought to be able to use foldr.
Here's an attempt:
els2 indexes list
= foldr comb undefined [0..] list indexes
comb _ _  _ = 
comb _ _ _  = 
comb j rec (x:xs) indexes@(i:is)
| j == i = x : rec xs is
| otherwise = rec xs indexes
Bonus marks for figuring out why I used "undefined".
Warning: this is largely untested code.
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