For comparison, here's a Haskell implementation: 
    change _      0  = [[]]
    change []     _  = fail "can't make change"
    change (c:cs) n
       | c > n       = change cs n
       | otherwise   = map (c:) (change (c:cs) (n-c)) ++ change cs n

[download]
Unlike the original, this one finds all of the solutions for a change problem: 
    > change [5,2] 16
    [[5,5,2,2,2],[2,2,2,2,2,2,2,2]]

[download]
Because Haskell computes lazily, if you ask for only the first solution, that's all the further Haskell searches. The following variation returns only the first solution, if any: 
    quickchange cs n = listToMaybe (change cs n)

    > quickchange [5,2] 16
    Just [5,5,2,2,2]

    > quickchange [7] 16
    Nothing

[download]
Further, we can use fractional coins if we want. How can we make change for 10/6 if we have only 1/2 and 1/3 coins? 
    > :m +Data.Ratio
    > quickchange [1%2, 1%3] (10%6)
    Just [1%2, 1%2, 1%3, 1%3]

[download]

Cheers,
Tom

Tom Moertel : Blog / Talks / CPAN / LectroTest / PXSL / Coffee / Movie Rating Decoder

In reply to Re: pathsearch/backtracking in Perl - how to solve a problem apparently requiring it? (comparison of Perl with Standard ML) by tmoertel
in thread pathsearch/backtracking in Perl - how to solve a problem apparently requiring it? (comparison of Perl with Standard ML) by metaperl

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