All Combinations Equal to a Sum in Perl and Prolog
A problem that comes up surprisingly often is that we want to determine lists of numbers which sum to some given number S.
For example, let's look at Perl Weekly Challenge 075
You are given a set of coins @C, assuming you have infinite amount of each coin in the set.
Write a script to find how many ways you make sum $S using the coins from the set @C.
@C = (1, 2, 4)
$S = 6
There are 6 possible ways to make sum 6.
a) (1, 1, 1, 1, 1, 1)
b) (1, 1, 1, 1, 2)
c) (1, 1, 2, 2)
d) (1, 1, 4)
e) (2, 2, 2)
f) (2, 4)
Such a problem is a nice application for a logic programming approach!
Here is a first attempt with a pure Prolog solution (with SWI-Prolog).
$ swipl -s prolog/ch-1.p -g main
We have some duplicate solutions to clean up, but since this is a Perl centric challenge let's do it in Perl and while we're at it, we'll also have Perl handle the input and output.
$ perl perl/ch-1.pl 6 "1,2,4"
The deduplication is handled by storing the resulting lists as hash keys. In order to use array references as hash keys we need to use Hash::MultiKey. The target sum and coin value list are set in the Prolog code via string substitution. Of course these could have been set as parameters and passed in the normal way but this seemed slightly more fun!