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annotate 3-Prolog/day2-maths.pl @ 103:98be775c533c default tip

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An odd "non-determinism" example from StackOverflow It is clever, but doesn't make much sense as to how it gets its results
author IBBoard <dev@ibboard.co.uk>
date Sun, 14 Jul 2019 13:44:13 +0100
parents f86bb0d669be
children
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f86bb0d669be Add a quick bit of maths code (list counting/summing)
IBBoard <dev@ibboard.co.uk>
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1 % First use of variable assignment!
f86bb0d669be Add a quick bit of maths code (list counting/summing)
IBBoard <dev@ibboard.co.uk>
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2 % We use a mix of rules and facts to do maths on lists.
f86bb0d669be Add a quick bit of maths code (list counting/summing)
IBBoard <dev@ibboard.co.uk>
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3 count(0, []).
f86bb0d669be Add a quick bit of maths code (list counting/summing)
IBBoard <dev@ibboard.co.uk>
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4 % This seems backwards, but we're saying "Count of a list with a head and a tail
f86bb0d669be Add a quick bit of maths code (list counting/summing)
IBBoard <dev@ibboard.co.uk>
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5 % is the count of its tail (which will be assigned to TailCount) plus one, which
f86bb0d669be Add a quick bit of maths code (list counting/summing)
IBBoard <dev@ibboard.co.uk>
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6 % is then assigned to Count, which gets passed back out"
f86bb0d669be Add a quick bit of maths code (list counting/summing)
IBBoard <dev@ibboard.co.uk>
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7 count(Count, [Head|Tail]) :- count(TailCount, Tail), Count is TailCount + 1.
f86bb0d669be Add a quick bit of maths code (list counting/summing)
IBBoard <dev@ibboard.co.uk>
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8
f86bb0d669be Add a quick bit of maths code (list counting/summing)
IBBoard <dev@ibboard.co.uk>
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9 sum(0, []).
f86bb0d669be Add a quick bit of maths code (list counting/summing)
IBBoard <dev@ibboard.co.uk>
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10 sum(Total, [Head|Tail]) :- count(Sum, Tail), Total is Head + Sum.
f86bb0d669be Add a quick bit of maths code (list counting/summing)
IBBoard <dev@ibboard.co.uk>
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11
f86bb0d669be Add a quick bit of maths code (list counting/summing)
IBBoard <dev@ibboard.co.uk>
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12 average(Average, List) :- sum(Sum, List), count(Count, List), Average is Sum / Count.