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annotate 3-Prolog/day2-concat.pl @ 60:643eec1f9e3a

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Add a custom implementation of append/3
author IBBoard <dev@ibboard.co.uk>
date Wed, 27 Sep 2017 20:48:28 +0100
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643eec1f9e3a Add a custom implementation of append/3
IBBoard <dev@ibboard.co.uk>
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1 % Rewriting "append" from scratch as "concat"
643eec1f9e3a Add a custom implementation of append/3
IBBoard <dev@ibboard.co.uk>
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2 % Append/3 is true when List1+List2 = List3. This can be used to:
643eec1f9e3a Add a custom implementation of append/3
IBBoard <dev@ibboard.co.uk>
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3 % * check whether two lists combine to a third (supply all args)
643eec1f9e3a Add a custom implementation of append/3
IBBoard <dev@ibboard.co.uk>
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4 % * combine lists (supply first two and a variable for the third)
643eec1f9e3a Add a custom implementation of append/3
IBBoard <dev@ibboard.co.uk>
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5 % * remove an item from a list (pass the first and third and the variable in 2)
643eec1f9e3a Add a custom implementation of append/3
IBBoard <dev@ibboard.co.uk>
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6 % * computing splits of the list (pass two variables and a list - possibly the least useful, normally)
643eec1f9e3a Add a custom implementation of append/3
IBBoard <dev@ibboard.co.uk>
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7 %
643eec1f9e3a Add a custom implementation of append/3
IBBoard <dev@ibboard.co.uk>
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8 % The following is written without looking at the code, just the list of features.
643eec1f9e3a Add a custom implementation of append/3
IBBoard <dev@ibboard.co.uk>
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9
643eec1f9e3a Add a custom implementation of append/3
IBBoard <dev@ibboard.co.uk>
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10 % Concat an empty list
643eec1f9e3a Add a custom implementation of append/3
IBBoard <dev@ibboard.co.uk>
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11 concatenate(List1, List2, List3) :- List2 = [], List3 = List1.
643eec1f9e3a Add a custom implementation of append/3
IBBoard <dev@ibboard.co.uk>
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12 % Concat one item from 1 on to 2
643eec1f9e3a Add a custom implementation of append/3
IBBoard <dev@ibboard.co.uk>
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13 concatenate(List1, List2, List3) :- List1 = [Head1], List3 = [Head1|List2].
643eec1f9e3a Add a custom implementation of append/3
IBBoard <dev@ibboard.co.uk>
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14 % Concat more items
643eec1f9e3a Add a custom implementation of append/3
IBBoard <dev@ibboard.co.uk>
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15 concatenate(List1, List2, List3) :- List1 = [Head1|Tail], concatenate(Tail, List2, TempTail), List3 = [Head1|TempTail].
643eec1f9e3a Add a custom implementation of append/3
IBBoard <dev@ibboard.co.uk>
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16 % See what we can generalise…
643eec1f9e3a Add a custom implementation of append/3
IBBoard <dev@ibboard.co.uk>
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17 % Erm, already seems rather general!
643eec1f9e3a Add a custom implementation of append/3
IBBoard <dev@ibboard.co.uk>
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18 %
643eec1f9e3a Add a custom implementation of append/3
IBBoard <dev@ibboard.co.uk>
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19 % Looking at the code, we can simplify by:
643eec1f9e3a Add a custom implementation of append/3
IBBoard <dev@ibboard.co.uk>
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20 % 1) making the first rule a fact (which I thought I might be able to do, but the wording implied I couldn't. And it worked so I left it)
643eec1f9e3a Add a custom implementation of append/3
IBBoard <dev@ibboard.co.uk>
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21 % 2) shifting the unifying from the right to the left
643eec1f9e3a Add a custom implementation of append/3
IBBoard <dev@ibboard.co.uk>
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22 % 3) taking advantage of "[Head|Tail]" matching an empty tail to combine the second two rules