| ITEC 380 |
| 2009fall |
| ibarland |
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; Lists in prolog: ; Scheme: Prolog: ; empty [] ; cons . ; list [ ..,..,..] ; cons* [ ..,..,.. | .. ] ; For example: mem_v2(_,[]) :- not(true). mem_v2(D,.(D,_)) :- true. mem_v2(D,.(_,R)) :- mem(D,R). % We can simplify this syntax, % once we internalize Prolog's mind-set: mem(X,[X|_]). mem(X,[_|R]) :- mem(X,R). % Other functions to write: % atLast(X,L) % removeLast(L1,L2) % reverse(L,R) % append (L1,L2,L) % zip(L1,L2,L) % e.g. zip( [a,b,c], [d,e,f], [a,d,b,e,c,f] ).
To do arithmetic (rather than pattern-matching),
we use a keyword: “
Var is Expr |
% A version of 'reverse' which requires 'removeLast' and 'last':
reverse([],[]).
reverse([F|R],L2) :- last(L2,F), removeLast(L2,L2less), reverse(R,L2less).
% removeLast, Last -- not included in notes,
% since it's part of hw05 !!
%% Alternate version: An accumulator variable:
reverse(L1,L2) :- reverseHelp( L1, [], L2 ).
%% [+,+,-]
%% reverseHelp(L1,L2,L3):
%% True when: L1 reversed, appended to L2, equals L3.
% First, a less idiomatic solution:
% reverseHelp([],RevSoFar,Soln) :- RevSoFar = Soln.
% reverseHelp(Lst, RevSoFar, Soln) :-
% Lst = [Frst,Rst],
% reverseHelp( Rst, [Frst,RevSoFar], Soln ).
% Here is the same thing, in idiomatic Prolog:
% We just move the "=" clauses into the definitions:
%
reverseHelp([], RevSoFar, RevSoFar).
reverseHelp([Frst|Rst], RevSoFar, Out) :-
reverseHelp(Rst, [Frst|RevSoFar], Out).
%% [+,+,-]
%% filterSmallerThan( Threshold, Lst, Result ):
%% True when 'Result' is all the elements of 'Lst' which
%% are less than Threshold.
% First, a less idiomatic solution:
% filterLessThan(Threshold,Lst,Answer) :- Lst=[], Answer=[].
% filterLessThan(Threshold,Lst,Answer) :-
% Lst = [Frst,Rst],
% Frst >= Threshold,
% filterLessThan(Threshold,Rst,SmallRest),
% Answer = SmallRest.
% filterLessThan(Threshold,Lst,Answer) :-
% Lst = [Frst,Rst],
% Frst < Threshold,
% filterLessThan(Threshold,Rst,SmallRest),
% Answer = [Frst|SmallRest].
% Here is the same thing, in idiomatic Prolog:
% We just move the "=" clauses into the definitions:
%
filterLessThan(Threshold,[],[]).
filterLessThan(Threshold,[Frst|Rst],SmallRest) :-
Frst >= Threshold,
filterLessThan(Threshold,Rst,SmallRest).
filterLessThan(Threshold,[Frst|Rst],[F|SmallRest]) :-
Frst < Threshold,
filterLessThan(Threshold,Rst,SmallRest).
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nth(0,[F|_],F). nth(I,[_|R],Answer) :- Iless1 is I-1, nth(Iless1,R,Answer). |
- briefly show a tracing of the search (w/o worrying 'bout the search alg)
- talk/reflect about how you can view parameters as input or output params.
- facts *are* just rules w/o variables.
If time:
- singleton variables; using '_' for 'don't-care'
- an example on processing a list: I do some, they do some:
removeOnce( Target, Data, Result ) (search)
contains(Target, Data) (search)
isAllEvens(Data) (search)
filterEvens(Data,Result) (filter)
squareEach(Data,Result) (map)
sum(Data,Total) (fold)
- talk/reflect about matching of sub-lists, in variables. Write 'third'.
(how cool is that?!
Cool enough that other languages copied it: e.g. http://www.radford.edu/itec380/2009fall-ibarland/Lectures/matching.ss )
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| ©2009, Ian Barland, Radford University Last modified 2009.Nov.06 (Fri) |
Please mail any suggestions (incl. typos, broken links) to iba�rland  rad�ford.edu |
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