60 lines
2.0 KiB
Perl
60 lines
2.0 KiB
Perl
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%% Exercises from chapter 4
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%%
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%% Exercise 4.3 Write a predicate second(X,List) which checks whether X is the
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%% second element of List .
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second(X, [_, X|_]).
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%% Exercise 4.4 Write a predicate swap12(List1,List2) which checks whether
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%% List1 is identical to List2 , except that the first two elements are
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%% exchanged.
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swap12([X,Y|T], [Y,X|T]).
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%% Exercise 4.5 Suppose we are given a knowledge base with the following
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%% facts:
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tran(eins,one).
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tran(zwei,two).
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tran(drei,three).
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tran(vier,four).
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tran(fuenf,five).
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tran(sechs,six).
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tran(sieben,seven).
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tran(acht,eight).
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tran(neun,nine).
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%% Write a predicate listtran(G,E) which translates a list of German number
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%% words to the corresponding list of English number words. For example:
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%% listtran([eins,neun,zwei],X).
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%% should give:
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%% X = [one,nine,two].
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%% Your program should also work in the other direction. For example, if you give it the query
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%% listtran(X,[one,seven,six,two]).
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%% it should return:
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%% X = [eins,sieben,sechs,zwei].
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%% (Hint: to answer this question, first ask yourself “How do I translate the
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%% empty list of number words?”. That’s the base case. For non-empty lists, first
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%% translate the head of the list, then use recursion to translate the tail.)
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listtran([], []).
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listtran([X|TX], [Y|TY]) :-
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tran(X, Y),
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listtran(TX, TY).
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%% Exercise 4.6 Write a predicate twice(In,Out) whose left argument is a list,
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%% and whose right argument is a list consisting of every element in the left
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%% list written twice. For example, the query
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%% twice([a,4,buggle],X).
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%% should return
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%% X = [a,a,4,4,buggle,buggle]).
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%% And the query
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%% twice([1,2,1,1],X).
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%% should return
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%% X = [1,1,2,2,1,1,1,1].
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%% (Hint: to answer this question, first ask yourself “What should happen when
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%% the first argument is the empty list?”. That’s the base case. For non-empty
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%% lists, think about what you should do with the head, and use recursion to
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%% handle the tail.)
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twice([], []).
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twice([X|T1], [X,X|T2]) :-
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twice(T1, T2).
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