CODEWITHSUNDEEP
prolog
JmRag
JmRag

Reputation: 1459

prolog generate list of numbers from a list[x,y]

Hello I want to generate a list as following. Given a list like [x,y] I want to generate a list that is x,x,...,x : y times eg [2,3]=[2,2,2] but I cannot figure out how.

This is my implementation so far:

generate([T,1],[T]).
generate([X,S],[X|T]):-S1 is S-1,generate([X,S1],[T]).

but for some reason it fails. Can you help me?

Upvotes: 2

Views: 690

Answers (2)

false
false

Reputation: 10102

generate([E,R], Es) :-
   length(Es, R),
   maplist(=(E), Es).

You said that your version fails. But in fact it does not:

?- generate([a,0], Xs).
   false.
?- generate([a,1], Xs).
   Xs = [a]
;  false.
?- generate([a,2], Xs).
   Xs = [a|a]
;  false.
?- generate([a,3], Xs).
   false.

It doesn't work for 0, seems to work for length 1, then, produces an incorrect solution Xs = [a|a] for length 2, and finally fails from length 3 on. [a|a] is a good hint that at someplace in your definition, lists and their elements are confused. To better distinguish them, use a variable in plural for a list, like Es which is the plural of E.

Upvotes: 2

Steven
Steven

Reputation: 2477

The problem is in your second clause. When you have [X|T], it means that T is a list. In the body you write generate([X,S1],[T]): by writing [T] you're now saying the second argument to generate is a list of which the only element is this list T. What you want to say is that it is simply this list T:

generate([T,1], [T]).
generate([X,S], [X|T]) :- S1 is S-1, generate([X,S1], T).

Upvotes: 0

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