删除Prolog列表中的前导零 [英] Remove leading zeros in list in Prolog

问题描述

我有一个列表，列表开头有一个未知的零，例如[0，0，0，1，2，0，3].我需要将此列表除去前导零，以便看起来像[1、2、0、3].

I have a list with an unknown number of zeros at the beginning of it, for example [0, 0, 0, 1, 2, 0, 3]. I need this list to be stripped of leading zeros, so that it would look like [1, 2, 0 , 3].

这就是我所拥有的:

lead([Head | _], _) :- Head =\= 0.
lead([0 | Tail], _) :- 
  lead(Tail, Tail).

其输出仅为True.读取跟踪表明它正在运行，直到它有一个没有前导零的列表，但是答案并没有传播回堆栈.我对Prolog还是很陌生，所以我不知道如何使它做到这一点.

The output of which is simply True. Reading the trace shows that it is running until it has a list with no leading zeros, but then the answer doesn't propagate back up the stack. I'm pretty new to Prolog, so I can't figure out how to make it do that.

推荐答案

以下是可在所有方向使用的解决方案:

Here is a solution that works in all directions:

lead([],[]).
lead([H|T],[H|T]) :-
    dif(H,0).
lead([0|T],T2) :-
    lead(T,T2).

一些查询:

?- lead([0,0,0,1,2,0,3], L).
L = [1, 2, 0, 3] ;
false.


?- lead(L, []).
L = [] ;
L = [0] ;
L = [0, 0] ;
L = [0, 0, 0] ;
...


?- lead(L0, L).
L0 = L, L = [] ;
L0 = L, L = [_G489|_G490],
dif(_G489, 0) ;
L0 = [0],
L = [] ;
L0 = [0, _G495|_G496],
L = [_G495|_G496],
dif(_G495, 0) ;
L0 = [0, 0],
L = [] ;
L0 = [0, 0, _G501|_G502],
L = [_G501|_G502],
dif(_G501, 0) ;
L0 = [0, 0, 0],
L = [] ;
...

编辑该谓词实际上不适用于例如lead(L0, [0,1,2]).

EDIT This predicate actually doesn't work for e.g. lead(L0, [0,1,2]).

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