序言 - 参数不够实例 [英] Prolog - Arguments are not sufficiently instantiated

问题描述

我写一个小程序，它计算列表中的很多元素是如何不是数字。
这里是我的code：

I am writing a little program which counts how many elements in a list are not numbers. Here is my code:

not_number([],0).
not_number([X|T],R):- 
    not(number(X)),
    R1 is R+1,  
    not_number(T,R1). 

not_number([_|Tail],Result):-
    not_number(Tail,Result).  

如果我执行code是这样的：

If I execute code like this :

?- not_number([1,2,3,5], R).

我得到了R = 0（因为它应该是）

I am getting that R = 0 (as it should be)

R = 0.

但是，如果我把一个角色列表：

But if I put a character in the list:

?- not_number([1,2,3,5,a], R).

然后我得到这个错误：

then I am getting this error:

ERROR: not_number/2: Arguments are not sufficiently instantiated
   Exception: (10) not_number([a], _G247) ? 

有人能解释什么是错用code？我是新来的序言。

Can someone explain whats wrong with code? I am new to prolog.

推荐答案

您的问题是，在这样的算术计算：

Your problem is that in arithmetic computation like this:

A是B

在右侧（B）一切都要已知的。没有变数存在。

everything on the right side (B) has to be already known. No variables there.

您可以做这样的事情：

not_number(X, Y) :- not_number(X, Y, 0).
not_number([], Y, Y).
not_number([H|T], Y, Z) :-
    \+ (number(H)), 
    Z1 is Z+1,
    not_number(T, Y, Z1).

not_number([H|T], Y, Z) :-
    number(H),
    not_number(T, Y, Z).

（本测试code现在，它的工作原理）。

(tested this code now, it works).

现在第三个参数是累加器。它计算多少不是号码有。当列表是空的，这第三个参数是统一的第二个，并且变得正确答案。

Now the third argument is an accumulator. It counts how many not-numbers there are. When the list is empty, this third argument is unified with the second one and it becomes the proper answer.

序言，有机会的时候，将通过所有可能的路由。如果你做这样的事情：

Prolog, when given the chance, will go through all the routes possible. If you do something like this:

cat(adam).
cat(eve).

，然后问：

?- cat(X).

您可以得到两个答案：X =亚当和X =前夜。
它适用于你的code太：注意，当列表的头是不是一个数字，你仍然可以做到这一点：

You could get both answers: X = adam and X = eve. It applies to your code too: notice that when the head of the list is not a number, you can still do this:

not_number([_|Tail],Result):-
    not_number(Tail,Result).  

这给你不想要的答案。你必须切断不感兴趣你的路由。在这种情况下，我想补充

which gives not the answer you would like. You have to cut off the routes that don't interest you. In this case, I would add

number(Head).

要确保我们跳过列表中的一个元素，而不由1增加计数器只在此元素不是一个数字。

to ensure that we skip an element in a list without increasing the counter by 1 only when this element is not a number.

要执行的Prolog找到一些其他的结果，必须preSS;键盘上的（就像这个亚当和夏娃的例子）。

To enforce Prolog to find some other results, you must press ";" on your keyboard (like in this adam and eve example).

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