计算一个数字在列表中出现的次数 [英] Count the number of occurrences of a number in a list

问题描述

我正在用 prolog 编写一个程序，计算一个数字在列表中出现的次数

I'm writing a program in prolog that count the number of occurrences of a number in a list

count([],X,0).
count([X|T],X,Y):- count(T,X,Z), Y is 1+Z.
count([_|T],X,Z):- count(T,X,Z).

这是输出

?- count([2,23,3,45,23,44,-20],X,Y).
X = 2,
Y = 1 ;
X = 23,
Y = 2 ;
X = 23,
Y = 1 ;
X = 3,
Y = 1 ;
X = 45,
Y = 1 ;
X = 23,
Y = 1 ;
X = 44,
Y = 1 ;
X = -20,
Y = 1 ;
false.

同一个数数了几遍

感谢任何帮助

推荐答案

使用另一个变量 X1 代替虚拟变量 _ 并确保它不与 X 统一.

Instead of the dummy variable _ just use another variable X1 and ensure it does not unify with X.

count([],X,0).
count([X|T],X,Y):- count(T,X,Z), Y is 1+Z.
count([X1|T],X,Z):- X1=X,count(T,X,Z).

但是请注意，第二个参数 X 应该被实例化.所以例如count([2,23,3,45,23,44,-20],23,C) 将 C 与 2 统一.如果您想要每个元素的计数，请使用

However note that the second argument X is supposed to be instantiated. So e.g. count([2,23,3,45,23,44,-20],23,C) will unify C with 2. If you want the count for every element use

:- use_module(library(lists)).

count([],X,0).
count([X|T],X,Y):- count(T,X,Z), Y is 1+Z.
count([X1|T],X,Z):- X1=X,count(T,X,Z).

countall(List,X,C) :-
    sort(List,List1),
    member(X,List1),
    count(List,X,C).

然后你得到

 ?- countall([2,23,3,45,23,44,-20],X,Y).
   X = -20,
   Y = 1 ? ;
   X = 2,
   Y = 1 ? ;
   X = 3,
   Y = 1 ? ;
   X = 23,
   Y = 2 ? ;
   X = 44,
   Y = 1 ? ;
   X = 45,
   Y = 1 ? ;
   no

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