list - Prolog:计算列表中的正元素

Question

我想计算列表中的正元素(VIsual Prolog)。所以我写了这个函数:

positiveCount([], C).
positiveCount([A], C) :- A > 0, C = C + 1.
positiveCount([H|T], C) :- H > 0,!,C = C+1,positiveCount(T,C); positiveCount(T,C).

错误:

The flow pattern '(o,i)' does not exist for '+' main.pro

据我从这个错误中了解到，我不能将 C 的 C=C+1 用作输入变量。

有什么办法可以修复我的代码吗？

Answer 1

以下代码使用clpfd在 swi-prolog ，所以不要期望它在 visual-prolog 上按原样运行:-(
不过，我希望它对你有用!

:- use_module(library(clpfd)).count_pos([], 0).count_pos([E|Es], C) :- E #=< 0,            count_pos(Es, C).count_pos([E|Es], C) :- E #>  0, C #= C0+1, count_pos(Es, C0).

Let's read the clauses in plain English in the direction of the "arrow" :-, that is "right to left".

1. count_pos([], 0).

   The number of positive arithmetic expressions contained in the empty list [] is zero.

2. count_pos([E|Es], C) :- E #=< 0, count_pos(Es, C).

   If list Es contains C positive arithmetic expressions
   and if some arithmetic expression E is not positive
   then conclude that [E|Es] also contains C positive arithmetic expressions.

3. count_pos([E|Es], C) :- E #> 0, C #= C0+1, count_pos(Es, C0).

   If list Es contains C0 positive arithmetic expressions
   and if some arithmetic expression E is positive
   then conclude that [E|Es] also contains C0+1 positive arithmetic expressions.

Sample query:

?- count_pos([1,2,3,0,-1,-2], C).
   C = 3
;  false.