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

正如我从这个错误中了解到的，我不能将 C=C+1 用于 C 作为输入变量。

有什么办法可以修复我的代码吗？

Answer 1

以下代码使用clpfd on swi-prolog, so don't expect it to run as-is on visual-prolog:-(
不过，希望对你有用！

:- 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).

让我们看一下在"arrow":-方向的通俗易懂的从句，即"right to left".

count_pos([], 0).

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

示例查询：

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

Prolog：计算列表中的正元素

Prolog: Count positive elems in list

list

prolog

clpfd

visual-prolog