计算List中某原子的个数---countif谓词

countif(+List, ?Atom, ?Count)

计算List中原子Atom的个数Count。

代码:

countif(List,Atom,Count):- (var(Count) -> ! ; true), bagof(I,nth1(I,List,Atom),R), length(R,Count).
countif(List,Atom,Count):- Count = 0.

例：

?- countif([atm, 123, atm, aabbcc], atm, Num).
传回 Num = 2.

?- countif([atm, 123, atm, aabbcc], atmzzz, Num).
传回 Num = 0.

?- countif([atm, 123, atm, aabbcc], String, 1).
传回
String = 123 ;
String = aabbcc ;
false.

?- countif([atm, 123, atm, aabbcc], String, Num).
传回
Str = 123,
Num = 1 ;
Str = aabbcc,
Num = 1 ;
Str = atm,
Num = 2.


适用的Prolog版本：SWI-Prolog