Lists&Member/2

一、List

1,定义

A list is a finite sequence of elements

Eg

[mia, vincent, jules, yolanda] [mia, robber(honeybunny), X, 2, mia] [ ] % a special list [mia, [vincent, jules], [butch, friend(butch)]] [[ ], dead(z), [2, [b,c]], [ ], Z, [2, [b,c]]]

2,特点

• List elements are enclosed in square brackets []

• The length of a list is the number of elements it has

•All sorts of Prolog terms can be elements of a list

二、Head and Tail

1，介绍

（1）A non-empty list can be thought of as consisting of two parts

– The head:The head is the first item in the list

– The tail :The tail is everything else

– The tail is the list that remains when we take the first element away

– The tail of a list is always a list

案例

[mia, vincent, jules, yolanda]
Head: mia Tail: [vincent, jules, yolanda]

[[ ], dead(z), [2, [b,c]], [ ], Z, [2, [b,c]]]
Head: [ ] Tail: [dead(z), [2, [b,c]], [ ], Z, [2, [b,c]]]

[dead(z)]
Head: dead(z) Tail: [ ]

（2）The empty list has neither a head nor a tail

•  For Prolog, [ ] is a special simple list without any internal structure

•  The empty list plays an important role in recursive predicates for list processing in Prolog

2，The built-in operator |

| which can be used to decompose a list into its head and tail

案例

3，Anonymous variable

The underscore is the anonymous variable

Is used when you need to use a variable, but you are not interested in what Prolog instantiates it to

三、member/2

1，member/2: a predicate that when given a term X and a list L, tells us whether or not X belongs to L

Rewriting member/2

member(X,[X|_]). member(X,[_|T]):- member(X,T).

2,

The member/2 predicate works by recursively working its way down a list
– doing something to the head, and then

– recursively doing the same thing to the tail

案例：

The predicate a2b/2 takes two lists as arguments and succeeds

– if the first argument is a list of a's, and

– the second argument is a list of b's of exactly the same length