问题描述
我想在Racket中实现此功能。如何在球拍中重写此功能?
我在PYTHON中的代码
# function to check transitive relation
def is_transitive(relation):
# for all (a,b) and (b,c) in Relation ; (a,c) must belong to Relation
for a,b in relation:
for c,d in relation:
if b == c and ((a,d) not in relation):
return False
return True
transitive?
将对列表作为唯一输入。该对列表表示二进制关系。如果该二进制关系是可传递的,则该函数应返回#t
,如下所示。
> (transitive? '((1 2) (2 3) (1 3)))
#t
> (transitive? '((1 3) (1 2) (2 3)))
#t
> (transitive? '((1 2) (2 3)))
#f
> (transitive? '((1 1) (1 2) (2 1) (2 2) (3 3)))
#t
> (transitive? '((1 2) (3 3) (2 2) (2 1) (1 1)))
#t
> (transitive? '((2 3) (3 3) (1 2) (1 1)))
#f
到目前为止,我在球拍中的位置是这样:
(define (get-all-relations-of x set)
(if (null? set)
'()
(if (equal? x (car (car set)))
(cons (cdr (car set))
(get-all-relations-of x (cdr set)))
(get-all-relations-of x (cdr set)))))
(define (exist-relation? r set)
(if (null? set)
#f
(if (equal? r (car set))
#t
(exist-relation? r (cdr set)))))
(define (exist-all-transitive-relations-of? x r set)
(if (null? r)
#t
(if (not (exist-relation? (cons x (car r)) set))
#f
(exist-all-transitive-relations-of? x (cdr r) set))))
(define (transitive? set)
(if (null? set)
#t
(if (and (not (null? (get-all-relations-of (cdr (car set)) set)))
(not (exist-all-transitive-relations-of?
(car (car set))
(get-all-relations-of (cdr (car set)) set)
set)))
#f
(transitive? (cdr set)))))
仅当(exist-all-transitive-relations-of?of?x r set)为true时有效。
这是我的输出:
>(exist-all-transitive-relations-of? 1 '(2 5 6) '((1 2) (1 5) (6 8)))))
> #t
>(define (transitive? '((1 2) (2 6)))
> #f
>(define (transitive? '((1 2) (2 6)(2 7)(1 7)))
> #f
>(define (transitive? '((1 2) (2 6)(2 7)(1 7)(1 6)))
> #t
如何修改我的代码?这样我就不必单独测试(定义(exist-all-transitive-relations-of?x r set)是否为真
解决方法
这里只有一些关于球拍的评论:
操作时:
(if (condition) #f (else))
您可以使用短路将其转换为
(and (condition) (else))
类似
(if (condition) #t (else))
做
(or (condition) (else))
最后,在考虑替换python中的循环时,考虑现有的列表抽象非常重要。在这种情况下,您将获取一个列表并将其压缩为一条数据,特别是如果该属性针对一个成员而损坏,则为布尔值。因此,您应该使用andmap
,这将大大减少您的代码:
(define (transitive? set)
(andmap (lambda (x) (andmap (lambda (y)
(local [(define a (first x))
(define b (second x))
(define c (first y))
(define d (second y))]
(not (and (= b c) (not (member `(,a,d) set)))))) set)) set))
注意:如果需要,(,d)
可以替换为(list a d)
(无准引用)。还有first
和second
以及car
和cadr
。