检查Cons列表中是否存在Cons

Question

我正在构建一个程序和lisp，需要检查cons是否存在于Cons列表中，但由于某种原因，它在if语句中总是返回nil，以下是我正在使用的当前代码。

(defun countVertexTriangles (graph numOfVertices)
    (findTriangle graph numOfVertices)
)

(defun findTriangle(graph numOfVertices)
    (loop for (x y) in graph do
            (loop for z from 1 to numOfVertices do
                (write graph)
                (terpri)
                (write (cons z (cons y nil)))
                (terpri)
                (write (cons z (cons x nil)))
                (terpri)
                ; (if (AND (member (cons z (cons y nil)) graph) (member (cons z (cons x nil)) graph))
                ;     then (write (cons y z))
                ; )
            )
        (terpri)
    )
)

; (defun findEdge(graph edge)
;     (loop for x in graph do
;         (write x)
;         (write edge)
;         (if (eql x edge)
;             (write "A")
;             (write "B")
;         )
;     )
; )

(defun testFunct ()
    (setf g1 '((1 2)(2 3)(1 3)(2 4)(3 4)(4 5)(3 5)))
    (countVertexTriangles g1 5)
)

(testFunct)

为什么成员(cons z(Cons Y nil))返回nil，即使在第一次迭代中我们可以看到(1，2)存在于列表中？

编辑：

目前，即使它是真的，它也会返回nil，为什么在给定以下代码时会出现这种情况？

(defun countVertexTriangles (graph numOfVertices)
    (findTriangle graph numOfVertices)
)

(defun findTriangle(graph numOfVertices)
    (loop for (x y) in graph do
            (loop for z from 1 to numOfVertices do
                ; (write graph)
                ; (terpri)
                ; (write (list z y ))
                ; (terpri)
                (write (findEdge graph (list z y)))
                (terpri)
                ; (if (AND (member (list z x) graph) (member (list z x ) graph))
                ;     then (write "TEST")
                ; )
            )
        (terpri)
    )
)

(defun findEdge(graph edge)
    (loop for x in graph do
        (if (equal x edge)
            (return-true)
            (return-false)
        )
    )
)

(defun return-true ()
   t)

(defun return-false ()
   nil)

(defun testFunct ()
    (setf g1 '((1 2)(2 3)(1 3)(2 4)(3 4)(4 5)(3 5)))
    (countVertexTriangles g1 5)
)

Answer 1

findEdge不返回任何内容。调用return-true和return-false不会从findEdge返回。在找到匹配项之前，您也不应该返回。

(defun findEdge(graph edge)
    (loop for x in graph do
        (if (equal x edge)
            (return-from findEdge (return-true))
        )
    )
    (return-false)
)

这可以简化为：

(defun findEdge (graph edge) 
  (member edge findEdge :test #'equal))