LISP函数，给定一个数字和一个列表，返回大于n的第一个偶数

我找不到我的错误。 这会不断返回

nil

：

(even-greater-n 5 '(1 2 3 4 5 6 7))

(defun even-greater-n (n L)
   (cond ((null L) nil)
         ((and (> (car L) n) (evenp n)) (car L))
         (t (even-greater-n n (cdr L)))))

你的错误 您正在传递到

n

而不是

（汽车L）

迭代 这是相对容易实现的使用 :

你的错误 您正在传递到

n

而不是

（汽车L）

迭代 这是相对容易实现的使用 :

---

您必须查找

（左车）

是否为偶数。

您必须查找

（左车）

是否为偶数。

使用

find if

和单个打开编码的lambda函数：

(defun even-greater (n list)
  (find-if (lambda (item) (and (> item n) (evenp item))) list)) 

使用函数组合器：

;; Combine multiple functions with AND:
;; Returns a function of one-argument which
;; passes that argument to the functions in the list,
;; one by one. If any function returns nil, it stops
;; and returns nil. Otherwise it returns the value
;; returned by the last function:
(defun andf (&rest functions)                                               
  (lambda (arg)
    (let (res)
      (dolist (f functions res)
        (unless (setf res (funcall f arg))
          (return))))))

 ;; Returns a one-argument function which tests
 ;; whether its argument is greater than quant.
 (defun greater (quant)
   (lambda (arg) (> arg quant)))

 ;; "find it, if it is greater than n, and even"
 (defun even-greater (n list)                                                
   (find-if (andf (greater n) #'evenp) list))

---

使用

find if

和单个开放编码lambda函数：

(defun even-greater (n list)
  (find-if (lambda (item) (and (> item n) (evenp item))) list)) 

使用函数组合器：

;; Combine multiple functions with AND:
;; Returns a function of one-argument which
;; passes that argument to the functions in the list,
;; one by one. If any function returns nil, it stops
;; and returns nil. Otherwise it returns the value
;; returned by the last function:
(defun andf (&rest functions)                                               
  (lambda (arg)
    (let (res)
      (dolist (f functions res)
        (unless (setf res (funcall f arg))
          (return))))))

 ;; Returns a one-argument function which tests
 ;; whether its argument is greater than quant.
 (defun greater (quant)
   (lambda (arg) (> arg quant)))

 ;; "find it, if it is greater than n, and even"
 (defun even-greater (n list)                                                
   (find-if (andf (greater n) #'evenp) list))

---

这是因为你在检查

n

是否为偶数，而在你的测试中它是

5

并且永远不会偶数这是因为你在检查

n

是否为偶数，而在你的测试中它是

5

并且永远不会偶数。。

;; Combine multiple functions with AND:
;; Returns a function of one-argument which
;; passes that argument to the functions in the list,
;; one by one. If any function returns nil, it stops
;; and returns nil. Otherwise it returns the value
;; returned by the last function:
(defun andf (&rest functions)                                               
  (lambda (arg)
    (let (res)
      (dolist (f functions res)
        (unless (setf res (funcall f arg))
          (return))))))

 ;; Returns a one-argument function which tests
 ;; whether its argument is greater than quant.
 (defun greater (quant)
   (lambda (arg) (> arg quant)))

 ;; "find it, if it is greater than n, and even"
 (defun even-greater (n list)                                                
   (find-if (andf (greater n) #'evenp) list))