我如何完成这个递归语句？

Question

def search_sequence( seq, item ):
""" Search a sequence for the given item. PROVIDE AN IMPLEMENTATION (TASK 
    #2). This function should use **car** and **cdr**.

    :param seq: the sequence to be searched.
    :param item: the item to be searched
    :type seq: tuple
    :type item: str
    :returns: True if the item is contained in the sequence, False 
     otherwise.
    :rtype: bool
    """

这是我们需要实现的功能。 seq 和 item 来自这里的这些测试函数。

def test_search_sequence_0(self):
    """ Search empty tuple """
    sandwich = ()
    self.assertEqual( search_sequence( sandwich, 'ham' ), False)

def test_search_sequence_size_1_1(self):
    """ Search  single-element tuple: successful search"""
    sandwich = ('mustard',)
    self.assertEqual( search_sequence( sandwich, 'mustard' ), True)

def test_search_sequence_size_1_2(self):
    """ Search single-element tuple: unsuccessful search"""
    sandwich = ('mustard',)
    self.assertEqual( search_sequence( sandwich, 'ham' ), False)

def test_search_sequence_size_7_1(self):
    """ Search 7-element tuple: successful search"""
    sandwich = ("jelly","butter", "mustard", "bread", "pickles", "jam", 
"cheese")
    self.assertEqual( search_sequence( sandwich, 'pickles'), True)

def test_search_sequence_size_7_2(self):
    """ Search 7-element tuple: unsuccessful search"""
    sandwich = ("jelly","butter", "mustard", "bread", "pickles", "jam", 
"cheese")
    self.assertEqual( search_sequence( sandwich, 'pear'), False)

我们在顶部有多个起点。我们得到了 car(lst) 函数：

def car(lst):
""" The first of the 3 primitive functions: return the first element of a sequence. 

.. note:: The Law of Car: The `car` function is only defined for non-empty lists.

:param lst: a non-empty sequence; passing an empty sequence will raise an exception.
:type lst: tuple
:returns: an object
:rtype: object
"""
if type(lst) is not tuple: 
    raise WrongTypeArgumentException("Argument is not a list.")
if len(lst)==0:
    raise WrongTypeArgumentException("List has no element") 
if len(lst)>=1:
    return lst[0]

我们还得到了 cdr(lst) 函数：

def cdr( lst ):
""" The second of the 3 primitive functions: return a sequence, minus the first element.

.. note:: The Law of Cdr: The `cdr` function is only defined for non-empty lists; the `cdr` of any non-empty list is always another list.


:param lst: a non-empty sequence; passing an empty sequence will raise an exception.
:type lst: tuple
:returns: a tuple; if the sequence has only one element, return an empty sequence.
:rtype: tuple
"""
if type(lst) is not tuple:
    raise WrongTypeArgumentException("Argument is not a list.")
if len(lst)==0:
    raise WrongTypeArgumentException("Cannot cdr on an empty list.")
if len(lst)==1:
    return ()
return lst[1:]

最后，我们得到了 cons 函数：

def cons( a, lst):
""" The third of the 3 primitive functions: return the sequence created by adding element `a` to the sequence `lst`.

.. note:: The Law of Cons: the primitive `cons` takes two arguments; the second argument to `cons` must be a list; the result is a list.

:param a: an object
:param lst: a tuple
:type a: object
:type lst: tuple
:returns: the tuple resulting from adding parameter `a` in front of sequence `lst`.
:rtype: tuple
"""
if type(lst) is not tuple:
    raise WrongTypeArgumentException("Argument is not a list.")
return (a,) + lst

我试图修复代码的实现，但是，出于某种原因，我的代码尝试 returns 这个错误：

 Traceback (most recent call last):
  File "C:\Users\MacKenzy\Desktop\pylisp_skeleton.py", line 223, in test_search_sequence_size_1_2
    self.assertEqual( search_sequence( sandwich, 'ham' ), False)
  File "C:\Users\MacKenzy\Desktop\pylisp_skeleton.py", line 133, in search_sequence
    return search_sequence(cdr(seq, item))
TypeError: cdr() takes 1 positional argument but 2 were given

这是我的代码：

    if seq == ():
        return False
    elif item == car(seq):
        return True
    return search_sequence(cdr(seq, item))

你们能告诉我我做错了什么吗？或者如何解决？非常感谢！！！

Answer 1

你的cdr()只接受一个参数，列表：

def cdr( lst ):
""" The second of the 3 primitive functions: return a sequence, minus the first element.

.. note:: The Law of Cdr: The `cdr` function is only defined for non-empty lists; the `cdr` of any non-empty list is always another list.


:param lst: a non-empty sequence; passing an empty sequence will raise an exception.
:type lst: tuple
:returns: a tuple; if the sequence has only one element, return an empty sequence.
:rtype: tuple
"""

但是您向它传递了两个参数：

return search_sequence(cdr(seq, item))

您可能打算将 item 作为参数传递给 search_sequence() 而不是 cdr():

return search_sequence(cdr(seq), item)

我如何完成这个递归语句？

How do I finish this recursive statement?

python

recursion

function

cardinality

recursive-backtracking