Python中的递归函数用于搜索列表中的项目[关闭]

Question

我正在尝试/需要编写一个递归函数，如果元素在排序列表中，则返回 True/False 值。我必须不使用“in”关键字来获得答案。我的代码在下面，我想我很接近，但我似乎无法让函数返回正确的值：

def listSplitter(listToProcess, numToFind):
    lenOfList = len(listToProcess)

    if lenOfList == 1:
        if numToFind == listToProcess[0]:
            return True
        else:
            return False
    else:

        if lenOfList % 2 == 0:
            list1 = listToProcess[0:int(lenOfList / 2)]
            list2 = listToProcess[int(lenOfList / 2):int(lenOfList)]

            if list1[-1] >= numToFind and list1[0] <= numToFind:
                return (list1, numToFind)
                #return
            elif list2[-1] >= numToFind and list2[0] <= numToFind:
                return listSplitter(list2, numToFind)
                #return
        else:
            list1 = listToProcess[0:int(lenOfList / 2)]
            list2 = listToProcess[int(lenOfList / 2):int(lenOfList)]

            if list1[-1] >= numToFind and list1[0] <= numToFind:
                return listSplitter(list1, numToFind)
                #return
            elif list2[-1] <= numToFind and list1[0] >= numToFind:
                return listSplitter(list2, numToFind)
                #return



def ordered_contains(S, x):
    return listSplitter(S,x)
    #result = listSplitter(S, x)
    #return result



A = [2, 16, 26, 32, 52, 71, 80, 88]

print("A contains 32: {}".format(ordered_contains(A, 32)))
print("A contains 7: {}".format(ordered_contains(A, 7)))
print("A contains -10: {}".format(ordered_contains(A, -10)))
print("\n(Did those results match the earlier example?)")

这是输出：

A contains 32: ([2, 16, 26, 32], 32)
A contains 7: ([2, 16, 26, 32], 7)
A contains -10: None

(Did those results match the earlier example?)

我错过了什么？

任何帮助将不胜感激。

谢谢

Answer 1

看来你在第 16 行犯了一个错误：

不应该是return (list1, numToFind)。

应该是return listSplitter(list1, numToFind)。

另外，不需要if lenOfList % 2 == 0:。列表的长度是偶数还是奇数不会改变你的逻辑。

完整的代码如下所示

def listSplitter(listToProcess, numToFind): lenOfList = len(listToProcess)

if lenOfList == 1:
    if numToFind == listToProcess[0]:
        return True
    else:
        return False
else:

    list1 = listToProcess[0:int(lenOfList / 2)]
    list2 = listToProcess[int(lenOfList / 2):int(lenOfList)]

    if list1[-1] >= numToFind and list1[0] <= numToFind:
        return listSplitter(list1, numToFind)
    elif list2[-1] >= numToFind and list2[0] <= numToFind:
        return listSplitter(list2, numToFind)
    else:
        return False   

Answer 2

虽然可以纠正错字，但我尝试以另一种方式实现相同的效果。我通过递归模拟了一个while循环。看起来简单多了。

In [0]: def traverse(index, array, toCompare):
   ...:     if (array[index] == toCompare):
   ...:         print(f"Found {toCompare} at {index + 1} position of the list")
   ...:         return # You can return a boolean True value and the index here and print it outside.
   ...:     if (index != len(array) - 1):
   ...:         traverse(index + 1, array, toCompare)
   ...:     else:
   ...:         print(f"Cannot find {toCompare} in list")
   ...:         return # You can return a boolean False value here and print it outside.
   ...:

In [1]: traverse(0, a, 77)
Cannot find 77 in list

In [2]: traverse(0, a, 88)
Found 88 at 8 position of the list

In [3]: a
Out[1]: [2, 16, 26, 32, 52, 71, 80, 88]

只是想我会分享这个，因为它以更简单的方式实现了同样的效果。

Answer 3

你的代码有一些问题：

1. 错别字：
   • 第 16 行：return (list1, numToFind) -> return listSplitter(list1, numToFind)
   • 第 28 行：elif list2[-1] <= numToFind and list1[0] >= numToFind: -> elif list2[-1] >= numToFind and list2[0] <= numToFind:（如果这不是错字，那么我认为它在逻辑上不正确）
2. 如果你更正上面的错别字，你会看到if lenOfList% 2 == 0:和else两种情况下的代码是一样的。因此，没有必要将这两种情况分开。
3. 的逻辑

   list1 = listToProcess[0:int(lenOfList / 2)]
   list2 = listToProcess[int(lenOfList / 2):int(lenOfList)]
   
   if list1[-1] >= numToFind and list1[0] <= numToFind:
       return listSplitter(list1, numToFind)
   elif list2[-1] >= numToFind and list2[0] <= numToFind:
       return listSplitter(list2, numToFind)
   

   将错过list1 和list2 有1 个元素并且它们都与numToFind 不同的情况。这就是为什么在某些情况下返回是None。要处理这种情况，只需在上面代码的末尾添加return False。

这是我纠正上述问题后的代码：

def listSplitter(listToProcess, numToFind):
    lenOfList = len(listToProcess)

    if lenOfList == 1:
        return numToFind == listToProcess[0]

    list1 = listToProcess[0:int(lenOfList / 2)]
    list2 = listToProcess[int(lenOfList / 2):int(lenOfList)]

    if list1[-1] >= numToFind and list1[0] <= numToFind:
        return listSplitter(list1, numToFind)
    elif list2[-1] >= numToFind and list2[0] <= numToFind:
        return listSplitter(list2, numToFind)
    
    return False


def ordered_contains(S, x):
    return listSplitter(S,x)


A = [2, 16, 26, 32, 52, 71, 80, 88]

print("A contains 32: {}".format(ordered_contains(A, 32)))
print("A contains 7: {}".format(ordered_contains(A, 7)))
print("A contains -10: {}".format(ordered_contains(A, -10)))
print("\n(Did those results match the earlier example?)")

输出：

A contains 32: True
A contains 7: False
A contains -10: False

(Did those results match the earlier example?)