Question

我在弄清楚为什么我的代码不起作用时遇到了一些麻烦，并且如果有人能指出我缺少的东西，我会很感激。这是一个基本的算法问题：给定一组不同的排序整数，确定是否存在a [i] = i（例如a [3] = 3）的元素。

我尝试使用print语句调试它，但它只调用FindIndex而不是递归。

以下是代码：

import math

def FindIndex(SetToSearch, beginningIndex, endingIndex):
    """Searches a list of sorted integers to see if there is some a[i] == i

    Keyword Arguments:
    SetToSearch -- a list of disctinct sorted integers 
    beginningIndex -- start point of index to search
    endingIndex -- end point to search """
    # calculate midpoint of set
    midpointIndex = math.ceil((beginningIndex + endingIndex) / 2)
    midpoint = SetToSearch[int(midpointIndex)]
    print "beginningIndex: %s, endingIndex: %s" %(beginningIndex,endingIndex)
    print "midpointIndex: %s, midpoint: %s" % (midpointIndex, midpoint)
    # check whether ending index is greater then beginning index
    if (endingIndex > beginningIndex):
        return "There is no value in this set such that a[i] = i"
    if (endingIndex == beginningIndex):
        if SetToSearch[beginningIndex] == SetToSearch[endingIndex]:
            return "a[%s] is equal to %s" % [beginningIndex, beginningIndex]
    if (midpoint == midpointIndex):
        return "The value at index %d" % midpointIndex
    if (midpoint > midpointIndex):
        print "midpoint > midpointIndex evaluated to true and executed this"
        return FindIndex(SetToSearch, 0, midpointIndex)
    if (midpoint < midpointIndex):
        print "midpoint < midpointIndex evaluated to true and executed this"
        return FindIndex(SetToSearch, midpointIndex + 1, len(SetToSearch) -1)
    else:
        "Something is wrong with your program, because you should never see this!"

sampleSet = [-10, -8, -6, -5, -3, 1, 2, 3, 4, 9 ]
lastIndex = len(sampleSet) - 1

FindIndex(sampleSet,0,lastIndex)

Answer 1

问题不在于递归。只是您的第一个条件始终为真：endingIndex始终大于beginningIndex。该条件在没有递归的情况下返回，因此函数在那里结束。

Answer 2

您可以使用for循环来执行此操作，如：

def find_index_equal_value(set):
    for i in range(0, len(set)):
        val = set[i]
        if(val == i):
           print "Index matches value."

Answer 3

首先，您必须将return添加到第48行的开头。
其次，将print添加到最后一行的开头。

Answer 4

首先，如果你看不到发生了什么，那是因为你需要打印返回的字符串：

print FindIndex(sampleSet,0,lastIndex)

现在，如果我运行它，我得到：

beginningIndex: 0, endingIndex: 9
midpointIndex: 4.0, midpoint: -3
There is no value in this set such that a[i] = i

表示此if匹配：

# check whether ending index is greater then beginning index
if (endingIndex > beginningIndex):
    return "There is no value in this set such that a[i] = i"

......好吧，当然它确实 - endingIndex 应该总是大于beginningIndex！

为了将来参考，你打印了字符串吗？您是否看到输出线而不理解为什么需要该分支？您是否尝试使用pdb单步执行代码？

基本的Python递归麻烦

4 个答案: