问题描述

现在，我已经为 Python(2.7 版)编写了二进制搜索.有时，它工作得很好，但有时它返回 None，尽管搜索的值在数组中.我已经尝试了每一种简单的方法来解决这个问题:我检查了函数返回的变量是否已定义，是否执行了 return 语句所在的工作流分支.并且:变量被定义，分支被执行.

Now, I have written a binary search for Python (version 2.7). Sometimes, it works just fine, but at other times it returns None although the searched value is in the array. I have tried every trivial way of fixing this: I have checked whether the variable the function returns is defined, whether the branch of the workflow in which the return statement is located is executed. And: the variable is defined, the branch is executed.

代码如下:

def binarySearch( array, desiderata, iMin, iMax ):
# Returns the index of the first instance of what we search
print 'min'
print iMin
print 'max'
print iMax

# If our search array is empty
if ( iMin > iMax ):
    return None

midP = (iMin + iMax)/2
curre = tapeNr( array[midP][TAPE_NUMBER] )
final = tapeNr( desiderata )
print 'curre'
print curre
print 'final'
print final
print 'midP'
print midP

if ( curre < final ):
    # print midP
    print "t: " + array[midP][TAPE_NUMBER] + ", d: " + desiderata
    binarySearch( array, desiderata, midP + 1, iMax )

else:
    if ( curre > final ):
        # print midP
        print "t: " + array[midP][TAPE_NUMBER] + ", d: " + desiderata
        binarySearch( array, desiderata, iMin, midP - 1 )

    else: 
        print 'hooray'
        # Now, find the first occurence of the value I need
        i = midP
        while ( array[i][TAPE_NUMBER] == desiderata ):
            i -= 1
            print i
        print (i + 1)
        return (i + 1)

由于我的调试，有很多打印"语句.最后一个，'print (i+1)'，实际上打印 (!) 我需要的东西的索引值，但函数仍然返回 None.

There are a lot of 'print' statements because of my debugging. The last of them, 'print (i+1)', actually prints (!) the index value of the thing I need, but the function still returns None.

你知道问题的根源吗?

推荐答案

你忽略了递归调用的返回值:

You ignore the return value of recursive calls:

binarySearch( array, desiderata, midP + 1, iMax )

和

binarySearch( array, desiderata, iMin, midP - 1 )

所以当 curre <最终是真:

if ( curre < final ):
    # print midP
    print "t: " + array[midP][TAPE_NUMBER] + ", d: " + desiderata
    binarySearch( array, desiderata, midP + 1, iMax )

你调用 binarySearch() 之后你的函数结束.如果没有显式返回，则意味着您的函数返回值设置为 None.

you call binarySearch() after which your function ends. Without an explicit return that means your function return value is set to None instead.

在这些行中添加 return 语句:

Add return statements to those lines:

return binarySearch( array, desiderata, midP + 1, iMax )

# ...

return binarySearch( array, desiderata, iMin, midP - 1 )

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