Python全局变量在递归函数中不起作用

Question

I wrote a quicksort program with Python and my goal here is to calculate total comparisons used. I declared a global variable named thesum . When I use thesum in the partition function, I can calculate the thesum correctly. However, when I tried to calculate the sum in the recursive function it gave wrong answers. Here are what I did respectively:

Method 1:

Calculate the sum in partition function:

def partition(listToSort, start, end):                                                                                                                 
    global thesum          
    thesum = thesum+end-start          

In the partition algorithm I'm using, I need to add m-1 when partitioning a m-length array.

Method 2:

calculate the sum in the recursive function qsort:

def qsort(listTo, start, end):
    if start >= end :
        return                                                                                                                                         
    else:
        index = partition(listTo, start, end)
        qsort(listTo, start, index-1)
        global thesum
        thesum = thesum + index-1-start
        qsort(listTo, index+1, end)
        thesum = thesum + end-index-1

In this method, thesum is not initialized to 0 but the length of the original array minus 1.

Things you may also need to know:

The algorithm I'm implementing is a simple version of quicksort. I have a list and need to sort it with this program. I use a global variable to denote the total comparisons the algorithm need to perform.

The problem and question

I think the two methods are equivalent but they gave different answers. After some testing by printing thesum , I found that this global variable didn't work as expected in the function qsort . for example when sorting a 10-elements array, thesum was initialized to 9 but later printed out as 8, which is strange. But why? I declare it as global in the function and it's used in the same way as in the function partition . All the difference I can think of is that qsort is a recursive function. But how does that make any difference? So global variables are not supposed to use in recursive functions?

Answer 1

这两种方法不执行等效操作。