使用最后一个元素作为枢轴在python中快速排序

Question

I have successfully tested my code.It works with last element as pivot. However, when i try to count the total no. of comparisons made, it shows incorrect count. I am counting through the global variable tot_comparisons .

Suggestions,where am i going wrong ? Is there some silly error that i am making?

def swap(A,i,k):
    temp=A[i]
    print "temp is "
    print temp
    A[i]=A[k]
    A[k]=temp

def partition(A,start,end): 
    pivot=A[end]   
    pivot_index=start
    #pivot=A[end]


    for i in range(start,end):   

        #if A[i]<=pivot:
        if A[i]<pivot:
            print 'HHHHHHHHHHHHHhh'
            swap(A,i,pivot_index)
            pivot_index+=1
    #swap(A,pivot_index,end)
    swap(A,pivot_index,end)
    return pivot_index

def quicksort(A,start,end):
    global tot_comparisons
    if start<end:

        pivot_index=partition(A,start,end)
        tot_comparisons+=end-start
        print "pivot_index"
        print pivot_index
        print "ENDS"


        quicksort(A, start,pivot_index-1)

        #tot_comparisons+=end-pivot_index
        #quicksort(A, pivot_index, end)

        quicksort(A, pivot_index+1, end)

#A=[45,21,23,4,65]

#A=[21,23,19,22,1,3,7,88,110]
#A=[1,22,3,4,66,7]

#A=[1, 3, 7, 19, 21, 22, 23, 88, 110]
#A=[7,2,1,6,8,5,3,4]

temp_list=[]
f=open('temp_list.txt','r')
for line in f:
    temp_list.append(int(line.strip()))
f.close()
print 'list is '
#print temp_list
print 'list ends'

tot_comparisons=0
#quicksort(A, 0, 7)
quicksort(temp_list, 0, 9999)
#quicksort(temp_list, 0, len(temp_list))
print 'hhh'
print temp_list
print tot_comparisons
#print A

Answer 1

I checked that your quicksort works, though it's slightly different from the algorithm given in popular algorithmic texts, in which the last element is switched to the first and then the partitioning ensues. This may change the ordering of the sort which has an effect on the number of comparisons.

For example, your code:

def partition(A,start,end):
    pivot=A[end]
    pivot_index=start
    for i in range(start,end):
        if A[i] < pivot:
            swap(A,i,pivot_index)
            pivot_index+=1
    swap(A,pivot_index,end)
    return pivot_index

can be switched to:

def partition(A,start,end):
    swap(A,start,end)
    pivot=A[start]
    pivot_index=start + 1
    for i in range(start+1,end+1):
        if A[i] < pivot:
            swap(A,i,pivot_index)
            pivot_index+=1
    swap(A,pivot_index-1,start)
    return pivot_index-1

Edited based on comment by OP.

Answer 2

I would suggest you to declare global variable at the top of your script, check this working basic example:

inside = 0
def qsort(l):
    global inside
    inside += 1
    if len(l) <= 1:
        return l
    pivot = l[-1]
    return qsort(filter(lambda x: x < pivot, l[:-1])) + [pivot] + qsort(filter(lambda x: x >= pivot, l[:-1]))

import random
l = [random.randint(0,100) for _ in xrange(100)]
print qsort(l)
print inside

>>> [1, 1, 2, 3, 7, 9, 10, 11, 11, 11, 13, 13, 13, 13, 17, 17, 17, 18, 18, 21, 22, 23, 26, 26, 28, 30, 30, 32, 32, 34, 35, 38, 40, 41, 42, 42, 42, 42, 43, 44, 45, 46, 47, 47, 48, 48, 49, 51, 51, 54, 55, 56, 56, 56, 58, 59, 60, 60, 61, 61, 62, 63, 65, 67, 67, 68, 68, 70, 70, 72, 73, 74, 77, 79, 80, 83, 85, 85, 85, 86, 87, 89, 90, 90, 90, 91, 91, 95, 96, 96, 96, 97, 97, 97, 98, 98, 98, 99, 99, 99]
>>> 135