I am trying to implement the inplace quick sort as explained in the http://en.wikipedia.org/wiki/Quicksort
Below is the python code, The partition function does not work as expected.
def swap(array, index1, index2):
tmp = array[index1]
array[index1] = array[index2]
array[index2] = tmp
def partition(array, left, right, pivotIndex):
pivotValue = array[pivotIndex]
swap(array, pivotIndex, right)
storeIndex = left
for i in range(left, right - 1):
if array[i] < pivotValue:
swap(array, i, storeIndex)
storeIndex = storeIndex + 1
print array, i
swap(array, storeIndex, right)
return storeIndex
def quicksort(array, left ,right):
if right > left:
print left, right
pivotIndex = left
pivotNewIndex = partition(array, left, right, pivotIndex)
quicksort(array, left, pivotNewIndex - 1)
quicksort(array, pivotNewIndex + 1, right)
if __name__ == '__main__':
array = [3,7,8,5,2,1,9,5,4]
partition(array, 0, len(array) - 1, 3) # 5 is pivot
print array # expecting all the elements to the left of pivot value(5) will be lesser or equal.
You need to make at least 2 fixes:
def partition(array, left, right, pivotIndex):
pivotValue = array[pivotIndex]
swap(array, pivotIndex, right)
storeIndex = left
for i in range(left, right): # range doesn't include right element already
if array[i] <= pivotValue: # need to check for equality (not really necessary for the sorting routine)
swap(array, i, storeIndex)
storeIndex = storeIndex + 1
print array, i
swap(array, storeIndex, right)
return storeIndex
range(left, right)
returns a list of items like this [left, left + 1, ..., right - 1]
, so there is no need to do generate a list with range(left, right -1)
, because thus we shall skip not only the last element of the list (where pivot is), but also the one before last (ie right - 2
).
If it is expected that after partition
the elements on left from pivot should be less than or equal , we should reflect it in the comparison during the array traversal ( array[i] <= pivotValue
).
is it not better way using less variables and clean approach
#!/usr/bin/python
Array = [1,2,3,4,5,4,3,23,4,5,4,3,2,1,2,3,4,3,4,1,412,2,4]
def swap(a,i,j):
temp=a[i]
a[i]=a[j]
a[j]=temp
def partition(a, left, right):
pivotIndex=right
for i in range(left,right):
if a[i] > a[pivotIndex]:
swap(a,i,pivotIndex)
return pivotIndex
def quicksort(array , left,right):
if left<right:
pivotIndex=partition(array,left,right)
quicksort(array, left, pivotIndex-1)
quicksort(array, pivotIndex+1, right)
def main():
quicksort(Array, 0 , len(Array) -1)
print (Array )
main()
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