Quick Sort Algorithm in Python
Quick sort algorithm was developed using the divide and conquer approach and it comes under comparison algorithm category.
This is also known as partition-exchange sort. This algorithm splits the array into two sub-arrays based on the pivot position and performs the sort operation recursively
till there is no more sub arrays exist. Quick sort is an easiest way of sorting technique, but it's not an efficient on large data sets.
Time Complexity
- Best complexity: n*log(n)
- Average complexity: n*log(n)
- Worst complexity: n^2
Quick Sort Implementation in Python
Python Input Screen
class QuickSortExample:
def __init__(self):
pass
def quickSort(self, arr, low, high):
if(low < high):
pivot = self.partition(arr, low, high)
self.quickSort(arr, low, pivot)
self.quickSort(arr, pivot + 1, high)
return arr
def partition(self, arr, low, high):
pivot = arr[low]
i = low
j = high
while(i < j):
while(arr[i] <= pivot and i < high):
i += 1
while(arr[j] > pivot and j > low):
j -= 1
if(i < j):
temp = arr[i]
arr[i] = arr[j]
arr[j] = temp
arr[low] = arr[j]
arr[j] = pivot
return j
def printArray(self, arr):
string = " ".join([str(item) for item in arr]);
print(string)
obj = QuickSortExample()
arr = [15, 3, 12, 6, -9, 9, 0]
print("Before Sorting: ")
obj.printArray(arr)
result = obj.quickSort(arr, 0, len(arr) - 1)
print("After Sorting: ")
obj.printArray(result)
Python Output Screen
Before Sorting:
15 3 12 6 -9 9 0
After Sorting:
-9 0 3 6 9 12 15
Quick sort implementation steps:
Let's take the input array as { 15, 3, 12, 6, -9, 9, 0 } and we can see how the Quick sort is sorting the array.
- Choose the lowest or highest index value as a pivot element, in our above example we chose the lowest index as a pivot element 15.
- Declare two variables (i.e. pointer) to store the lowest and highest index position except the pivot element, so the lowest index position is 1 points the value 3 and the highest index position is 6 which is pointing the value 0
- Move the left side pointer towards right until the value of the pointer is less than or equal to the pivot element.
- Move the right side pointer towards left until the value of the pointer is greater than the pivot element.
- Interchange the values of left side pointer and right side pointer.
- Interchange the values of leftmost index (i.e. position 0 has the pivot element 15) and right side pointer.
- The right side pointer is a new pivot position for the value 15
- Split the array into two sub-arrays with the help of pivot point
- Index from 0 to pivot
- Index from (pivot + 1) to (n - 1), where n is the length of an array
- Apply the step 1 to 8 until there is no more elements to split as an array.
Quick Sort Implementation in Python with explanation
Python Input Screen
class QuickSortExample:
def __init__(self):
pass
def quickSort(self, arr, low, high):
if(low < high):
# Find the pivot index of an lower bound of an array
pivot = self.partition(arr, low, high)
# Apply Divide and conquer stratagy
# to sort the left side and right side of an array
# respective to the pivot position
# Left hand side array
self.quickSort(arr, low, pivot)
#Right hand side array
self.quickSort(arr, pivot + 1, high)
return arr
def partition(self, arr, low, high):
# Pick a lowest bound element as an pivot value
pivot = arr[low]
i = low
j = high
while(i < j):
# Increment the index value (i.e. i )
# till the piovt value is greater than or equal the left most values
while(arr[i] <= pivot and i < high):
i += 1
# Increment the index value (i.e. j )
# till the piovt value is less than the right most values
while(arr[j] > pivot and j > low):
j -= 1
# Interchange the values of present
# in the index i and j of an array only if i is less than j
if(i < j):
temp = arr[i]
arr[i] = arr[j]
arr[j] = temp
# Interchange the element in j's poisition to the lower bound of an array
# and place the Pivot element to the j's position
arr[low] = arr[j]
arr[j] = pivot
return j
def printArray(self, arr):
# Loop the item and print each item in the console with white space seperated
string = " ".join([str(item) for item in arr])
print(string)
obj = QuickSortExample()
arr = [15, 3, 12, 6, -9, 9, 0]
print("Before Sorting: ")
obj.printArray(arr)
result = obj.quickSort(arr, 0, len(arr) - 1)
print("After Sorting: ")
obj.printArray(result)
Python Output Screen
Before Sorting:
15 3 12 6 -9 9 0
After Sorting:
-9 0 3 6 9 12 15
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