Aim:
To write a Python Program to perform Merge sort.
Algorithm:
1. Create a function named mergesort
2. Find the mid of the list
3. Assign lefthalf = alist[:mid] and righthalf = alist[mid:]
4. Initialise i=j=k=0
5. while i < len(lefthalf) and j < len(righthalf), perform the following
if lefthalf[i] < righthalf[j]:
alist[k]=lefthalf[i]
Increment i
else
alist[k]=righthalf[j]
Increment j
Increment k
6. while i < len(lefthalf),perform the following
alist[k]=lefthalf[i]
Increment i
Increment k
7. while j < len(righthalf), perform the following
alist[k]=righthalf[j]
Increment j
Increment k
8. Print the sorted list
Program:
# Python program for implementation of MergeSort
# Merges two subarrays of arr[].
# First subarray is arr[l..m]
# Second subarray is arr[m+1..r]
def merge(arr, l, m, r):
n1 = m - l + 1
n2 = r- m
# create temp arrays
L = [0] * (n1)
R = [0] * (n2)
# Copy data to temp arrays L[] and R[]
for i in range(0 , n1):
L[i] = arr[l + i]
for j in range(0 , n2):
R[j] = arr[m + 1 + j]
# Merge the temp arrays back into arr[l..r]
i = 0 # Initial index of first subarray
j = 0 # Initial index of second subarray
k = l # Initial index of merged subarray
while i < n1 and j < n2 :
if L[i] <= R[j]:
arr[k] = L[i]
i += 1
else:
arr[k] = R[j]
j += 1
k += 1
# Copy the remaining elements of L[], if there
# are any
while i < n1:
arr[k] = L[i]
i += 1
k += 1
# Copy the remaining elements of R[], if there
# are any
while j < n2:
arr[k] = R[j]
j += 1
k += 1
# l is for left index and r is right index of the
# sub-array of arr to be sorted
def mergeSort(arr,l,r):
if l < r:
# Same as (l+r)/2, but avoids overflow for
# large l and h
m = (l+(r-1))/2
# Sort first and second halves
mergeSort(arr, l, m)
mergeSort(arr, m+1, r)
merge(arr, l, m, r)
# Driver code to test above
arr = [12, 11, 13, 5, 6, 7]
n = len(arr)
print ("Given array is")
for i in range(n):
print ("%d" %arr[i]),
mergeSort(arr,0,n-1)
print ("\n\nSorted array is")
for i in range(n):
print ("%d" %arr[i]),
Sample Output:
$python main.py Given array is 12 11 13 5 6 7 Sorted array is 5 6 7 11 12 13