Question

Show that mergesort is a stable algorithm by modifiying the algorithm to sort a list of tuples (first and last names) by their second position. (Modify the merge section to break ties) Modify in python

def mergesort(mlist):
if len(mlist) < 2:
return mlist
else:
mid = len(mlist)//2
return merge( mergesort(mlist[:mid]), mergesort(mlist[mid:]))

# merge two sorted lists
def merge(left, right):
if left == []:
return right
elif right == []:
return left
elif left[0] < right[0]:
return [left[0]] + merge(left[1:], right)
else:
return [right[0]] + merge(left, right[1:])

Answer 1

Python code :

The above python code in question can be modified to sort the list with first name and last name by making it stable sort. The code is as given below with output :

======================================

def mergesort(mlist):

if len(mlist) < 2 :

return mlist

else :

mid = int(len(mlist)/2)

return merge(mergesort(mlist[:mid]), mergesort(mlist[mid:]))

def merge(left, right):

if left == [] :

return right

elif right == [] :

return left

elif left[0][0] < right[0][0]:

return [left[0]] + merge(left[1:], right)

elif left[0][0] > right[0][0]:

return [right[0]] + merge(left, right[1:])

elif left[0][0] == right[0][0] :

if(left[0][1] <= right[0][1]):

return [left[0]] + merge(left[1:], right)

else :

return [right[0]] + merge(left, right[1:])

# sort the list

tuple = [("abc", "aaa"), ("abc", "bbb"), ("abc", "aaa"), ("bcd", "aaa"), ("edfg", "abcd"), ("bc", "aaa")]

print("tuple before sorting : "+ str(tuple))

result = mergesort(tuple)

print("list after sorting : "+ str(result))

===============================================

The output of above code :

tuple before sorting : [('abc', 'aaa'), ('abc', 'bbb'), ('abc', 'aaa'), ('bcd', 'aaa'), ('edfg', 'abcd'), ('bc', 'aaa')]
list after sorting : [('abc', 'aaa'), ('abc', 'aaa'), ('abc', 'bbb'), ('bc', 'aaa'), ('bcd', 'aaa'), ('edfg', 'abcd')]

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