Better mergesort.py

[LuisEnMarroquin]

Allow the function to be executed without having to pass the start and end indexes of the array

Describe your change:

Added default values to mergesort function to automatically calculate the start and end indexes

Includes @cclauss revision

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?
• Improved an algorithm?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[realDuYuanChao]

@cclauss can you take a look why Travic CI build error. Thanks in advance.

[spamegg1]

@cclauss can you take a look why Travic CI build error. Thanks in advance.

Build log says

E _pytest.pathlib.ImportPathMismatchError: ('merge_sort', '/home/travis/build/TheAlgorithms/Python/divide_and_conquer/merge_sort.py', PosixPath('/home/travis/build/TheAlgorithms/Python/sorts/merge_sort.py'))

There are two files named merge_sort.py so Travis is confused. There is this:
https://github.com/TheAlgorithms/Python/blob/master/sorts/merge_sort.py
And there is this:
https://github.com/TheAlgorithms/Python/blob/master/divide_and_conquer/mergesort.py
You changed the name of the second one, so now they are both named merge_sort.py.
It's possible that whoever wrote these two files named them differently on purpose to avoid this issue. I'm guessing changing the name back to mergesort.py will fix the issue?

[cclauss]

Reverting the name change will fix the issue. It is bugging me to have two different merge sorts in two different directories. Maybe we should just have one in the sorts directory.

[cclauss]

Suggested change

	if left is None or left is False or left < 0:
	if (left or -1) < 0:

[cclauss]

Suggested change

	if right is None or right is False or right < 0:
	if (right or -1) < 0:

[LuisEnMarroquin]

@cclauss That conditional won't work, value will always be -1

Try running this:

def mergesort(num, left=None):
    yyy = (left or -1)
    if (left or -1) < 0:
        print('inside if', num, yyy)

mergesort(0)
mergesort(1, None)
mergesort(2, False)
mergesort(3, -1)
mergesort(4, 0)
mergesort(5, 1)

And you will get:

inside if 0 -1
inside if 1 -1
inside if 2 -1
inside if 3 -1
inside if 4 -1

[cclauss]

>>> for i in range(-5, 5):
...     left = i
...     if (left or -1) < 0:
...         left = 0
...     print(i, left)
... 
-5 0
-4 0
-3 0
-2 0
-1 0
0 0
1 1
2 2
3 3
4 4

[LuisEnMarroquin]

@cclauss That conditional won't work, value will always be -1

Try running this:

def mergesort(num, left=None):
    yyy = (left or -1)
    if (left or -1) < 0:
        print('inside if', num, yyy)

mergesort(0)
mergesort(1, None)
mergesort(2, False)
mergesort(3, -1)
mergesort(4, 0)
mergesort(5, 1)

And you will get:

inside if 0 -1
inside if 1 -1
inside if 2 -1
inside if 3 -1
inside if 4 -1

Please run this, or operator doesn't work the same way inside def

Executed on:

• Ubuntu Server - Python 3.6.9
• Windows 10 Pro - Python 3.8.5

[LuisEnMarroquin]

After adding your suggested changes I get maximum recursion depth exceeded

Here is the file with your changes applied @cclauss

def merge(arr, left, mid, right):
    # overall array will divided into 2 array
    # left_arr contains the left portion of array from left to mid
    # right_arr contains the right portion of array from mid + 1 to right
    left_arr = arr[left : mid + 1]
    right_arr = arr[mid + 1 : right + 1]
    k = left
    i = 0
    j = 0
    while i < len(left_arr) and j < len(right_arr):
        # change sign for Descending order
        if left_arr[i] < right_arr[j]:
            arr[k] = left_arr[i]
            i += 1
        else:
            arr[k] = right_arr[j]
            j += 1
        k += 1
    while i < len(left_arr):
        arr[k] = left_arr[i]
        i += 1
        k += 1
    while j < len(right_arr):
        arr[k] = right_arr[j]
        j += 1
        k += 1
    return arr


def mergesort(arr, left=None, right=None):
    """
    >>> mergesort([3, 2, 1])
    [1, 2, 3]
    >>> mergesort([3, 2, 1, 0, 1, 2, 3, 5, 4])
    [0, 1, 1, 2, 2, 3, 3, 4, 5]
    >>> mergesort([3, 2, 1], -1, -3.1)
    [1, 2, 3]
    >>> mergesort([3, 2, 1], None, False)
    [1, 2, 3]
    """
    if (left or -1) < 0:
        left = 0
    if (right or -1) < 0:
        right = len(arr) - 1
    if left < right:
        mid = (left + right) // 2
        # print("ms1",a,b,m)
        mergesort(arr, left, mid)
        # print("ms2",a,m+1,e)
        mergesort(arr, mid + 1, right)
        # print("m",a,b,m,e)
        merge(arr, left, mid, right)
        return arr


if __name__ == "__main__":
    import doctest

    doctest.testmod()

[stale]

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[stale]

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