Maximum Subarray in Python

Published on (Updated: 14 October 2020)

Welcome to the Maximum Subarray in Python page! Here, you'll find the source code for this program as well as a description of how the program works.

Current Solution

import sys
from typing import List


def error_handling(argv: List) -> List[int]:
    str_input = (','.join(i for i in argv[1:])).strip()
    if str_input == "":
        print('Usage: Please provide a list of at least two integers to sort in the format: "1, 2, 3, 4, 5"')
        sys.exit(1)
    nums = [int(num) for num in str_input.split(',')]
    return nums


def maximum_subarray(nums: List[int]) -> int:
    local_max = 0
    global_max = nums[0]
    for num in nums:
        local_max += num
        if (local_max < 0):
            local_max = 0
        elif global_max < local_max:
            global_max = local_max
    return global_max


if __name__ == "__main__":
    nums = error_handling(sys.argv)
    print(maximum_subarray(nums))

Maximum Subarray in Python was written by:

Divyansh Agarwal
Jeremy Grifski

If you see anything you'd like to change or update, please consider contributing.

Note: The solution shown above is the current solution in the Sample Programs repository as of May 12 2022 20:19:17. The solution was first committed on Nov 03 2020 08:44:37. As a result, documentation below may be outdated.

How to Implement the Solution

Let's look at the code in detail:

code for maximum_subarray.py:-

#!/usr/bin/env python
def maximum_subarray():
   # takes care of both empty input and no input
    str_input = (','.join(i for i in sys.argv[1:])).strip()
    if str_input == "":
        print("Usage: Please provide a list of at least two integers to sort in the format: '1, 2, 3, 4, 5'")
        return

    # split comma separated input string into list of integers
    arr = [int(num) for num in str_input.split(',')]
    ans = 0
    curr_sum = 0
    for i in range(len(arr)):
        if (curr_sum + arr[i] > 0):
            curr_sum += arr[i]
        else:
            curr_sum = 0
        ans = max(ans, curr_sum)
    print(ans)
    return

if __name__ == "__main__":
    maximum_subarray()  # call function to carry out kadane's algorithm

The main Function

Let us breakdown the code in smaller parts,

if __name__ == "__main__":
    maximum_subarray()

This bit of code checks to see if this is the main module run. If true, then it calls the maximum_subarray() function which carries out the kadane's algorithm which includes taking array input from standard input(via sys args through the terminal) and computing max subarray sum value and printing the result to standard output.

def maximum_subarray():
    # takes care of both empty input and no input
    str_input = (','.join(i for i in sys.argv[1:])).strip()
    if str_input == "":
        print("Usage: Please provide a list of at least two integers to sort in the format: '1, 2, 3, 4, 5'")
        return

    # split comma separated input string into list of integers
    arr = [int(num) for num in str_input.split(',')]
    ans = 0
    curr_sum = 0
    for i in range(len(arr)):
        if (curr_sum + arr[i] > 0):
            curr_sum += arr[i]
        else:
            curr_sum = 0
        ans = max(ans, curr_sum)
    print(ans)
    return

This is the main function of this file which implements all the algorithm logic. It takes a comma separated string of integers from the standard input (via the sys args). It then converts the string into an array of integers and carries out Kadane's algorithm for that array and prints the result. It also deals with the edge case which arises when there is no input or the input string is empty(blank string).

If the string is not empty, the function initialises ans (final value to be returned from the function) and curr_sum as 0. It then iterates through the array, keeps adding values to curr_sum until curr_sum > 0. If curr_sum goes to less than 0, then curr_sum is set to 0 again and process is carried on for the remaining elements of the array. Finally, ans is set to the maximum of 0 and curr_sum, and it's value is printed.

For example, if `` is the input:

First we compare, if str_input == ""
True
print "Usage: Please provide a list of at least two integers to sort in the format: '1, 2, 3, 4, 5'"
return

For example, if -1, -2, 1, 2, 3 is the input:

First we compare, if str_input == ""
False
Convert str_input into array of integers
Initialise curr_sum and ans to 0.
Now while iterating through the array, index i goes from 0 to 4.
For i = 0 and 1, the initial value of curr_sum is 0. 0 + (-1) < 0 and 0 + (-2) < 0, so -1 and -2 are skipped.
For i = 2,3,4 the values 1,2,3 are added to curr_sum iteratively and finally curr_sum value is 1 + 2 + 3 = 6
Max of 0 and 6 is 6. So ans gets the value 6.
Print 6 and return from function.

How to Run the Solution

If you want to run this program, you can download a copy of Maximum Subarray in Python.

Next, make sure you have the latest Python interpreter (latest stable version of Python 3 will do).

Finally, open a terminal in the directory of the downloaded file and run the following command:

python maximum_subarray.py 1,2,3,4

Replace 1,2,3,4 with input of your choice(comma separated integers)

Alternatively, copy the solution into an online Python interpreter and hit run.