Find the Median Index in Python

🐍 Question: How to find the index of the median element in a Python list? Something like argmedian()?

In case you need a quick refresher, I have written this tutorial on finding the median itself (but not yet its index):

👉 Recommended: 6 Ways to Get the Median of a Python List

Solution

A multi-liner solution to get the index of the median index is to first sort the list, take the mid-element from the sorted list, and search the index of the median using the list.index() method. This finds the index of the first occurrence of the median.

Here’s a code snippet that does that:

def median_index(lst):
    sorted_lst = sorted(lst)
    mid = len(lst) // 2
    median = sorted_lst[mid]
    return lst.index(median)


print(median_index([10, 1, 5]))
# 2

print(median_index([10, 1, 5, 99, 100, 100]))
# 3

print(median_index([10, 1, 5, 99, 100, 100, 0]))
# 0

This is a Python function that takes a list of numbers as input and returns the index of the median element of the list. Here’s how the code works:

1. def median_index(lst): This line defines a function called median_index that takes a list lst as input.
2. sorted_lst = sorted(lst): This line creates a new list called sorted_lst that contains the same elements as the input list lst, but sorted in ascending order using the built-in sorted() function.
3. mid = len(lst) // 2: This line calculates the index of the median element in the original unsorted list lst by dividing the length of the list by 2 using the floor division operator //.
4. median = sorted_lst[mid]: This line assigns the value of the median element in the sorted list sorted_lst to a new variable called median. If the length of the list is odd, this will be the element at the index mid. If the length of the list is even, mid will be the index of the element to the left of the middle two elements, and median will be the leftmost of those two elements.
5. return lst.index(median): This line returns the index of the median element in the original unsorted list lst by using the built-in index() function to find the index of the first occurrence of the median value in lst.
6. The print() statements at the end of the code call the median_index() function with different input lists and print the output to the console. For example, print(median_index([10, 1, 5])) will print 2 because the median element of the input list [10, 1, 5] is 5, and its index in the list is 2.

Python One-Liner to Find the Index of Median

I love Python one-liners. ♥️

So, here’s the same code condensed in a single line of Python code that finds the index of the median element in the lst using lst.index(sorted(lst)[len(lst)//2])

def median_index(lst):
    return lst.index(sorted(lst)[len(lst)//2])

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