middle number without using median function, Python

Question

I have been looking for how to find the middle number in the list so that I do not use the median function, but cannot find the information how to do that.

I need to write a code which takes middle(L) function (have to define it), makes a list L as its argument, and returns the item in the middle position of L. (In order that the middle is well-defined, i should assume that L has odd length.)

It is all i have right now and actually have no idea how to do that.

def middle (L):
    i= len((L)[0:-1])/2
    return i
print (middle)

@RyneEverett Sounds like a school assignment, and that was one of the requirements. Not that uncommon. They often have students re-implement features just to have a better understanding of how they work (e.g. various sorts, trees, hashing, etc). — Cory Kramer
– Cory Kramer, Commented Oct 13, 2014 at 18:59

Cristian Ciupitu · Accepted Answer · 2015-06-07 12:13:07Z

4

To find the median, just sort the list and return the number in the middle position or (if the list has even number of elements), return the average of the 2 elements in middle:

def middle(L):
    L = sorted(L)
    n = len(L)
    m = n - 1
    return (L[n/2] + L[m/2]) / 2.0

Example:

>>> print middle([1, 2, 3, 4, 5])
3.0
>>> print middle([1, 2, 3, 4, 5, 6])
3.5

edited Jun 7, 2015 at 12:13

Cristian Ciupitu

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answered Oct 13, 2014 at 18:53

NPE

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Padraic Cunningham · Accepted Answer · 2014-10-13 19:20:01Z

1

As NPE's answer suggests you just have to get the middle element of a sorted list when the list has an uneven number of elements, if it has an even number of elements you take the average of the middle two elements:

def median(l):
    srt = sorted(l)
    mid = len(l)//2
    if len(l) % 2: # f list length mod 2 has a remainder the list is an odd lenght
            return srt[mid]
    else:
        med = (srt[mid] + srt[mid-1]) / 2  # in a list [1,2,3,4] srt[mid]-> 2, srt[mid-1] -> 3
        return med

answered Oct 13, 2014 at 19:20

Padraic Cunningham

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4 Comments

abarnert Over a year ago

This isn't necessary; as the question says, "In order that the middle is well-defined, i should assume that L has odd length." It might still be useful for other people with different assignments, but for this one, he'd probably get dinged for not reading the instructions.

Padraic Cunningham Over a year ago

@abarnert, I am aware of that but I still think adding how to to get the median of a list of values when the list is even is useful

abarnert Over a year ago

That's why I said "It might still be useful for other people with different assignments".

Padraic Cunningham Over a year ago

@abarnert, well I guess we agree it is useful then

Jason Zhang · Accepted Answer · 2018-07-10 22:36:46Z

0

For optimization, we should use binary search to detect the median, rather than to sort all numbers. For details, please check: https://www.quora.com/Given-a-list-of-unsorted-numbers-how-would-you-find-the-median-without-sorting-the-original-array

For the code, please check: https://medium.com/@nxtchg/calculating-median-without-sorting-eaa639cedb9f

There are two well-known ways to calculate median:

naive way (sort, pick the middle)
using quickselect (or similar algorithm for weighted median)

Hope it helps.

answered Jul 10, 2018 at 22:36

Jason Zhang

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