IndexError: list assignment index out of range Python

Question

def mode(given_list):
    highest_list = []
    highest = 0
    index = 0
    for x in range(0, len(given_list)):
        occurrences = given_list.count(given_list[x])
        if occurrences > highest:
            highest = occurrences
            highest_list[0] = given_list[x]
        elif occurrences == highest:
            highest_list.append(given_list[x])

The code is meant to work out the mode of a given list. I do not understand where I am going wrong.

Exact Error I am receiving.

line 30, in mode
    highest_list[0] = given_list[x]
IndexError: list assignment index out of range

wim · Accepted Answer · 2016-01-22 22:45:47Z

1

The problem is that you have an empty list originally:

highest_list = []

And then in the loop you try to access it at index 0:

highest_list[0] = ...

It's impossible, because it's an empty list and so is not indexable at position 0.

A better way to find the mode of a list is to use a collections.Counter object:

>>> from collections import Counter
>>> L = [1,2,3,3,4]
>>> counter = Counter(L)
>>> max(counter, key=counter.get)
3
>>> [(mode, n_occurrences)] = counter.most_common(1)
>>> mode, n_occurrences
(3, 2)

answered Jan 22, 2016 at 22:45

wim

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Comments

Garrett R · Accepted Answer · 2016-01-22 22:48:28Z

1

As far as getting the mode, you can just use a Counter from the collections library

from collections import Counter
x = [0, 1, 2, 0, 1, 0] #0 is the mode
g = Counter(x)
mode = max(g, key = lambda x: g[x])

answered Jan 22, 2016 at 22:48

Garrett R

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1 Comment

TigerhawkT3 Over a year ago

You can use g.get instead of lambda x: g[x].

TigerhawkT3 · Accepted Answer · 2016-01-22 22:50:39Z

0

At that point, at the start of the loop, highest_list is empty, so there's no first index. You can initialize highest_list as [0] so that there is always at least one "highest value."

That said, you can accomplish this more simply as follows:

def mode(given_list):
    return max(set(given_list), key=given_list.count)

This will find the highest item in the passed given_list, based on each item's count() in it. Making a set first ensures that each item is only counted once.

answered Jan 22, 2016 at 22:50

TigerhawkT3

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