Finding the mode of a list

Given a list of items, recall that the mode of the list is the item that occurs most often.

I would like to know how to create a function that can find the mode of a list but that displays a message if the list does not have a mode (e.g., all the items in the list only appear once). I want to make this function without importing any functions. I'm trying to make my own function from scratch.

Solution 1:

You can use the max function and a key. Have a look at python max function using 'key' and lambda expression.

max(set(lst), key=lst.count)

Solution 2:

You can use the Counter supplied in the collections package which has a mode-esque function

from collections import Counter
data = Counter(your_list_in_here)
data.most_common()   # Returns all unique items and their counts
data.most_common(1)  # Returns the highest occurring item

Note: Counter is new in python 2.7 and is not available in earlier versions.

Solution 3:

Python 3.4 includes the method statistics.mode, so it is straightforward:

>>> from statistics import mode
>>> mode([1, 1, 2, 3, 3, 3, 3, 4])
 3

You can have any type of elements in the list, not just numeric:

>>> mode(["red", "blue", "blue", "red", "green", "red", "red"])
 'red'

Solution 4:

Taking a leaf from some statistics software, namely SciPy and MATLAB, these just return the smallest most common value, so if two values occur equally often, the smallest of these are returned. Hopefully an example will help:

>>> from scipy.stats import mode

>>> mode([1, 2, 3, 4, 5])
(array([ 1.]), array([ 1.]))

>>> mode([1, 2, 2, 3, 3, 4, 5])
(array([ 2.]), array([ 2.]))

>>> mode([1, 2, 2, -3, -3, 4, 5])
(array([-3.]), array([ 2.]))

Is there any reason why you can 't follow this convention?