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Allen Qin
Allen Qin

Reputation: 19947

Python - Get percentage of minority class in a list

Suppose I have a list, how can I write an elegant one-liner to calculate the percentage of the minority class in a list?

For example, for list1 = [1,1,1,-1,-1], the minority class is -1. The percentage of -1 in the list will be 2/5=0.4

For another list list2 = [1,-1,-1,-1,-1], the minority class is 1. The percentage of 1 in the list will be 1/5=0.2

Upvotes: 1

Views: 381

Answers (2)

SparkAndShine
SparkAndShine

Reputation: 18017

Use collections.Counter,

import collections

list1 = [1,1,1,-1,-1]
p = min(collections.Counter(list1).values())*1.0/len(list1)
print(p)
# Output
0.4

collections.Counter(list1) returns a dict with the key:value pair element:frequency. In this case, it will be Counter({1: 3, -1: 2}).

Upvotes: 3

DeepSpace
DeepSpace

Reputation: 81614

You can use a Counter:

from collections import Counter

nums = [1,-1,-1,-1,-1]
# if using Python 2 use float(len(nums))
print(Counter(nums).most_common()[-1][-1] / len(nums))
>> 0.2

Counter.most_common() returns a list of tuples of the form (element, count) ordered from most to least common, so most_common()[-1][-1] returns the element that is the least common.

If there are several minorities, one of them will be chosen arbitrarily for the calculation. For example, using my code with nums = [3, 3, -1, 1, 2] will also return 0.2, using either -1, 1or 2 for the calculation.

Upvotes: 2

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