How to Average a List of Lists in Python?

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Problem: You have a list of lists and you want to calculate the average of the different columns.

Example: Given the following list of lists with four rows and three columns.

data = [[0, 1, 0],
        [1, 1, 1],
        [0, 0, 0],
        [1, 1, 0]]

You want to have the average values of the three columns:

[average_col_1, average_col_2, average_col_3]

There are three methods that solve this problem. You can play with them in the interactive shell and read more details below:

Method 1: Average in Python (No Library)

A simple one-liner with list comprehension in combination with the zip() function on the unpacked list to transpose the list of lists does the job in Python.

data = [[0, 1, 0],
        [1, 1, 1],
        [0, 0, 0],
        [1, 1, 0]]


# Method 1: Pure Python
res = [sum(x) / len(x) for x in zip(*data)]
print(res)
# [0.5, 0.75, 0.25]

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You can visualize the code execution and memory objects of this code in the following tool (just click “Next” to see how one step of the code unfolds).

Method 2: Average with NumPy Library

You create a NumPy array out of the data and pass it to the np.average() function.

data = [[0, 1, 0],
        [1, 1, 1],
        [0, 0, 0],
        [1, 1, 0]]

# Method 2: NumPy
import numpy as np
a = np.array(data)
res = np.average(a, axis=0)
print(res)
# [0.5  0.75 0.25]

The axis argument of the average function defines along which axis you want to calculate the average value. If you want to average columns, define axis=0. If you want to average rows, define axis=1. If you want to average over all values, skip this argument.

Method 3: Mean Statistics Library + Map()

Just to show you another alternative, here’s one using the map() function and our zip(*data) trick to transpose the “matrix” data.

data = [[0, 1, 0],
        [1, 1, 1],
        [0, 0, 0],
        [1, 1, 0]]

# Method 3: Statistics + Map()
import statistics
res = map(statistics.mean, zip(*data))
print(list(res))
# [0.5, 0.75, 0.25]

The map(function, iterable) function applies function to each element in iterable. As an alternative, you can also use list comprehension as shown in method 1 in this tutorial. In fact, Guido van Rossum, the creator of Python and Python’s benevolent dictator for life (BDFL), prefers list comprehension over the map() function.

Where to Go From Here?

Enough theory. Let’s get some practice!

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