5 Best Ways to Return the Minimum of an Array Along Axis 1 or Minimum Ignoring Any NaNs in Python

by

💡 Problem Formulation: When working with numerical data in Python, it’s common to encounter the challenge of finding the minimum value in rows of an array. This becomes more complex when the array contains NaN (Not a Number) values, which can disrupt statistical calculations. For instance, given an input like [[3, NaN, 1], [2, 5, NaN]], we seek methods to find the row-wise minima, such as [1, 2], excluding any NaNs present.

Method 1: Using NumPy and np.nanmin()

NumPy’s np.nanmin() function is designed to handle computations involving NaN values by ignoring them. This method effectively calculates the minimum of an array along the specified axis while excluding NaNs from consideration, making it straightforward for such scenarios.

Here’s an example:

import numpy as np

array_with_nans = np.array([[3, np.nan, 1], [2, 5, np.nan]])
min_values = np.nanmin(array_with_nans, axis=1)
print(min_values)

Output:

[1. 2.]

This code snippet imports the NumPy library and applies the np.nanmin() function to the given 2D array along axis 1 (rows). The NaN values are ignored, and the smallest value per row is returned as the output array.

Method 2: Using List Comprehension with min() and math.isnan()

Combining Python’s list comprehension with min() function and math.isnan() from the math module can be used to filter out NaN values explicitly and calculate row-wise minima for arrays.

Here’s an example:

import math

array_with_nans = [[3, float('nan'), 1], [2, 5, float('nan')]]
min_values = [min(x for x in row if not math.isnan(x)) for row in array_with_nans]
print(min_values)

Output:

[1, 2]

In this snippet, list comprehension iterates over each row of the array filtering out NaNs using math.isnan(), then computes the minimum value per row using the min() function to generate the results.

Method 3: Using Pandas and DataFrame.min()

Pandas offers a convenient and high-level approach to handle NaNs while computing statistical functions. The DataFrame.min() method, when applied along the correct axis, computes the minimum value for each row while automatically ignoring NaNs.

Here’s an example:

import pandas as pd

array_with_nans = pd.DataFrame([[3, None, 1], [2, 5, None]])
min_values = array_with_nans.min(axis=1)
print(min_values)

Output:

0    1.0
1    2.0
dtype: float64

After converting the 2D list with NaN values into a Pandas DataFrame, the DataFrame.min() method is called along the row axis (axis=1). This method automatically handles NaNs, returning the minimum values for each row.

Method 4: Using the SciPy library and scipy.stats.mstats.nanmin()

The SciPy library, which extends NumPy, provides the scipy.stats.mstats.nanmin() function specifically for such tasks. Similar to NumPy’s np.nanmin() but located within the statistics module of SciPy, it’s well-suited for scientific computations involving missing data.

Here’s an example:

from scipy.stats.mstats import nanmin

array_with_nans = np.array([[3, np.nan, 1], [2, 5, np.nan]])
min_values = nanmin(array_with_nans, axis=1)
print(min_values)

Output:

[1. 2.]

The demonstration uses SciPy’s nanmin() function in a manner similar to NumPy’s equivalent. This approach is particularly useful if your array is part of a larger SciPy-based statistical analysis workflow.

Bonus One-Liner Method 5: Using NumPy with Infinity Replacement

This approach involves replacing NaN values with positive infinity and then calling NumPy’s regular min() function. By doing so, NaN values become non-issues as they are treated as infinitely large numbers.

Here’s an example:

import numpy as np

array_with_nans = np.array([[3, np.nan, 1], [2, 5, np.nan]])
min_values = np.min(np.where(np.isnan(array_with_nans), np.inf, array_with_nans), axis=1)
print(min_values)

Output:

[1. 2.]

This one-liner substitutes NaNs with np.inf using np.where() before applying the standard np.min() function. Such an approach cleverly navigates around NaNs while achieving the desired outcome.

Summary/Discussion

  • Method 1: NumPy with np.nanmin(). Strong at handling NaNs efficiently. Best for NumPy arrays.
  • Method 2: List comprehension with min(). More Pythonic. Good for lists but not as efficient with large datasets.
  • Method 3: Pandas with DataFrame.min(). High-level and concise. Ideal for Pandas DataFrame objects.
  • Method 4: SciPy with nanmin(). Scientifically oriented. Blends well into SciPy statistical workflows.
  • Method 5: NumPy with Infinity Replacement. A unique one-liner. Works well in conjunction with NumPy’s comprehensive functionality.