Python – Sort Matrix by Row Median

When working with matrices in Python, you may need to sort rows based on their median values. Python's statistics module provides a median() function that can be used with the sort() method's key parameter to achieve this.

Syntax

from statistics import median

matrix.sort(key=lambda row: median(row))

Example

Here's how to sort a matrix by row median using a custom function ?

from statistics import median

def median_row(row):
    return median(row)

my_matrix = [[43, 14, 27], [13, 27, 24], [32, 56, 18], [34, 62, 55]]

print("Original matrix:")
print(my_matrix)

# Calculate median for each row to understand sorting
for i, row in enumerate(my_matrix):
    print(f"Row {i}: {row} ? Median: {median(row)}")

my_matrix.sort(key=median_row)

print("\nSorted matrix by row median:")
print(my_matrix)

Original matrix:
[[43, 14, 27], [13, 27, 24], [32, 56, 18], [34, 62, 55]]
Row 0: [43, 14, 27] ? Median: 27
Row 1: [13, 27, 24] ? Median: 24
Row 2: [32, 56, 18] ? Median: 32
Row 3: [34, 62, 55] ? Median: 55

Sorted matrix by row median:
[[13, 27, 24], [43, 14, 27], [32, 56, 18], [34, 62, 55]]

Using Lambda Function

You can also use a lambda function for more concise code ?

from statistics import median

my_matrix = [[43, 14, 27], [13, 27, 24], [32, 56, 18], [34, 62, 55]]

print("Original matrix:")
print(my_matrix)

# Sort using lambda function
my_matrix.sort(key=lambda row: median(row))

print("Sorted matrix:")
print(my_matrix)

Original matrix:
[[43, 14, 27], [13, 27, 24], [32, 56, 18], [34, 62, 55]]
Sorted matrix:
[[13, 27, 24], [43, 14, 27], [32, 56, 18], [34, 62, 55]]

Sorting in Descending Order

To sort by median in descending order, use the reverse=True parameter ?

from statistics import median

my_matrix = [[43, 14, 27], [13, 27, 24], [32, 56, 18], [34, 62, 55]]

print("Original matrix:")
print(my_matrix)

# Sort in descending order
my_matrix.sort(key=lambda row: median(row), reverse=True)

print("Sorted matrix (descending):")
print(my_matrix)

Original matrix:
[[43, 14, 27], [13, 27, 24], [32, 56, 18], [34, 62, 55]]
Sorted matrix (descending):
[[34, 62, 55], [32, 56, 18], [43, 14, 27], [13, 27, 24]]

How It Works

The sort() method uses the key parameter to determine the sorting criteria:

• The median() function calculates the middle value of each row

• The sort() method compares these median values to order the rows

• Rows with smaller medians appear first in ascending order

• The original matrix is modified in-place

Conclusion

Use sort(key=lambda row: median(row)) to sort a matrix by row median values. The statistics.median() function handles the median calculation, while the key parameter tells sort() how to compare rows.