Python - Reverse sort Matrix Row by Kth Column

Python - Reverse sort Matrix Row by Kth Column

Let's delve into the task of reverse sorting the rows of a matrix based on the elements in a given column (the kth column).

Problem Statement:

Given a matrix (2D list) and an index k, we want to sort the rows of the matrix in descending order based on the elements in the kth column.

Example:

For the matrix:

[ [4, 7, 2], [1, 9, 5], [8, 3, 6] ]

and k=1 (2nd column, since indexing starts from 0), the sorted matrix will be:

[ [1, 9, 5], [4, 7, 2], [8, 3, 6] ]

Tutorial:

Using the sorted() function:

We'll leverage the sorted() function, specifying a custom sorting key to sort the rows based on the kth column elements.

def sort_matrix_by_kth_column(matrix, k): return sorted(matrix, key=lambda x: x[k], reverse=True) # Example usage: matrix = [ [4, 7, 2], [1, 9, 5], [8, 3, 6] ] k = 1 sorted_matrix = sort_matrix_by_kth_column(matrix, k) for row in sorted_matrix: print(row)

Explanation:

  • We provide a lambda function lambda x: x[k] as the sorting key to sorted(). This ensures that the sorting is based on the elements in the kth column.

  • The reverse=True argument ensures that the sorting is in descending order.

Additional Note:

The sorted() function returns a new sorted list and doesn't modify the original list in place. If you'd prefer to sort the matrix in place, you can use the sort() method of lists:

def sort_matrix_by_kth_column_inplace(matrix, k): matrix.sort(key=lambda x: x[k], reverse=True) # Example usage: matrix = [ [4, 7, 2], [1, 9, 5], [8, 3, 6] ] k = 1 sort_matrix_by_kth_column_inplace(matrix, k) for row in matrix: print(row)

Conclusion:

Sorting the rows of a matrix based on the elements in a given column is a common operation, especially in data processing tasks. In Python, this can be done succinctly and efficiently using the built-in sorting functionality combined with a custom sorting key.

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