How to Sort a NumPy Array's Rows by a Specific Column?

Sorting Arrays in NumPy by Column

Sorting arrays by columns is a common task in data analysis and manipulation. NumPy provides efficient ways to perform this operation.

Question:

Given a NumPy array with multiple columns, how can you sort its rows by a specific column?

Example:

Consider the following array a:

a = np.array([[9, 2, 3],
              [4, 5, 6],
              [7, 0, 5]])

We want to sort the rows of a by the second column (column 1, indexed from 0) to obtain:

array([[7, 0, 5],
       [9, 2, 3],
       [4, 5, 6]])

Answer:

To sort a by its second column, use the following code:

a[a[:, 1].argsort()]

This expression achieves the desired result because:

1. a[:, 1] selects the second column of a.
2. argsort() returns the indices that would sort the column in ascending order.
3. Indexing a with these indices sorts the rows of a accordingly.

Therefore, this approach provides a convenient and efficient way to sort NumPy arrays by any specified column.

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