Working with columns of NumPy matrices

Question

I've been unable to figure out how to access, add, multiply, replace, etc. single columns of a NumPy matrix. I can do this via looping over individual elements of the column, but I'd like to treat the column as a unit, something that I can do with rows.

When I've tried to search I'm usually directed to answers handling NumPy arrays, but this is not the same thing.

Answer 1

Can you provide code that's giving trouble? The operations on columns that you list are among the most basic operations that are supported and optimized in NumPy. Consider looking over the tutorial on NumPy for MATLAB users, which has many examples of accessing rows or columns, performing vectorized operations on them, and modifying them with copies or in-place.

NumPy for MATLAB Users

Just to clarify, suppose you have a 2-dimensional NumPy ndarray or matrix called a. Then a[:, 0] would access the first column just the same as a[0] or a[0, :] would access the first row. Any operations that work for rows should work for columns as well, with some caveats for broadcasting rules and certain mathematical operations that depend upon array alignment. You can also use the numpy.transpose(a) function (which is also exposed with a.T) to transpose a making the columns become rows.