Growing matrices columnwise in NumPy

Question

In pure Python you can grow matrices column by column pretty easily:

data = []
for i in something:
    newColumn = getColumnDataAsList(i)
    data.append(newColu         


        

Answer 1

Generally it is expensive to keep reallocating the NumPy array - so your third solution is really the best performance wise.

However I think hstack will do what you want - the cue is in the error message,

ValueError: arrays must have same number of dimensions`

I'm guessing that newColumn has two dimensions (rather than a 1D vector), so you need data to also have two dimensions..., for example, data = np.array([[]]) - or alternatively make newColumn a 1D vector (generally if things are 1D it is better to keep them 1D in NumPy, so broadcasting, etc. work better). in which case use np.squeeze(newColumn) and hstack or vstack should work with your original definition of the data.