M. Klugerford M. Klugerford - 5 months ago 23
Python Question

Make numpy matrix with insufficient length of data

I have some data, say a list of 10 numbers and I have to convert that list to a matrix of shape (3,4). What would be the best way to do so, if I say I wanted the data to fill by columns/rows and the unfilled spots to have some default value like -1.


data = [0,4,1,3,2,5,9,6,7,8]
>>> output
array([[ 0, 4, 1, 3],
[ 2, 5, 9, 6],
[ 7, 8, -1, -1]])

What I thought of doing is

data += [-1]*(row*col - len(data))
output = np.array(data).reshape((row, col))

Is there a simpler method that allows me to achieve the same result without having to modify the original data or sending in
data + [-1]*remaining
to the


I'm sure there are various ways of doing this. My first inclination is to make a output array filled with the 'fill', and copy the data to it. Since the fill is 'ragged', not a full column or row, I'd start out 1d and reshape to the final shape.

In [730]: row,col = 3,4
In [731]: data = [0,4,1,3,2,5,9,6,7,8]
In [732]: output=np.zeros(row*col,dtype=int)-1
In [733]: output[:len(data)]=data
In [734]: output = output.reshape(3,4)
In [735]: output
array([[ 0,  4,  1,  3],
       [ 2,  5,  9,  6],
       [ 7,  8, -1, -1]])

Regardless of whether data starts as a list or a 1d array, it will have to be copied to output. With a change in the total number of characters we can't just reshape it.

This isn't that different from your approach of adding the extra values via [-1]*n.

There is a pad function, but it works on whole columns or rows, and internally is quite complex because it's written for general cases.