How to declare and fill an array in NumPy?

Question

I need to create an empty array in Python and fill it in a loop method.

data1 = np.array([ra,dec,[]])

Here is what I have. The ra and dec portions are from another array I've imported. What I am having trouble with is filling the other columns. Example. Lets say to fill the 3rd column I do this:

for i in range (0,56):
    data1[i,3] = 32

The error I am getting is:

IndexError: invalid index for the second line in the aforementioned code sample.

Additionally, when I check the shape of the array I created, it will come out at (3,). The data that I have already entered into this is intended to be two columns with 56 rows of data.

So where am I messing up here? Should I transpose the array?

Answer 1

You could do:

data1 = np.zeros((56,4))

to get a 56 by 4 array. If you don't like to start the array with 0, you could use np.ones or np.empty or np.ones((56, 4)) * np.nan

Then, in most cases it is best not to python-loop if not needed for performance reasons. So as an example this would do your loop:

data[:, 3] = 32

Answer 2

data1 = np.array([ra,dec,[32]*len(ra)])

Gives a single-line solution to your problem; but for efficiency, allocating an empty array first and then copying in the relevant parts would be preferable, so you avoid the construction of the dummy list.