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?

deinonychusaur · Accepted Answer · 2014-07-30 16:48:03Z

6

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

edited Jul 30, 2014 at 16:48

answered Jul 30, 2014 at 16:42

deinonychusaur

7,3543 gold badges32 silver badges43 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

handroski Over a year ago

I would need to enter a unique value for each element. Would I be able do enter that value using something like data1[2,3] = 32

deinonychusaur Over a year ago

If that which should be entered is in an iterable I you could simply do data1[:, 3] = I

Eelco Hoogendoorn · Accepted Answer · 2014-07-30 16:46:40Z

3

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.

answered Jul 30, 2014 at 16:46

Eelco Hoogendoorn

10.8k1 gold badge46 silver badges43 bronze badges

Comments

punyidea · Accepted Answer · 2014-08-04 20:55:16Z

2

One thing that nobody has mentioned is that in Python, indexing starts at 0, not 1.

This means that if you want to look at the third column of the array, you actually should address [:,2], not [:,3].

Good luck!

answered Aug 4, 2014 at 20:55

punyidea

1731 silver badge9 bronze badges

Comments

Peter · Accepted Answer · 2014-07-31 06:55:10Z

0

Assuming ra and dec are vectors (1-d):

data1 = np.concatenate([ra[:, None], dec[:, None], np.zeros((len(ra), 1))+32], axis=1)

Or 

data1 = np.empty((len(ra), 3))
data[:, 0] = ra
data[:, 1] = dec
data[:, 2] = 32

answered Jul 31, 2014 at 6:55

Peter

13.8k11 gold badges82 silver badges99 bronze badges

Comments

João Rebelo · Accepted Answer · 2021-11-05 21:16:20Z

-1

hey guys if u want to fill an array with just the same number just

x_2 = np.ones((1000))+1

exemple for 1000 numbers 2

answered Nov 5, 2021 at 21:16

João Rebelo

1

Collectives™ on Stack Overflow

How to declare and fill an array in NumPy?

5 Answers 5

2 Comments

Comments

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

2 Comments

Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related