Insert 1D array into a 2D array

Question

I am trying to insert a column into a 2D array. Currently I have a 2D array generated using itertools.

sample_points=[-1.5, -.8]
base_points = itertools.combinations_with_replacement(sample_points, 3)
base_points_list=list(base_points)
base_points_array=np.asarray(base_points_list)

Then I get an array which looks like this:

>>> base_points_array
array([[-1.5, -1.5, -1.5],
       [-1.5, -1.5, -0.8],
       [-1.5, -0.8, -0.8],
       [-0.8, -0.8, -0.8]])

I want to add a column at the beginning so that the array looks like this:

[[1 -1.5 -1.5 -1.5]
 [1 -1.5 -1.5 -0.8]
 [1 -1.5 -0.8 -0.8]
 [1 -0.8 -0.8 -0.8]]

So I used the command: np.insert(base_points_array,0,1,1) Because it should be able to do that using broadcasting. but I get something completely different. the number of rows have changes:

array([[ 1. , -1.5, -1.5, -1.5, -0.8],
       [ 1. , -1.5, -1.5, -0.8, -0.8],
       [ 1. , -1.5, -0.8, -0.8, -0.8]])

What am I doing wrong?

I cannot reproduce the problem. When I try np.insert(base_points_array,0,1,1), I get the expected result, and not the claimed result. Do keep in mind that np.insert creates a new array and does not modify the original. — Karl Knechtel
– Karl Knechtel, Commented Jul 2, 2022 at 20:18
Executing np.insert(base_points_array.T,0,1,1) gives the result you see. Any chance you accidentally transposed your array in between creating it and doing the insert? — slothrop
– slothrop, Commented Jul 2, 2022 at 20:19
Agreed, it looks as though the array somehow got transposed first. As a side note, it isn't necessary to make a temporary list: see also stackoverflow.com/questions/367565/… — Karl Knechtel
– Karl Knechtel, Commented Jul 2, 2022 at 20:22

Yusuf Syam · Accepted Answer · 2022-07-03 05:17:50Z

2

Using the np.append . But if your array to insert is 1D array

insert_array= [1, 1, 1, 1]

You need to expand the dimension of your inserting array by 1 first, you can do it with

insert_array= np.expand_dims(insert_array, 1)

And then you can use the append method

base_points_array= np.append(insert_array, base_points_array, 1)

edited Jul 3, 2022 at 5:17

answered Jul 2, 2022 at 21:15

Yusuf Syam

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4 Comments

eeqesri Over a year ago

I intially gave an up arrow for the answer. but then I realized, that it didn't work somehow. So wanted to cancel the up arrow and clicked on the down arrow, although I didn't want to give a down arrow. I am really sorry about that. Is there a way I can remove the downvote?

Yusuf Syam Over a year ago

@eeqesri It didnt work for you? maybe because you did not assign the np.append() to a new array, since it does not modify the array but make a new array instead.

eeqesri Over a year ago

Indeed you are right. It worked now after saving it in a new array and then printing it.

Yusuf Syam Over a year ago

@eeqesri all right, that is actually my bad for not give a detailed answer, ill edit it

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