Initialize 2D array within loop

Question

I want to create 2D array with data from loop. Each loop iteration should add array within the array. As an example start from [ ] => [ [2,3] ] => [ [2,3] , [3,4] ] => [ [2,3] , [3,4] , [7,3] ] and likewise.

import numpy as np

output_arr = np.array([])

for i in range(0,4): 
   temp_arr = np.ones(2)
   print temp_arr.shape
   output = np.append((output_arr, temp_arr))

print output_arr.shape

Here np.append is sample code where I need to concatenate/append/hstack arrays together... (np.append didn't work.)
How to populate 2D array within the loop?

How to append that it would make 2D array... np.append doesn't work... — udarabibile
– udarabibile, Commented Aug 7, 2017 at 6:28
Stick with list append that you show at the start. Then make the array from the nested list. — hpaulj
– hpaulj, Commented Aug 7, 2017 at 6:28
Forget np.append. Learn to use np.concatenate instead. Look at the np.append code to understand why I say that. — hpaulj
– hpaulj, Commented Aug 7, 2017 at 6:56

cs95 · Accepted Answer · 2017-08-07 06:31:24Z

2

Start with an empty list:

output_arr = []

Append within the loop:

for _ in range(5):
    output_arr.append([1, 1])

Outside the loop, just call np.array:

X = np.array(output_arr)

answered Aug 7, 2017 at 6:31

cs95

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3 Comments

udarabibile Over a year ago

Thanks. This works. But is there a way to do this with arrays only, not involving lists. Just asking...

cs95 Over a year ago

@Chathuranga94 yes. If you append arrays instead of lists to the outer list, it will work. However, appending arrays to arrays is not simple, because numpy arrays are immutable. Feel free to mark accepted if it helped.

SwiftsNamesake Over a year ago

Your answer is technically what the OP asked for, but this could be done with a single list comprehension.

Daniel F · Accepted Answer · 2017-08-07 07:29:56Z

0

If you really just want to construct with numpy, construct an np.empty array at the beginning, and then if you need to operate on the currently filled part of the array, use output[:i+1]

import numpy as np

output_arr = np.empty((5,2))

for i in range(0,4): 
   temp_arr = np.ones(2)
   print temp_arr.shape
   output_arr[i] = temp_arr
   print output_arr[:i + 1].shape

print output_arr.shape

edited Aug 7, 2017 at 7:29

answered Aug 7, 2017 at 7:23

Daniel F

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