Append columns to the beginning and end of a 2D array

Question

I would like to take two 1D arrays

Ne_initial = Ne[:,0]
Ne_final = Ne[:,-1]

and append them to an existing 2D array as the first (Ne_initial) and last (Ne_final) rows. I am unsure how to do this. Can anyone help?

@Ywapom could you use either of those to cleanly create a new "column" in a python nested list? — roganjosh, Jun 15 '18 at 19:25
@roganjosh yes, see: https://stackoverflow.com/questions/9285414/appending-to-a-nested-list — Ywapom, Jun 15 '18 at 19:26
Possible duplicate of [How to add an extra column to a numpy array](https://stackoverflow.com/questions/8486294/how-to-add-an-extra-column-to-a-numpy-array) — roganjosh, Jun 15 '18 at 19:36
It is different from the duplicate, once you've clarified rows and columns. If the case is just to add rows then I'm curious about any efforts you've made to solve this problem. — roganjosh, Jun 15 '18 at 19:54
@roganjosh I've attempted np.insert for the first and np.append for the last, but I am getting "ValueError: array is not broadcastable to correct shape" — Brandon, Jun 15 '18 at 20:07
Update: problem solved using @mad_ 's method, with a bit of reshaping — Brandon, Jun 15 '18 at 21:07

Amir Dezhkam · Answer 1 · 2018-06-15T19:43:22.507

0

You can use this code:

my_2d_array = []

my_2d_array.insert(0, Ne_initial)
my_2d_array.append(Ne_final)

edited Jun 15 '18 at 19:43

answered Jun 15 '18 at 19:32

Amir Dezhkam

39
4

That only works if the 2D array is empty, which it isnt. My question is more so how to add these two 1D arrays to the beginning and then the end of the pre-existing 2D array – Brandon Jun 15 '18 at 19:35
this method also does not work if objects are not lists. My objects are np.arrays, not lists. – Brandon Jun 15 '18 at 19:55

score 0 · Answer 2 · answered Jun 15 '18 at 19:36

The following example should accomplish what you want:

IN:

import numpy as np

existing2Darray = np.matrix([[8, 3, 6, 1],[2, 5, 4, 2],[7, 2, 5, 1]])
Ne_initial = existing2Darray[:,0]
Ne_last = existing2Darray[:,-1]

OUT:

 [[8 3 6 1]
 [2 5 4 2]
 [7 2 5 1]]

[[8]
 [2]
 [7]]

[[1]
 [2]
 [1]]

IN:

np.append(existing2Darray, Ne_initial, axis=1))

OUT:

[[8 3 6 1 8]
 [2 5 4 2 2]
 [7 2 5 1 7]]

IN:

np.insert(existing2Darray, [0], Ne_last, axis=1))

OUT:

[[1 8 3 6 1]
 [2 2 5 4 2]
 [1 7 2 5 1]]

score 0 · Accepted Answer · answered Jun 15 '18 at 19:38

0

import numpy as np 
a = np.array([[1,2],[3,4]]) 

print 'First array:' 
print a 
print '\n'  
b = np.array([[5,6],[7,8]]) 

print 'Second array:' 
print b 
print '\n'

print 'Vertical stacking:' 
c = np.vstack((a,b)) 
print c # creating a 2-D array
col1=np.array([2,3,4,5])
col2=np.array([1,1,1,1])
d=np.column_stack((a1,b2))
np.concatenate((d,c)) # adding back to existing array

answered Jun 15 '18 at 19:38

mad_

8,121
2
25
40

I had to tweak your answer a bit for it to work: instead of np.vstack I had to use np.hstack, and for it to work properly I had to reshape the initial and final 1D arrays so that their shapes were (#,1) instead of (#,). with those edits, this works! – Brandon Jun 15 '18 at 20:59

Append columns to the beginning and end of a 2D array

3 Answers3