CODEWITHSUNDEEP
python
Ricardo Rod
Ricardo Rod

Reputation: 8911

How can I add an additional row and column to an array?

I need to add a column and a row to an existing Numpy array at a defined position.

Upvotes: 13

Views: 120477

Answers (3)

Erdem Şen
Erdem Şen

Reputation: 99

I would suggest sympy Matrix object to do it:

a = [[ 2,   1,  180],
     [ 1,   3,  300],
     [-1,  -4,    0]]

b = [[1,0],
     [0,0],
     [0,1]]

import sympy as sp

a = sp.Matrix(a).col_insert(-2, sp.Matrix(b))
a.tolist()

Output:

[[2, 1, 0, 1, 180], 
 [1, 0, 0, 3, 300], 
 [-1, 0, 1, -4, 0]]

And to continue with a Numpy array, you can use np.asarray(a) instead of a.tolist() (assuming you have imported Numpy as np)

Upvotes: 1

doug
doug

Reputation: 70038

There are many ways to do this in numpy, but not all of them let you add the row/column to the target array at any location (e.g., append only allows addition after the last row/column). If you want a single method/function to append either a row or column at any position in a target array, i would go with 'insert':

T = NP.random.randint(0, 10, 20).reshape(5, 4)
c = NP.random.randint(0, 10, 5)
r = NP.random.randint(0, 10, 4)
# add a column to T, at the front:
NP.insert(T, 0, c, axis=1)
# add a column to T, at the end:
NP.insert(T, 4, c, axis=1)
# add a row to T between the first two rows:
NP.insert(T, 2, r, axis=0)

Upvotes: 7

Stefan Kendall
Stefan Kendall

Reputation: 67832

I assume your column and rows are just a list of lists?

That is, you have the following?

L = [[1,2,3],
     [4,5,6]]

To add another row, use the append method of a list.

L.append([7,8,9])

giving

L = [[1,2,3],
     [4,5,6],
     [7,8,9]]

To add another column, you would have to loop over each row. An easy way to do this is with a list comprehension.

L = [x + [0] for x in L]

giving

L = [[1,2,3,0],
     [4,5,6,0]]

Upvotes: 27

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