Appending a new row to a numpy array

Question

I am trying to append a new row to an existing numpy array in a loop. I have tried the methods involving append, concatenate and also vstack none of them end up giving me the result I want.

I have tried the following:

for _ in col_change:
if (item + 2 < len(col_change)):
    arr=[col_change[item], col_change[item + 1], col_change[item + 2]]
    array=np.concatenate((array,arr),axis=0)
    item+=1

I have also tried it in the most basic format and it still gives me an empty array.

array=np.array([])
newrow = [1, 2, 3]
newrow1 = [4, 5, 6]
np.concatenate((array,newrow), axis=0)
np.concatenate((array,newrow1), axis=0)
print(array)

I want the output to be [[1,2,3][4,5,6]...]

array = np.concatenate((array,newrow), axis=0) and array = np.concatenate((array,newrow1), axis=0). It doesn't work by reference. You need to have array = before the operations. — Abdou
– Abdou, Commented Jan 8, 2017 at 14:00
Already answered here : [1]: stackoverflow.com/questions/3881453/numpy-add-row-to-array — Gyanshu
– Gyanshu, Commented Jan 8, 2017 at 14:06
I tried that too but it ends up adding to the end of the row like this [ 1. 2. 3. 4. 5. 6.] — blaziken105
– blaziken105, Commented Jan 8, 2017 at 14:08
Just try array = np.vstack([np.concatenate([array, newrow]), newrow1]) then. All those options are available in the thread posted by Gyanshu. — Abdou
– Abdou, Commented Jan 8, 2017 at 14:24

hpaulj · Accepted Answer · 2017-01-08 16:37:01Z

4

The correct way to build an array incrementally is to not start with an array:

alist = []
alist.append([1, 2, 3])
alist.append([4, 5, 6])
arr = np.array(alist)

This is essentially the same as

arr = np.array([ [1,2,3], [4,5,6] ])

the most common way of making a small (or large) sample array.

Even if you have good reason to use some version of concatenate (hstack, vstack, etc), it is better to collect the components in a list, and perform the concatante once.

answered Jan 8, 2017 at 16:37

hpaulj

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MSeifert · Accepted Answer · 2017-01-08 14:11:17Z

1

If you want [[1,2,3],[4,5,6]] I could present you an alternative without append: np.arange and then reshape it:

>>> import numpy as np

>>> np.arange(1,7).reshape(2, 3)
array([[1, 2, 3],
       [4, 5, 6]])

Or create a big array and fill it manually (or in a loop):

>>> array = np.empty((2, 3), int)
>>> array[0] = [1,2,3]
>>> array[1] = [4,5,6]
>>> array
array([[1, 2, 3],
       [4, 5, 6]])

A note on your examples:

In the second one you forgot to save the result, make it array = np.concatenate((array,newrow1), axis=0) and it works (not exactly like you want it but the array is not empty anymore). The first example seems badly indented and without know the variables and/or the problem there it's hard to debug.

edited Jan 8, 2017 at 14:11

answered Jan 8, 2017 at 14:04

MSeifert

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1 Comment

blaziken105 Over a year ago

I concatenated the array and then reshaped it, works fine now.

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