I have a numpy array and want to add a row to it and modify one column. This is my array:
import numpy as np
small_array = np.array ([[[3., 2., 5.],
[3., 2., 2.]],
[[3., 2., 7.],
[3., 2., 5.]]])
Then, firstly I wnat to add a fixed value (e.g. 2.
) to last column. I did it this way:
chg = 2.
small_array[:,:,-1] = chg
next thing that I want to do is adding another row to each aubarray. Added row should have the same first and second columns but third column shold be different. This time chg x 2. should be subtracted from the existing value in third column:
big_array = np.array ([[[3., 2., 7.],
[3., 2., 4.],
[3., 2., 0.]], # this row is added
[[3., 2., 9.],
[3., 2., 7.],
[3., 2., 3.]]]) # this row is added
I very much appreciate any help to do it.
CodePudding user response:
I believe the operation you are looking for is np.concatenate
, which can construct a new array by concatenating two arrays.
Simple example, we can add a row of zeroes like this:
>>> np.concatenate((small_array, np.zeros((2,1,3))), axis=1)
array([[[3., 2., 7.],
[3., 2., 4.],
[0., 0., 0.]],
[[3., 2., 9.],
[3., 2., 7.],
[0., 0., 0.]]])
Now, instead of zeros, we can get the values from the first row in each matrix:
>>> np.concatenate((small_array, small_array[:,:1,:]), axis=1)
array([[[3., 2., 7.],
[3., 2., 4.],
[3., 2., 7.]],
[[3., 2., 9.],
[3., 2., 7.],
[3., 2., 9.]]])
At this point, you can modify the value in the third column of the new rows as needed.
The axis
parameter is important here, it tells concatenate()
along which axis I want to concatenate the two input arrays.
Documentation: https://numpy.org/doc/stable/reference/generated/numpy.concatenate.html
CodePudding user response:
have you tried using numpy.resize ?
https://numpy.org/doc/stable/reference/generated/numpy.resize.html
you will have to decide for yourself if this is too computationally expensive, and if a numpy.list makes more sense for your purposes.
for example,
import numpy as np
a = np.array([[1,2], [3,4]])
a = np.resize(a, (3,2))
then use the index to edit the value of the array at the latest position
a[-1]=[8,9]
the final output of this example should be
a
array([[1,2],
[3,4]
[8,9]])