I have a MATLAB code that looks like this:
dindex =find(H2(:,1)>=2400)
H2 =H2(dindex,:);
I find all the rows of a 2-d array, H2, in which its 1st column is bigger than 2400.
How can this be done in Python?
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1 Answer
I would recommend using numpy for this, it's a very powerful python library that makes this sort of task trivial.
numpy.where() is what you are looking for here:
>>> import numpy as np
>>> arr = np.array([[2500, 1, 1, 1, 1], [2300, 1, 1, 1, 1], [2600, 1, 1, 1, 1]])
>>> arr
array([[2500, 1, 1, 1, 1],
[2300, 1, 1, 1, 1],
[2600, 1, 1, 1, 1]])
>>> np.where(arr[:,0] >= 2400)
(array([0, 2], dtype=int64),)
If you only want to keep the rows that meet this condition, you can do this with numpy indexing:
>>> arr = np.array([[2500, 1, 1, 1, 1], [2300, 1, 1, 1, 1], [2600, 1, 1, 1, 1]])
>>> arr
array([[2500, 1, 1, 1, 1],
[2300, 1, 1, 1, 1],
[2600, 1, 1, 1, 1]])
>>> arr = arr[np.where(arr[:,0] >= 2400)]
>>> arr
array([[2500, 1, 1, 1, 1],
[2600, 1, 1, 1, 1]])
2 Comments
Eric Kim
i've never used numpy before, so i am a bit confused. I want to replace the original 2-d array with only rows with its 1st column bigger than 2400. How can I do that, after your code? So, in the list 'arr', I want to remove the middle row, by using the index obtained from numpy.
user3483203
Updated my answer.