How do you remove a range of rows in a python ndarray matrix?

Question

Suppose I have a 100x100 matrix in Python like below:

import numpy as np
A = np.linspace(0,100*100-1,100*100).reshape([100,100,])


print(A)

You can see A below:

[[0.000e+00 1.000e+00 2.000e+00 ... 9.700e+01 9.800e+01 9.900e+01]
 [1.000e+02 1.010e+02 1.020e+02 ... 1.970e+02 1.980e+02 1.990e+02]
 [2.000e+02 2.010e+02 2.020e+02 ... 2.970e+02 2.980e+02 2.990e+02]
 ...
 [9.700e+03 9.701e+03 9.702e+03 ... 9.797e+03 9.798e+03 9.799e+03]
 [9.800e+03 9.801e+03 9.802e+03 ... 9.897e+03 9.898e+03 9.899e+03]
 [9.900e+03 9.901e+03 9.902e+03 ... 9.997e+03 9.998e+03 9.999e+03]]

How do I delete a range of rows (like rows 5 - 50) in A?

RoseGod · Accepted Answer · 2021-12-03 15:10:05Z

2

create a mask array of the elements you want to keep and then just index the array.

Code:

import numpy as np
A = np.linspace(0,100*100-1,100*100).reshape([100,100,])
mask = np.ones(len(A), dtype=bool)
mask[5:50] = False
A = A[mask]

Shape before:

(100, 100)

Shape after:

(55, 100)

answered Dec 3, 2021 at 15:10

RoseGod

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Comments

jeoc · Accepted Answer · 2021-12-03 15:41:08Z

1

@RoseGod's answer is probably more pythonic but I thought I'd post this anyway:

A = np.asarray([*A[:5], *A[50:]])

answered Dec 3, 2021 at 15:41

jeoc

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2 Comments

Kaz Over a year ago

@RoseGod's answer is probably more pythonic, but above all more efficient. I measured a factor of 2 between the two methods.

jeoc Over a year ago

Definitely! Good point

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