Set Values on 2D NumPy Array

Question

If I have a NumPy array and a list of row indices:

import numpy as np

x = np.random.rand(50).reshape(10,5)
row_idx = [0, 1, 4]

How do I set all of the values in the rows in row_idx that are less that 0.5 to zero?

I've tried:

x[row_idx][x[row_idx] < 0.5] = 0.0

But this didn't do anything

Randy · Accepted Answer · 2021-05-18 01:15:52Z

1

You can do it with np.where:

In [26]: x[row_idx] = np.where(x[row_idx] < 0.5, 0, x[row_idx])

In [27]: x
Out[27]:
array([[0.94870486, 0.        , 0.        , 0.89030411, 0.50505295],
       [0.56803186, 0.90804518, 0.69843535, 0.77174293, 0.        ],
       [0.1318847 , 0.95940137, 0.92036048, 0.669007  , 0.15404623],
       [0.90021311, 0.72959638, 0.82705006, 0.65329554, 0.3714969 ],
       [0.56293165, 0.        , 0.        , 0.        , 0.        ],
       [0.46015752, 0.96294812, 0.0678065 , 0.66693152, 0.69825679],
       [0.63310433, 0.59532105, 0.75913618, 0.60258213, 0.48668606],
       [0.69935925, 0.15807776, 0.8589115 , 0.37657828, 0.69651669],
       [0.87587399, 0.68772743, 0.59854082, 0.67857679, 0.34182774],
       [0.3734155 , 0.06255165, 0.02622334, 0.17993743, 0.1783275 ]])

answered May 18, 2021 at 1:15

Randy

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2 Comments

Julien Over a year ago

Any explanation as to why OP's code doesn't work when both x[row_idx] = 0.0 and x[x < 0.5] = 0.0 do work?

Randy Over a year ago

His second indexing turns it into a 1D array, which you can see if you exclude the assignment.

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