How to Use Boolean Array to Replace Specific Indices in Python

Question

I have a numpy and a boolean array:

nparray = [ 12.66  12.75  12.01  13.51  13.67 ]
bool = [ True False False True True ]

I would like to replace all the values in nparray by the same value divided by 3 where bool is False.

I am a student, and I'm reasonably new to python indexing. Any advice or suggestions are greatly appreciated!

Grr · Accepted Answer · 2016-05-12 17:04:53Z

2

naming an array bool might not be the best idea. As ayhan did try renaming it to bl or something else.

You can use numpy.where see the docs here

nparray2 = np.where(bl == False, nparray/3, nparray)

answered May 12, 2016 at 17:04

Grr

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1 Comment

morepenguins Over a year ago

Thank you so much! This is really helpful.

user2285236 · Accepted Answer · 2016-05-12 16:53:44Z

1

Use boolean indexing with ~ as the negation operator:

arr = np.array([12.66, 12.75, 12.01, 13.51, 13.67 ])
bl = np.array([True, False, False, True ,True])

arr[~bl] = arr[~bl] / 3
array([ 12.66      ,   4.25      ,   4.00333333,  13.51      ,  13.67      ])

answered May 12, 2016 at 16:53

user2285236

2 Comments

morepenguins Over a year ago

Thank you so much! This is a really helpful tool.

evn Over a year ago

It may be worth noting that even though you may set values in the original array while accessing it through Boolean Indexing, this same Boolean Indexing notation cannot be used to store a view of the original array in a variable. Instead the variable will become a copy of this view. See this answer: stackoverflow.com/a/47671637

diegueus9 · Accepted Answer · 2016-05-12 17:10:12Z

0

With just python you can do it like this:

nparray = [12.66, 12.75, 12.01, 13.51, 13.67]
bool = [True, False, False, True, True]
map(lambda x, y: x if y else x/3.0, nparray, bool)

And the result is:

[12.66, 4.25, 4.003333333333333, 13.51, 13.67]

answered May 12, 2016 at 17:10

diegueus9

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