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I have numpy array with random numbers. For example like this [7 1 2 0 2 3 4 0 5] and I want to replace every number at the same time if number from this array = 7, I want to replace it with 2 , also if number = 2, replace it with 3. So it will be like [2 1 3 0 3 3 4 0 5] . I have tried it with np.where but can't change any of them.

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    what does your code look like? Commented Jun 10, 2021 at 20:07
  • Have a look at this answer - you would probably have to run two replace operations, so maybe something like myarray[myarray == 2] = 3 then myarray[myarray == 7] = 2 so that the values changed by the second condition aren't altered by the first replacement (unless that is the intent). Commented Jun 10, 2021 at 20:09
  • @Chris Hello. I am trying to write code that work like this tool sndeep.info/en/tools/checksum_calculator . And in the second section I have to change values. Commented Jun 10, 2021 at 20:20

2 Answers 2

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It's better to use np.select if you've multiple conditions:

a = np.array([7, 1, 2, 0, 2, 3, 4, 0, 5])
a = np.select([a == 7, a == 2], [2, 3], a)

OUTPUT:

[2 1 3 0 3 3 4 0 5]
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3 Comments

Thank you @Nk03 for your answer. What if when this array is generated randomly and I have formula for number change? I mean I want to do multiple changes but not always know which number will be first my be 7 will be second , so it will be replaced with 3 that I do not want that. I hope you understand what I mean.
I'm hardcoding both the values here. If you want dynamic values don't hardcode the values and replace them with suitable formula.
short and precise.
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Numpy provide the comparison to a scalar with the standard == operator, such that arr == v return a boolean array, often called a mask. The masked selection arr[arr == v] takes the subset of arr where the condition is met, so this snippet should work.

import numpy as np
arr = np.array([7, 1, 2, 0, 2, 3, 4, 0, 5])
arr[arr == 7] = 2
arr
array([2, 1, 2, 0, 2, 3, 4, 0, 5])
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2 Comments

what if I have multiple numbers to change , not two of them but every number in given array with some formula ? Thank you.
In this case it's possible to either join the masks, using and | : arr[(arr == 7) | (arr == 5)] = 2, or do it sequentially with statements similar to the ones in the answer.

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