CODEWITHSUNDEEP
pythonnumpy
Dan C
Dan C

Reputation: 41

Adding numpy array elements to new array only if conditions met

I need to copy elements from one numpy array to another, but only if a condition is met. Let's say I have two arrays:

x = ([1,2,3,4,5,6,7,8,9])
y = ([])

I want to add numbers from x to y, but only if they match a condition, lets say check if they are divisible by two. I know I can do the following:

y = x%2 == 0

which makes y an array of values 'true' and 'false'. This is not what I am trying to accomplish however, I want the actual values (0,2,4,6,8) and only those that evaluate to true.

Upvotes: 1

Views: 2232

Answers (1)

sacuL
sacuL

Reputation: 51335

You can get the values you want like this:

import numpy as np
x = np.array([1,2,3,4,5,6,7,8,9])
# array([1, 2, 3, 4, 5, 6, 7, 8, 9])

y = x[x%2==0]
# y is now: array([2, 4, 6, 8])

And, you can sum them like this:

np.sum(x[x%2==0])
# 20

Explanation: As you noticed, x%2==0 gives you a boolean array array([False, True, False, True, False, True, False, True, False], dtype=bool). You can use this as a "mask" on your original array, by indexing it with x[x%2==0], returning the values of x where your "mask" is True. Take a look at the numpy indexing documentation for more info.

Upvotes: 2

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