Change Values in One Array, Based On Value from Column of Second Array

Question

I have a one dimensional array called Y_train that contains a series of 1's and 0's. I have another array called sample_weight that is an array of all 1's that has the shape of Y_train, defined as:

sample_weight = np.ones(Y_train.shape, dtype=int)

I'm trying to change the values in sample_weight to a 2, where the corresponding value in Y_train == 0. So initially side by side it looks like:

Y_train        sample_weight
0              1
0              1
1              1
1              1
0              1
1              1

and I'd like it to look like this after the transformation:

Y_train        sample_weight
0              2
0              2
1              1
1              1
0              2
1              1

What I tried was to use a for loop (shown below) but none of the 1's are changing to 2's in sample_weight. I'd like to somehow use the np.where() function if possible, but it's not crucial, just would like to avoid a for loop:

sample_weight = np.ones(Y_train.shape, dtype=int)
for num, i in enumerate(Y_train):
    if i == 0:
        sample_weight[num] == 2

I tried using the solution shown here but to no success with the second array. Any ideas??? Thanks!

offeltoffel · Accepted Answer · 2020-01-18 01:01:35Z

2

import numpy as np

Y_train = np.array([0,0,1,1,0,1])
sample_weight = np.where(Y_train == 0, 2, Y_train)

>> print(sample_weight)
[2 2 1 1 2 1]

The np.where basically works just like Excel's "IF":

np.where(condition, then, else)

Works for transposed arrays, too:

Y_train = np.array([[0,0,1,1,0,1]]).T
sample_weight = np.where(Y_train == 0, 2, Y_train)

>> print(sample_weight)
[[2]
 [2]
 [1]
 [1]
 [2]
 [1]]

answered Jan 18, 2020 at 1:01

offeltoffel

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1 Comment

wildcat89 Over a year ago

Worked like a charm, and thank you for the quick lesson on where(). I learned proper syntax now. Thanks!

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