operate only on filtered elements in an array in python

Question

Consider this simple problem: in a list of integers I need multiply all even number by 10. I can certainly do element-wise operation such as:

[if x%2==0: x=x*10 for x in arr]

But what if I the operation has to be operated on the array level? The trouble I am having is after the operation on the filtered array, how do I nicely put them back to the original array?

Code example:

arr=np.arange(1,10) # the original array array([1, 2, 3, 4, 5, 6, 7, 8, 9])
filter1 = arr%2==0  # the filter 
arr1=arr[filter1]   # the filtered array array([2, 4, 6, 8])
arr1=arr1*10        # the 'array'-wise operation array([20, 40, 60, 80])


# this is the part I am trying to improve
i=0
j=0
arr2=[]
for f in filter1:
    if f:
        arr2.append(arr1[i])
        i=i+1
    else:
        arr2.append(arr[j])
    j=j+1

# output arr2: [1, 20, 3, 40, 5, 60, 7, 80, 9]

actual_panda · Accepted Answer · 2020-04-20 11:14:57Z

1

>>> a = np.arange(10)
>>> a
array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
>>> a[a%2 == 0] *= 10
>>> a
array([ 0,  1, 20,  3, 40,  5, 60,  7, 80,  9])

answered Apr 20, 2020 at 11:14

actual_panda

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Austin · Accepted Answer · 2020-04-20 11:15:35Z

1

This is how you can do element-wise operation on array based on a condition:

arr[arr%2 == 0] *= 10

Note this does not create new array but modifies the array.

If you want a new array as well, you can copy the array after the operation:

arr2 = arr.copy()

answered Apr 20, 2020 at 11:15

Austin

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