[FIXED] array copy and view in numpy python

Issue

I am new to numpy.Recently only I started learning.I am doing one practice problem and getting error. Question is to replace all even elements in the array by -1.

import numpy as np
np.random.seed(123)
array6 = np.random.randint(1,50,20)
slicing_array6 = array6[array6 %2==0]
print(slicing_array6)
slicing_array6[:]= -1
print(slicing_array6)
print("Answer is:")
print(array6)

I am getting output as :

[46 18 20 34 48 10 48 26 20]
[-1 -1 -1 -1 -1 -1 -1 -1 -1]
Answer is:
[46  3 29 35 39 18 20 43 23 34 33 48 10 33 47 33 48 26 20 15]

My doubt is why original array not replaced? Thank you in advance for help

Solution

Explanation

Let's move step by step. We start with

array6 = np.array([46,  3, 29, 35, 39, 18, 20, 43, 23, 34, 33, 48, 10, 33, 47, 33, 48, 26, 20, 15])
print(array6 %2==0)

# array([ True, False, False, False, False,  True,  True, False, False,
#         True, False,  True,  True, False, False, False,  True,  True,
#         True, False])

You just made a mask (%2==0) to array6. Then you apply the mask to it and assign to a new variable:

slicing_array6 = array6[array6 %2==0]
print(slicing_array6)

# [46 18 20 34 48 10 48 26 20]

Note that this returns a new array:

print(id(array6))
# 2643833531968

print(id(slicing_array6))
# 2643833588112

print(array6)
# [46  3 29 35 39 18 20 43 23 34 33 48 10 33 47 33 48 26 20 15]
# unchanged !!

Final step, you assign all elements in slicing_array6 to -1:

slicing_array6[:]= -1
print(slicing_array6)
# [-1 -1 -1 -1 -1 -1 -1 -1 -1]

---

Solution

Instead of assigning masked array to a new variable, you apply new value directly to the original array:

array6[array6 %2==0] = -1
print(array6)
print(id(array6))

# [-1  3 29 35 39 -1 -1 43 23 -1 33 -1 -1 33 47 33 -1 -1 -1 15]
# 2643833531968
# same id !!

Answered By - AcaNg