How to replace all the elements of a numpy array?

Question

Given a numpy array with multiple arrays inside, how do I replace all the values of the array with values from another array?

For example:

import numpy
first_array = numpy.array([[1,2],[3,4],[5,6],[7,8],[9,10]])

second_array = numpy.array([0.1, 0.2, 0.3, 0.4, 0.5, 0.6,
 0.7, 0.8, 0.9, 1])

Given these arrays, How do I replace 1,2 with 0.1, 0.2 and etc?

first_array = second_array.reshape((5,2)) is the easy way. As long as the total number of elements is the same, just reshape it. — Tim Roberts
– Tim Roberts, Commented Jul 29, 2021 at 20:50
What is the rule that tells you how to do the replacement? What is the exact desired result in this case, and how is it different from just using second_array directly? — Karl Knechtel
– Karl Knechtel, Commented Jul 29, 2021 at 20:50
@TimRoberts Probably better to use first_array.shape than hard-code (5,2). [EDIT: I now see that an answer has been posted, which does exactly this.] — alani
– alani, Commented Jul 29, 2021 at 20:58
first_array dtype is int, so you can't put floats in it. In any case reshaping the 2nd array is more efficient. — hpaulj
– hpaulj, Commented Jul 29, 2021 at 22:31

Corralien · Accepted Answer · 2021-07-29 20:55:09Z

1

Use np.reshape

# import numpy as np

>>> m
array([[ 1,  2],
       [ 3,  4],
       [ 5,  6],
       [ 7,  8],
       [ 9, 10]])

>>> n
array([0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1. ])

>>> n.reshape(m.shape)
array([[0.1, 0.2],
       [0.3, 0.4],
       [0.5, 0.6],
       [0.7, 0.8],
       [0.9, 1. ]])

answered Jul 29, 2021 at 20:55

Corralien

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Karina · Accepted Answer · 2021-07-29 21:03:30Z

0

first_array = np.array([[1,2],[3,4],[5,6],[7,8],[9,10]])
second_array = np.array([0.1, 0.2, 0.3, 0.4, 0.5, 0.6,0.7, 0.8, 0.9, 1])

np.set_printoptions(formatter={'float':"{0:0.1f}".format})
first_array = first_array.astype(float)

for i in range(np.shape(first_array)[0]):
    for j in range(np.shape(first_array)[1]):
        first_array[i][j] = second_array[2*i+j]

print(first_array)

Output:

[[0.1 0.2]
 [0.3 0.4]
 [0.5 0.6]
 [0.7 0.8]
 [0.9 1.0]]

answered Jul 29, 2021 at 21:03

Karina

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3 Comments

Karina Over a year ago

Of course, replacing a whole array with another array is silly, but what if I purposely create a np.zeros array, and then replacing it element by element with element from another calculation, I mean instead of append just do element replacement. (maybe the asker just write the question as a whole array with another array replacement, but actually what he wanted is just to know generally how to do element by element replacement array[i][j] =)

Tim Roberts Over a year ago

There's a debate to be had. If you're working with Python lists, then it's almost always way better to create a new list from scratch rather than replacing each element one by one. With numpy arrays, it's not always that straightforward, but the reshape solution is obviously much better than element-wise replacement.

Karina Over a year ago

You are right, it's almost always way better, until the homework explicitly state that we need to do element by element replacement (real example in my case: making hysteresis loop with 1000 MonteCarlo steps, where the initial array keep changing over time)

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