Styjun

I'm having problem with this code. I think I'm doing something wrong.

 import numpy as np

 array = np.zeros(10)

 arrays = []

 for i in range(len(array)):
 array[i] = 1
 arrays.append(array)

 print(arrays[0])

I was expecting to get:[1. 0. 0. 0. 0. 0. 0. 0. 0. 0.]
But I'm getting:[1. 1. 1. 1. 1. 1. 1. 1. 1. 1.]

That is the last array I appended to arrays, and not the first one. Why is that happening and more important what can I do to get the desired output?

I think you are expecting:

arrays.append(array)

to add a COPY of your main array to the arrays list. But that's not what you're doing. You're pushing another reference to the same array each time you do:

arrays.append(array)

so at the end of your loop, you have the list arrays with 10 references to the same original array you created. By then, you've set every value of that ONE ARRAY to 1. So you get that the first value in arrays contains an array with every value set to 1 because every array in arrays is that same array.

If you actually copy a new array each time into arrays, I bet you'll get what you expected. To do that, change that line to:

arrays.append(array.copy())

Here's a complete version of your program with this fix. I changed it also to print all 10 of the arrays in arrays:

def main():
 import numpy as np

 array = np.zeros(10)

 arrays = []

 for i in range(len(array)):
 array[i] = 1
 arrays.append(array.copy())

 for array in arrays:
 print(array)

Result:

[1. 0. 0. 0. 0. 0. 0. 0. 0. 0.]
[1. 1. 0. 0. 0. 0. 0. 0. 0. 0.]
[1. 1. 1. 0. 0. 0. 0. 0. 0. 0.]
[1. 1. 1. 1. 0. 0. 0. 0. 0. 0.]
[1. 1. 1. 1. 1. 0. 0. 0. 0. 0.]
[1. 1. 1. 1. 1. 1. 0. 0. 0. 0.]
[1. 1. 1. 1. 1. 1. 1. 0. 0. 0.]
[1. 1. 1. 1. 1. 1. 1. 1. 0. 0.]
[1. 1. 1. 1. 1. 1. 1. 1. 1. 0.]
[1. 1. 1. 1. 1. 1. 1. 1. 1. 1.]

just add this change:

arrays.append(np.array(array))

The actual way to do this in numpy is with np.tri():

np.tri(10)
Out[]:
array([[ 1., 0., 0., ..., 0., 0., 0.],
 [ 1., 1., 0., ..., 0., 0., 0.],
 [ 1., 1., 1., ..., 0., 0., 0.],
 ..., 
 [ 1., 1., 1., ..., 1., 0., 0.],
 [ 1., 1., 1., ..., 1., 1., 0.],
 [ 1., 1., 1., ..., 1., 1., 1.]])

Maybe you are looking for this , just added if condition in your code

import numpy as np

array = np.zeros(10)

arrays = []

for i in range(len(array)):
 if i==0:
 array[i] = 1
 arrays.append(array)

print(arrays[0])
out: [1. 0. 0. 0. 0. 0. 0. 0. 0. 0.]

You can use array.copy() a method defined on numpy arrays as @Steve has suggested.

As it has been already used in one of the answer (@Steve's answer) to this problem so I choose another approach i.e. deepcopy() functionto obtain the result.

import numpy as np 
from copy import deepcopy

array = np.zeros(10)
arrays = []

for i in range(len(array)):
 array[i] = 1
 arrays.append(deepcopy(array))

print(arrays)
# [array([1., 0., 0., 0., 0., 0., 0., 0., 0., 0.]), array([1., 1., 0., 0., 0., 0., 0., 0., 0., 0.]), array([1., 1., 1., 0., 0., 0., 0., 0., 0., 0.]), array([1., 1., 1., 1., 0., 0., 0., 0., 0., 0.]), array([1., 1., 1., 1., 1., 0., 0., 0., 0., 0.]), array([1., 1., 1., 1., 1., 1., 0., 0., 0., 0.]), array([1., 1., 1., 1., 1., 1., 1., 0., 0., 0.]), array([1., 1., 1., 1., 1., 1., 1., 1., 0., 0.]), array([1., 1., 1., 1., 1., 1., 1., 1., 1., 0.]), array([1., 1., 1., 1., 1., 1., 1., 1., 1., 1.])]

print(arrays[0])
# [1. 0. 0. 0. 0. 0. 0. 0. 0. 0.]

print(arrays[-1])
# [1. 1. 1. 1. 1. 1. 1. 1. 1. 1.]