Can't append matrix to array in python

Question

I've been writing some code in Python that should return an array of matrices from one starting matrix. Basically, you give the function a matrix and there are some if conditions that change certain numbers in it, then I add that matrix to the list of all matrices but it turns out that every time I add a new matrix I add the same exact one.

Here is my code:

arr = []
arr.append(staring_mat)

test = arr[:] # copying arr

matrix = staring_mat

for k in range(10):
  temporary = matrix
  for i in range(20):
    for j in range(20):
      number = do_something(i,j,temporary)
      if number < 15:
        temporary[i][j] = 12
      if number > 60:
        temporary[i][j] = 54
        .
        .
        .
  test.append(temporary)
  
  for num in range(len(test)):
    print(test[num])
    print('\n')
  
  matrix = temporary
  
 arr.extend(test)

JLeno46 · Accepted Answer · 2020-10-29 17:45:40Z

2

matrix = staring_mat
temporary = matrix

By doing this you're creating a shallow copy, changes to the copy also affects the original. You should create a deep copy, here's the docs.

test = arr[:]

Here you're creating a deep copy.

answered Oct 29, 2020 at 17:45

JLeno46

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