I have a list of lists of numpy arrays. I would like to loop through the list of lists and change certain elements. Below is an example, but just with integers: The i==j element has been changed to something else. When I run the exact same code logic, but with 2x2 numpy arrays, I cannot get it to do the

Changing specific elements in a list of list of numpy arrays

I have a list of lists of numpy arrays. I would like to loop through the list of lists and change certain elements. Below is an example, but just with integers:

    a = [[1, 1, 1], [1, 1, 1], [1, 1, 1]]

    print(a)

    for i in range(len(a)):
         for j in range(i+1):
                if i == j:
                    a[i][j] = 100

    print(a)

This is the output:[[1, 1, 1], [1, 1, 1], [1, 1, 1]][[100, 1, 1], [1, 100, 1], [1, 1, 100]]

The i==j element has been changed to something else. When I run the exact same code logic, but with 2×2 numpy arrays, I cannot get it to do the same thing:

    L=3
    I = np.eye(2, dtype = int)
    sigplus = np.array([[0,2],[0,0]])
    plusoperatorlist=[[I]*L]*L

    print(plusoperatorlist)
    for i in range(len(plusoperatorlist)):
        for j in range(i+1):
                if i == j:
                    plusoperatorlist[i][j] = sigplus
    
    print(plusoperatorlist)

This is the output:[[array([[1, 0],[0, 1]]), array([[1, 0],[0, 1]]), array([[1, 0],[0, 1]])], [array([[1, 0],[0, 1]]), array([[1, 0],[0, 1]]), array([[1, 0],[0, 1]])], [array([[1, 0],[0, 1]]), array([[1, 0],[0, 1]]), array([[1, 0],[0, 1]])]][[array([[0, 2],[0, 0]]), array([[0, 2],[0, 0]]), array([[0, 2],[0, 0]])], [array([[0, 2],[0, 0]]), array([[0, 2],[0, 0]]), array([[0, 2],[0, 0]])], [array([[0, 2],[0, 0]]), array([[0, 2],[0, 0]]), array([[0, 2],[0, 0]])]]

It changes every element to the matrix sigplus that I defined. The desired output is to still have the list of lists of identity matrices, but have the sigplus matrix at the i==j position in the list of lists. Help is much appreciated :)

I have outlined what I tried above.

Answer

The issue is that you created a nested list by multiplication of the original list. The reference to the original list will remain the same under the hood, which means seemingly changing one of the lists will actually change all of them. Try generating your nested list like:

plusoperatorlist= [[I for _ in range(L)] for _ in range(L)]

Now, every nested list has its own reference and can be changed independently.

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Answer