How do I change a copy of a cell array without changing the original cell array?

I have a 1x2 cell array of two collisionBox objects.

I want to create a copy of the cell array and then change the Pose matrices in the collisionBox objects.

However, when I change the copy, the original changes as well.

How to create a copy of a cell array that is not linked to the original?

ws_ = ws;
ws_{1}{1}.Pose(1,4) = -ws{1}{1}.Pose(1,4);
ws_{1}{1}.Pose(3,4) = -ws{1}{1}.Pose(3,4);
ws_{1}{2}.Pose(1,4) = -ws{1}{2}.Pose(1,4);
ws_{1}{2}.Pose(3,4) = -ws{1}{2}.Pose(3,4);

[Rik]

Your code should not change anything inside ws. What makes you think it does?

[Robert Pastor]

This is what I'm getting. I would expect ws not to be changed. But it returns the same value as ws_.

>> ws{1}{1}.Pose

ans =

    1.0000         0         0   -0.4800
         0    1.0000         0         0
         0         0    1.0000   -0.7800
         0         0         0    1.0000

>> ws_ = ws;
ws_{1}{1}.Pose(1,4) = -ws{1}{1}.Pose(1,4);
ws_{1}{1}.Pose(3,4) = -ws{1}{1}.Pose(3,4);
>> ws{1}{1}.Pose

ans =

    1.0000         0         0    0.4800
         0    1.0000         0         0
         0         0    1.0000    0.7800
         0         0         0    1.0000

>> ws_{1}{1}.Pose

ans =

    1.0000         0         0    0.4800
         0    1.0000         0         0
         0         0    1.0000    0.7800
         0         0         0    1.0000

[Rik]

As you can see below, I'm unable to reproduce your issue in the latest release. If I can't reproduce it, I can't troubleshoot.

The only possible way I can think of, is if ws{1}{1} contains an object of a handle class, not a normal double. Then the Pose method could be used to modify/return the exact same array, leading to this behavior.

ws{1}{1}.Pose=eye(4);
ws{1}{1}.Pose(1,4)=-0.4;
ws{1}{1}.Pose(3,4)=-0.78;

ws_ = ws;
ws_{1}{1}.Pose(1,4) = -ws{1}{1}.Pose(1,4);
ws_{1}{1}.Pose(3,4) = -ws{1}{1}.Pose(3,4);

ws{1}{1}.Pose

ans = 4×4

1.0000 0 0 -0.4000 0 1.0000 0 0 0 0 1.0000 -0.7800 0 0 0 1.0000

ws_{1}{1}.Pose

ans = 4×4

1.0000 0 0 0.4000 0 1.0000 0 0 0 0 1.0000 0.7800 0 0 0 1.0000

[Robert Pastor]

Yes, ws{1}{1} contains a collisionBox object from Robotics System Toolbox.

>> class(ws)

ans =

    'cell'

>> class(ws{1})

ans =

    'cell'

>> class(ws{1}{1})

ans =

    'collisionBox'

>> class(ws{1}{1}.Pose)

ans =

    'double'

[Robert Pastor]

After learning that this is not cell array specific, but rather handle class specific, I found this workaround here

copyStream = getByteStreamFromArray(objToCopy);
objCopy = getArrayFromByteStream(copyStream);

Works for cel array as well.

[Rik]

This has indeed nothing to do with the cell. There are two fundamental kinds of variables: handle classes and data classes. A handle will only contain a pointer to the data itself, so copying the variable will only create a new pointer to the same data. Copying a data variable will create a copy of the data itself (although for memory efficiency Matlab uses a copy-on-write system where a variable is only actually duplicated when you change an element).