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POMatrix View in Function Doesn't Have Side Effects
 

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    <p><strong>Edit</strong>: I've found what the problem boils down to:</p> <p>If you run this code:</p> <pre><code>A = ones((10,4)) view = A[:,1] view.fill(7) A </code></pre> <p>or </p> <pre><code>A = ones((10,4)) view = A[:,1:3] view.fill(7) A </code></pre> <p>You'll see that the columns of A change</p> <p>If you run this:</p> <pre><code>A = ones((10,4)) view = A[:,(1,2)] view.fill(7) A </code></pre> <p>There's no side effects on A. Is this behavior on purpose or a bug?</p> <p>I have a function that calculates the amount I have to rotate certain columns of x,y points in a matrix. The function only takes one input - a matrix mat:</p> <pre><code>def rotate(mat): </code></pre> <p>In the function, I create views to make working with each section easier:</p> <pre><code>rot_mat = mat[:,(col,col+1)] </code></pre> <p>Then, I calculate a rotation angle and apply it back on the view that I had created before:</p> <pre><code>rot_mat[row,0] = cos(rot)*x - sin(rot)*y rot_mat[row,1] = sin(rot)*x + cos(rot)*y </code></pre> <p>If I perform this in the main body of my program, the changes to my rot_mat view would propagate to the original matrix mat. When I turned it into a function, the views stopped having side effects on the original matrix. What's the reasoning for this and is there any way to get around it? I should also note that it isn't changing mat within the function itself. At the end, I just try to return mat but no changes have been made.</p> <p>Full code for function:</p> <pre><code>def rotate(mat): # Get a reference shape ref_sh = 2*random.choice(range(len(filelist))) print 'Reference shape is ' print (ref_sh/2) # Create a copy of the reference point matrix ref_mat = mat.take([ref_sh,ref_sh+1],axis=1) # Calculate rotation for each set of points for col in range(len(filelist)): col = col * 2 # Account for the two point columns rot_mat = mat[:,(col,col+1)] # Numerator = sum of wi*yi - zi*xi numer = inner(ref_mat[:,0],rot_mat[:,1]) - inner(ref_mat[:,1],rot_mat[:,0]) # Denominator = sum of wi*xi + zi*yi denom = inner(ref_mat[:,0],rot_mat[:,0]) + inner(ref_mat[:,1],rot_mat[:,1]) rot = arctan(numer/denom) # Rotate the points in rot_mat. As it's a view of mat, the effects are # propagated. for row in range(num_points): x = rot_mat[row,0] y = rot_mat[row,1] rot_mat[row,0] = cos(rot)*x - sin(rot)*y rot_mat[row,1] = sin(rot)*x + cos(rot)*y return mat </code></pre>
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    1. COWhether or not the code is in a function should make no difference. Could you include a runnable example that demonstrates this behaviour?
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    2. CO@aix I added the full code for my function. I'm working on producing a runnable example now.
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    3. COThank you. I think that'll make it much easier for us to figure out what's going on.
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