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Comment by imurray | Hacker News Reader

A different (probably slightly cheaper) hack to get a safe copy of an array is:

    A = X; A(1) = A(1);

Then one can evilly edit A in-place in a mex file without accidentally editing other arrays like X. However, the docs tell us never to edit arrays from input arguments in place, so a future version of Matlab could be cleverer and still break things. I wish their mex API would provide a supported way to edit things in-place. Native Matlab code does allow inplace editing:

   A = fun(A);

If function fun is defined that way too, A can be altered in-place by native matlab code, but not (officially) by mex functions.

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What Matlab function edits its inputs in-place? I cannot think of any.

Actually Matlab's pass by value is one of its best properties (as a language). This is how functions work in math and is much easier to reason about.

I really dislike Julia's pass by reference semantics. Even if it enables better performance.

Here's an example:

    A = plus_diag(A, x);  % Using [1] below

Adds a vector x onto the diagonal elements of a huge matrix A. In old versions of Matlab, 2*huge memory would temporarily be allocated. Now the memory usage doesn't increase: A is altered in place. This optimization happens automatically when Matlab notices it can be done. The code has to be in a function though, not run from the command-line or a script.

[1] http://homepages.inf.ed.ac.uk/imurray2/code/imurray-matlab/p...

The fact that A is updated in-place is an optimization that happens only if you pass a variable and assign the output to the same variable.

It is not a characteristic of the function `plus_diag`. The function itself is completely pure.

I'm well aware of how and when it works. It is the recognized idiom to perform in-place operations in practice in Matlab code [1].

It would be nice to take advantage of the same idea in mex code where possible. For example an API function that indicates if it is safe to over-write an input array and return it as the corresponding output or not. I don't think Mathworks currently provide documented and future-safe means to do so however. (If one controls all the code, updating things in-place is possible in practice if careful, using the A(1)=A(1); trick I gave in a parent post. Such code is just not guaranteed to be future-proof.)

[1] http://blogs.mathworks.com/loren/2007/03/22/in-place-operati...

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