In-Place Quicksort in matlab

Question

I wrote a small quicksort implementation in matlab to sort some custom data. Because I am sorting a cell-array and I need the indexes of the sort-order and do not want to re

Answer 1

In this post, I only explain MATLAB function-calling convention, and am not discussing the quick-sort algorithm implementation.

When calling functions, MATLAB passes built-in data types by-value, and any changes made to such arguments are not visible outside the function.

function y = myFunc(x)
    x = x .* 2;         %# pass-by-value, changes only visible inside function
    y = x;
end

This could be inefficient for large data especially if they are not modified inside the functions. Therefore MATLAB internally implements a copy-on-write mechanism: for example when a vector is copied, only some meta-data is copied, while the data itself is shared between the two copies of the vector. And it is only when one of them is modified, that the data is actually duplicated.

function y = myFunc(x)
    %# x was never changed, thus passed-by-reference avoiding making a copy
    y = x .* 2;
end

Note that for cell-arrays and structures, only the cells/fields modified are passed-by-value (this is because cells/fields are internally stored separately), which makes copying more efficient for such data structures. For more information, read this blog post.

In addition, versions R2007 and upward (I think) detects in-place operations on data and optimizes such cases.

function x = myFunc(x)
    x = x.*2;
end

Obviously when calling such function, the LHS must be the same as the RHS (x = myFunc(x);). Also in order to take advantage of this optimization, in-place functions must be called from inside another function.

In MEX-functions, although it is possible to change input variables without making copies, it is not officially supported and might yield unexpected results...

For user-defined types (OOP), MATLAB introduced the concept of value object vs. handle object supporting reference semantics.