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Asked by Matt J
on 30 Dec 2012

Any theories as to why SORT + row re-ordering outperforms SORTROWS when sorting with respect to a single column:

x=rand(18e6,7);

tic; [~,idx]=sort(x(:,1)); y1=x(idx,:); toc %Elapsed time is 3.589928 seconds.

tic; y2=sortrows(x,1); toc %Elapsed time is 14.869106 seconds.

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Answer by Jan Simon
on 30 Dec 2012

Edited by Jan Simon
on 30 Dec 2012

Accepted answer

The profile shows, that the additional time is spent in the C-Mex function `sortrowsc.mexw64`. At least in R2009a the source code has been shipped with Matlab. Inside the C-Mex, the not stable `qsort` is called, which is obviously slower than Matlab's `sort` algorithm.

Copy `sortrows.m` and change the line 74 of from:

ndx = sortrowsc(x_sub, col);

to:

ndx = sort_back_to_front(x_sub, col);

to let the main work be done by `sort`. For a productive use, cells must be considered also, see some lines later for the general case in the `else` branch. It is ironic, that this is the "old Matlab 6.0" fallback, which has been obviously much faster.

Please send an enhancement request to TMW.

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