Matlab’s built-in cellfun function has traditionally enabled several named (string) processing functions such as ‘isempty’. The relevant code would look like this:
data = cellfun('isempty',cellArray);
In recent years, newer Matlab releases has added support for function handles, so the previous code snippet can now be written as follows:
data = cellfun(@isempty,cellArray);
The newer function-handle format is “cleaner” and more extensive than the former format, accepting any function, not just the limited list of pre-specified processing function names (‘isreal’, ‘islogical’, ‘length’, ‘ndims’, ‘prodofsize’). Some have even reported that the older format has limitations vis-a-vis compilation etc.
All this is well known and documented. However, it turns out that, counter-intuitively (and undocumented), the older format is actually much faster than the newer format for those pre-specified processing function names. The reason appears to be that ‘isempty’ (as well as the other predefined string functions) uses specific code-branches optimized for performance:
>> c = mat2cell(1:1e6,1,repmat(1,1,1e6)); >> tic, d=cellfun('isempty',c); toc Elapsed time is 0.115583 seconds. >> tic, d=cellfun(@isempty,c); toc Elapsed time is 7.493989 seconds.
Perhaps a future Matlab release will improve cellfun’s internal code, to check for function-handle equality to the optimized functions, and use the optimized code branch if possible. When I posted this issue today as a correction to a reader’s misconception, Matlab’s Loren Shure commented as follows:
We could improve cellfun to check function handles to see if they match specified strings. Even then MATLAB would have to be careful in case the user has overridden the built-in version of whatever the string points to.
While this comment seems to imply that the performance boost feature will be maintained and possibly improved in future releases, users should note that this is not guarantied. One could even argue that future code optimizations would be applied to the new (function-handle) format rather than the old string format. The performance pendulum might also change based on user platform. Therefore, users for whom performance is critical should always test both versions on their target system: ‘isempty’ vs. @isempty etc. (note that the corresponding function for ‘prodofsize’ is @numel).