(octave.info)Miscellaneous Techniques

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19.6 Miscellaneous Techniques
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Here are some other ways of improving the execution speed of Octave
programs.

   • Avoid computing costly intermediate results multiple times.  Octave
     currently does not eliminate common subexpressions.  Also, certain
     internal computation results are cached for variables.  For
     instance, if a matrix variable is used multiple times as an index,
     checking the indices (and internal conversion to integers) is only
     done once.

   • Be aware of lazy copies (copy-on-write).  When a copy of an object
     is created, the data is not immediately copied, but rather shared.
     The actual copying is postponed until the copied data needs to be
     modified.  For example:

          a = zeros (1000); # create a 1000x1000 matrix
          b = a; # no copying done here
          b(1) = 1; # copying done here

     Lazy copying applies to whole Octave objects such as matrices,
     cells, struct, and also individual cell or struct elements (not
     array elements).

     Additionally, index expressions also use lazy copying when Octave
     can determine that the indexed portion is contiguous in memory.
     For example:

          a = zeros (1000); # create a 1000x1000 matrix
          b = a(:,10:100);  # no copying done here
          b = a(10:100,:);  # copying done here

     This applies to arrays (matrices), cell arrays, and structs indexed
     using ‘()’.  Index expressions generating comma-separated lists can
     also benefit from shallow copying in some cases.  In particular,
     when A is a struct array, expressions like ‘{a.x}, {a(:,2).x}’ will
     use lazy copying, so that data can be shared between a struct array
     and a cell array.

     Most indexing expressions do not live longer than their parent
     objects.  In rare cases, however, a lazily copied slice outlasts
     its parent, in which case it becomes orphaned, still occupying
     unnecessarily more memory than needed.  To provide a remedy working
     in most real cases, Octave checks for orphaned lazy slices at
     certain situations, when a value is stored into a "permanent"
     location, such as a named variable or cell or struct element, and
     possibly economizes them.  For example:

          a = zeros (1000); # create a 1000x1000 matrix
          b = a(:,10:100);  # lazy slice
          a = []; # the original "a" array is still allocated
          c{1} = b; # b is reallocated at this point

   • Avoid deep recursion.  Function calls to m-file functions carry a
     relatively significant overhead, so rewriting a recursion as a loop
     often helps.  Also, note that the maximum level of recursion is
     limited.

   • Avoid resizing matrices unnecessarily.  When building a single
     result matrix from a series of calculations, set the size of the
     result matrix first, then insert values into it.  Write

          result = zeros (big_n, big_m)
          for i = over:and_over
            ridx = ...
            cidx = ...
            result(ridx, cidx) = new_value ();
          endfor

     instead of

          result = [];
          for i = ever:and_ever
            result = [ result, new_value() ];
          endfor

     Sometimes the number of items can not be computed in advance, and
     stack-like operations are needed.  When elements are being
     repeatedly inserted or removed from the end of an array, Octave
     detects it as stack usage and attempts to use a smarter memory
     management strategy by pre-allocating the array in bigger chunks.
     This strategy is also applied to cell and struct arrays.

          a = [];
          while (condition)
            ...
            a(end+1) = value; # "push" operation
            ...
            a(end) = []; # "pop" operation
            ...
          endwhile

   • Avoid calling ‘eval’ or ‘feval’ excessively.  Parsing input or
     looking up the name of a function in the symbol table are
     relatively expensive operations.

     If you are using ‘eval’ merely as an exception handling mechanism,
     and not because you need to execute some arbitrary text, use the
     ‘try’ statement instead.  Note: The try Statement.

   • Use ‘ignore_function_time_stamp’ when appropriate.  If you are
     calling lots of functions, and none of them will need to change
     during your run, set the variable ‘ignore_function_time_stamp’ to
     "all".  This will stop Octave from checking the time stamp of a
     function file to see if it has been updated while the program is
     being run.