## 10 Porting programs from Matlab to Octave

People often ask

“I wrote some code for Matlab, and I want to get it running under Octave. Is there anything I should watch out for?”

or alternatively

“I wrote some code in Octave, and want to share it with Matlab users. Is there anything I should watch out for?”

which is not quite the same thing. There are still a number of differences between Octave and Matlab, however in general differences between the two are considered as bugs. Octave might consider that the bug is in Matlab and do nothing about it, but generally functionality is almost identical. If you find a difference between Octave behavior and Matlab, then you should send a description of this difference (with code illustrating the difference, if possible) to bug@octave.org.

Furthermore, Octave adds a few syntactical extensions to Matlab that might cause some issues when exchanging files between Matlab and Octave users. As both Octave and Matlab are under constant development the information in this section is subject to change at anytime.

You should also look at the page http://octave.sourceforge.net/packages.html and http://octave.sourceforge.net/doc/ that has a function reference that is up to date. You can use this function reference to see the number of octave function that are available and their Matlab compatibility.

The major differences between Octave 2.9.N and Matlab R2007a are:

- Nested Functions

  Octave doesn't yet have nested functions. That if

                function y = foo (x)
                   y = bar(x)
                   function y = bar (x)
                      y = ...;
                   end
                end

  There was discussion in Octave of having these even prior to Matlab, and the decision was made not to have these in Octave at the time for compatibility. The above written with sub-functions functions would be

                function y = foo (x)
                   y = bar(x)
                end
                function y = bar (x)
                   y = ...;
                end

  Now that Matlab has recently introduced nested functions, Octave will probably have them soon as well. Till then nested functions in Octave are treated as sub-functions with the same scoping rules as sub-functions.

  The authors of Octave consider the nested function scoping rules of Matlab to be more problems than they are worth as they introduce diffiult to find bugs as inadvertantly modifying a variable in a nested function that is also used in the parent is particularly easy.

- Differences in core syntax

  There a few core Matlab syntaxes that are not accepted by Octave, these being

  - Some limitations on the use of function handles. The major difference is related to nested function scoping rules (as above) and there use with function handles.
  - Some limitations of variable argument lists on the LHS of an expression, though the most common types are accepted.
  - Matlab object-oriented classes with overloading etc through the subasgn and "@" directories is not treated by Octave. There is work to remove this restriction in the future.
  - Matlab private directories are not treated, though as for classes, this will change in the near future. This restriction alleviated by using addpath on the private directories, but this makes the private directories visible in the global scope.
- Differences in core functions

  A large number of the Matlab core functions (ie those that are in the core and not a toolbox) are implemented, and certainly all of the commonly used ones. There are a few functions that aren't implemented, for example condest or to do with specific missing Octave functionality (gui, dll, java, activex, dde, web, and serial functions). Some of the core functions have limitations that aren't in the Matlab version. For example the `sprandn` function can not force a particular condition number for the matrix like Matlab can.

- Just-In-Time compiler

  Matlab includes a "Just-In-Time" compiler. This compiler allows the acceleration of for-loops in Matlab to almost native performance with certain restrictions. The JIT must know the return type of all functions called in the loops and so you can't include user functions in the loop of JIT optimized loops. Octave doesn't have a JIT and so to some might seem slower than Matlab. For this reason you must vectorize your code as much as possible. Matlab themselves produces a good document discussing vectorization at http://www.mathworks.com/support/tech-notes/1100/1109.html.

- Compiler

  On a related point, there is no Octave compiler, and so you can't convert your Octave code into a binary for additional speed or distribution. There is an example of how to do this at http://www.stud.tu-ilmenau.de/~rueckn/, but this is a very early example code and would need lots of work to complete it.

- Graphic Handles

  Up to Octave 2.9.9 there was no support for graphic handles in Octave itself. There is in Octave 2.9.10 and later, and so the graphics between Octave and Matlab are currently in the process of converging to a common interface. Note that the basic graphic handle stuff is in place since 2.9.10, but not graphics objects like "patch", "barseries", etc.

- GUI

  There is no Matlab compatible GUI functions. There are a number of bindings from Octave to tcl/tk, vtk and zenity included in the octave-forge project (http://octave.sourceforge.net) for example that can be used for a GUI, but these are not Matlab compatible. This might be an issue if you intend to exchange Octave code with Matlab users.

- Simulink

  Octave itself includes no Simulink support. Typically the simulink models lag research and are less flexible, so shouldn't really be used in a research environment. However, some Matlab users that try to use Octave complain about this lack. There is a similar package to simulink for the Octave and R projects available at http://www.scicraft.org/

- Mex-Files

  Octave includes an API to the matlab MEX interface. However, as MEX is an API to the internals of Matlab and the internals of Octave differ to Matlab, there is necessarily a manipulation of the data to convert from a MEX interface to the Octave equivalent. This is notable for all complex matrices, where matlab stores complex arrays as real and imaginary parts, whereas octave respects the C99/C++ standards of co-locating the real/imag parts in memory. Also due to the way matlab allows access to the arrays passed through a pointer, the MEX interface might require that copies of arrays (even non complex ones). There were some serious memory leaks in the MEX API in Octave up to version 2.9.9, and version 2.9.10 or later should be used if posible.

- Block comments

  Matlab recently included the possibility to have block comments. With the "%{" and "%}" markers. Octave doesn't yet have block comments, but might in the future.

- Mat-File format

  There are some differences in the mat v5 file format accepted by Octave. Matlab recently introduced the "-V7.3" save option which is an hdf5 format which is particularly useful for 64-bit platforms where the standard matlab format can not correctly save variables.. Octave accepts hdf5 files, but is not yet compatible with the "-v7.3" versions produced by Matlab.

  Up to version 2.9.10, Octave can't load/save the inline or function handles saved in mat-files by Matlab (though can in its own format). In 2.9.11 and later Octave can load inline and function handles saved in the mat-file formats, but can't save them to mat-files.

  Finally, Some multi-byte unicode characters aren't yet treated in mat-files.

- Profiler

  Octave doesn't have a profiler. Though there is a patch for a flat profiler, that might become a real profiler sometime in the future. see the thread

  http://www.cae.wisc.edu/pipermail/octave-maintainers/2007-January/001685.html

  for more details

- Toolboxes

  Octave is a community project and so the toolboxes that exist are donated by those interested in them through the octave-forge website (http://octave.sourceforge.net). These might be lacking in certain functionality relative to the Matlab toolboxes, and might not exactly duplicate the matlab functionality or interface.

- Short-circuit & and | operators

  The & and | operators in Matlab short-circuit when included in an if statemant and not otherwise. Whereas in Octave only the && and || short circuit. This is due to the absence of && and || in Matlab till recently. Note that this means that

                  if (a | b)
                    ...
                  end

  and

                  t = a | b;
                  if t
                    ...
                  end

  are different in Matlab. This is really a Matlab bug, but there is too much code out there that relies on this behavior to change it. Prefer the || and && operators in if statements if possible.

  Note that the difference is also significant when either argument is a function with side effects or if the first argument is a scalar and the second argument is an empty matrix. For example, note the difference between

                  t = 1 | [];          ## results in [], so...
                  if (t) 1, end        ## in if ([]), this is false.

  and

                  if (1 | []) 1, end   ## short circuits so condition is true.

  Another case that is documented in the Matlab manuals is that

                  t = [1, 1] | [1, 2, 3];          ## error
                  if ([1, 1] | [1, 2, 3]) 1, end   ## OK

  Also Matlab requires the operands of && and || to be scalar values but Octave does not (it just applies the rule that for an operand to be considered true, every element of the object must be nonzero or logically true).

  Finally, note the inconsistence of thinking of the condition of an if statement as being equivalent to `all(X(:))` when X is a matrix. This is true for all cases EXCEPT empty matrices:

                  if ([0, 1]) == if (all ([0, 1]))   ==>  i.e., condition is false.
                  if ([1, 1]) == if (all ([1, 1]))   ==>  i.e., condition is true.

  However,

                  if ([]) != if (all ([]))

  because `all ([]) == 1` (because, despite the name, it is really returning true if none of the elements of the matrix are zero, and since there are no elements, well, none of them are zero). But, somewhere along the line, someone decided that if `([])` should be false. Mathworks probably thought it just looks wrong to have `[]` be true in this context even if you can use logical gymnastics to convince yourself that "all" the elements of a matrix that doesn't actually have any elements are nonzero. Octave however duplicates this behavior for if statements containing empty matrices.

- Octave extensions

  The extensions in Octave over Matlab syntax are very useful, but might cause issues when sharing with Matlab users. A list of the major extensions that should be avoided to be compatible with Matlab are

  - Comments in octave can be marked with "#". This allows POSIX systems to have the first line as "#! octave -q" and mark the script itself executable. Matlab doesn't have this feature due to the absence of comments starting with "#"
  - Code blocks like if, for, while, etc can be terminated with block specific terminations like "endif". Matlab doesn't have this and all blocks must be terminated with "end"
  - Octave has a lisp like unwind_protect block that allows blocks of code that terminate in an error to ensure that the variables that are touched are restored. You can do something similar with try/catch combined with rethrow(lasterror()) in Matlab, however rethrow and lasterror are only available in Octave 2.9.10 and later.

    Note that using try/catch combined with rethrow(lasterror()) can not guarentee that global variables will be correctly reset, as it won't catch user interrupts with Ctrl-C. For example

                         global a
                         a = 1;
                         try
                           _a = a;
                           a = 2
                           while true
                           end
                         catch
                           fprintf ('caught interrupt\n');
                           a = _a;
                           rethrow (lasterror());
                         end

    compared to

                         global a
                         a = 1;
                         unwind_protect
                           _a = a;
                           a = 2
                           while true
                           end
                         unwind_protect_cleanup
                           fprintf ('caught interrupt\n');
                           a = _a;
                         end

    Typing Ctrl-C in the first case returns the user directly to the prompt, and the variable "a" is not reset to the save value. In the second case the variable "a" is reset correctly. Therefore matlab gives no save way of temporarily changing global variables.

  - Indexing can be applied to all objects in Octave and not just variable. Therefore `sin(x)(1:10);` for example is perfectly valid in Octave but not Matlab. To do the same in Matlab you must do `y = sin(x); y = y([1:10]);`
  - Octave has the operators "++", "–", "-=", "+=", "*=", etc. As Matlab doesn't, if you are sharing code these should be avoided.
  - Strings in Octave can be denoted with double or single quotes. There is a subtle difference between the two in that escaped characters like \n, etc are interpreted in double quoted strings but not single quoted strings. This difference is important on Windows platforms where the "\" character is used in path names, and so single quoted strings should be used in paths. Matlab doesn't have double quoted strings and so they should be avoided if the code will be transfered to a matlab user.