MATLAB Programming/Differences between Octave and MATLAB

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Octave has been mainly built with MATLAB compatibility in mind. It essentially shares a lot of features in common with MATLAB:

1. Matrices as fundamental data type.
2. Built-in support for complex numbers.
3. Powerful built-in math functions and extensive function libraries.
4. Extensibility in the form of user-defined functions.

Some of the differences that do exist between Octave and MATLAB can be worked around using "user preference variables."^[1]

GNU Octave is mostly compatible with Matlab. However, Octave's parser allows some (often very useful) syntax that Matlab's does not, so programs written for Octave might not run in Matlab. For example, Octave supports the use of both single and double quotes. Matlab only supports single quotes, which means parsing errors will occur if you try to use double quotes (e.g. in an Octave script when run on Matlab). Octave and Matlab users who must collaborate with each other need to take note of these issues and program accordingly.

Note: Octave can be run in "traditional mode" (by including the --traditional flag when starting Octave) which makes it behave in a slightly more Matlab-compatible way.

This chapter documents instances where Matlab's parser will fail to run code that will run in Octave, and instances where Octave's parser will fail to run code that will run in Matlab. This page also contains notes on differences between things that are different between Octave (in traditional mode) and Matlab.

Tip for sharing .mat files between Octave and Matlab: always use save -V6 if you are using Matlab 7.X. Octave version 2.1.x cannot read Matlab 7.X .mat files. Octave 2.9.x can read Matlab 7.X .mat files.

Contents 1 load 2 C-Style Autoincrement and Assignment operators 3 product of booleans 4 nargin 5 startup.m 6 ['abc ';'abc'] 7 Calling Shells 8 hist 9 Getting Version Information 10 Format of printing to screen 11 Attempting to load empty files 12 fprintf and printf 13 Whitespace 14 Line continuation 15 Logical operators (And, Or, Not) 16 Some other differences 17 References

[edit] load

For compatablility, it is best to specify absolute paths of files for LOAD. Matlab (7.0) vs Octave (2.1.71): paths are not searched for .mat files in the same way as .m files:

Matlab

 a = 1; save /tmp/a.mat a ; addpath('/tmp'); load a.mat 
 % OK
 

Octave

 a = 1; save /tmp/a.mat a ;   
 LOADPATH=['/tmp:',LOADPATH];
 load a.mat 
 % error: load: nonexistent file: `a.mat'

For any other purpose, don't use absolute paths. It is bad programming style. Don't do it. It causes many problems.

[edit] C-Style Autoincrement and Assignment operators

Octave (3.0.1) supports C-style autoincrement and assignment operators:

  i++; ++i; i+=1; etc.

MatLab (7.0) does not.

[edit] product of booleans

Matlab (7.0) and Octave (3.0.2) responds differently when computing the product of boolean values:

  X = ones(2,2) ; prod(size(X)==1) 

  Matlab: ??? Function 'prod' is not defined for values of class 'logical'.
  Octave: ans = 0

[edit] nargin

Matlab (7.0) will not allow the following; Octave (2.1.71) will.

 function a = testfun(c)
  if (nargin == 1)
   nargin = 2;
  end

[edit] startup.m

Matlab will execute a file named startup.m in the directory it was called from on the command line. Octave does not. It will, however, execute a file named .octaverc.

[edit] ['abc ';'abc']

['abc ';'abc'] is allowed in Octave; Matlab returns: ?? Error using ==> vertcat

In Octave the result will be a 2 by 4 matrix where the last element of the last row is a space.

[edit] Calling Shells

the "! STRING" syntax calls a shell with command STRING in Matlab. Octave does not recognize !. Always use system(STRING) for compatibility.

If you really miss the one-character shortcut, for convenience on the command line you can create a similar shortcut by defining the following in your 2.9.x octave startup file:

  function S(a), system(a); end
  mark_as_rawcommand('S')

Now "S STRING" will evaluate the string in the shell.

[edit] hist

hist.m in Octave has a normalization input, Matlab does not.

[edit] Getting Version Information

Octave (2.1.7X) uses "OCTAVE_VERSION", Matlab uses "ver" for version informaton. (Octave 2.9.5 and later has "ver", so is compatible in this sense.)

[edit] Format of printing to screen

Cell arrays and structures print to screen in different format. There may be switches to make them the same, for example, struct_levels_to_print. None are set in --traditional mode, however.

[edit] Attempting to load empty files

MATLAB lets you load empty files, OCTAVE does not.

system('touch emptyfile') ; A = load('emptyfile')

  Matlab 6.5    : A=[]
  Octave 2.1.71 : error: load: file `emptyfile' seems to be empty!
                  error: load: unable to extract matrix size from file `emptyfile'

[edit] fprintf and printf

Matlab doesn't support `printf' as a command for printing to the screen.

   foo = 5;
   printf('My result is: %d\n', foo)

works in Octave, but not Matlab. If using Matlab, `fprintf' covers writing both to the screen and to a file:

   foo = 5;
   fprintf('My result is: %d\n', foo)

In Matlab, the omission of the optional file-handle argument to fprintf (or using the special value 1 for standard output or 2 for standard error) causes the text to be printed to the screen rather than to a file.

[edit] Whitespace

Matlab doesn't allow whitespace before the transpose operator.

   [0 1]'

works in Matlab, but

   [0 1] '

doesn't. Octave allows both cases.

[edit] Line continuation

Matlab always requires `...' for line continuation.

   rand (1, ...
         2)

while both

   rand (1,
         2)

and

   rand (1, \
         2)

work in Octave, in addition to `...'.

[edit] Logical operators (And, Or, Not)

Octave allows users to use two different group of logical operators: the ones used in Matlab, or the ones familiar to C/Java/etc programmers. If you use the latter, however, you'll be writing code that Matlab will not accept, so try to use the Matlab-compatible ones:

• For not-equal comparison, Octave can use '~=' or '!='. Matlab requires '~=' .

• For a logical-and, Octave can use `&' or `&&'; Matlab requires `&' . (note: Matlab supports `&&' and `||' as short-circuit logical operators since version 6.5.)

• For a logical-or, Octave can use `|' or `||'; Matlab requires `|' . (note: Octave's '||' and '&&' return a scalar, '|' and '&' return matrices)

[edit] Some other differences

• There used to be a difference in datestr between Octave-forge and Matlab. Has this been changed?

• MATLAB uses the percent sign to begin a comment. Octave uses both the pound sign and the percent sign interchangeably.

• MATLAB has no fputs function. Call fprintf instead.

• load in Matlab = load --force in Octave (should --force be default in --traditional?)

• page_output_immediately = 1 should this be default in --traditional?

• For exponentiation, Octave can use `^' or `**'; Matlab requires `^'.

• For string delimiters, Octave can use ` or "; Matlab requires '.

• For ends, Octave can use `end{if,for, ...}'; Matlab requires `end'.

• For "dbstep in" use "dbstep"; for "dbstep" use "dbnext"

• For "eig(A,B)" use "qz(A,B)"

• For gallery, compan, and hadamard install http://www.ma.man.ac.uk/~higham/testmat.html

• For subspace try http://www.mathworks.com/support/ftp/linalgv5.shtml

• For datetick use gnuplot commands:

 __gnuplot_set xdata time 
 __gnuplot_set timefmt "%d/%m" 
 __gnuplot_set format x "%b %d"

• If something (like Netlab) need a function called fcnchk you can just put the following into a file called fcnchk.m and put it somewhere Octave can find it:

function f=fcnchk(x, n)
  f = x;
end

[edit] References

• http://wiki.octave.org/wiki.pl?MatlabOctaveCompatibility