MATLAB Programming/Differences between Octave and MATLAB

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.

1 load
2 prod
3 nargin
4 startup.m
5 ['abc ';'abc']
6 Calling Shells
7 String comparisons
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 Subtracting the mean
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] prod

Matlab (7.0) and Octave (2.1.71) respond differently to the following (which was a typo - it's meaningless to ask for the prod of a boolean value):

  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 in --traditional mode; Matlab rerturns: ?? Error using ==> vertcat

[edit] Calling Shells

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

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] String comparisons

In MATLAB you must use special comparison functions for strings, rather than standard comparison functions, because the strings are treated as arrays of characters and unlike other arrays, they are often of significantly varying length. OCTAVE automatically adjusts to this without use of special functions. For example, you can do this in OCTAVE but not in MATLAB.

b=find('abc' == '\n')

   Matlab 6.5    : ??? Error using ==> eq Array dimensions must match for binary array op.
   Octave 2.1.71 : b=[]

[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 ths 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] Subtracting the mean

Matlab understands the intuitive concept of subtracting the mean from each row/column, even though the result of the "mean" function gives a differently-shaped matrix:

x = z - mean(z,2)

In Octave (v2.9.9 tested) you must make more explicit how the matrices should be combined, by repeating the "mean" matrix appropriately:

x = z - repmat(mean(z,2), 1, size(z,2))

This specific example is also covered in both Matlab and Octave by the "center" function.

[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 a logical-and, Octave can use `&' or `&&'; Matlab requires `&'.

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

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"

[edit] References

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

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MATLAB Programming/Differences between Octave and MATLAB

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