GNU Octave Compatibility Reference

Table of Contents

1. Overview
2. Syntax Differences
3. Operator Differences
4. Function Differences
5. Features Unique to Octave
6. Features Missing in Octave
7. Writing Compatible Code
8. Octave Packages

Overview

GNU Octave is a free, open-source alternative to MATLAB with high compatibility. Most MATLAB scripts run in Octave with no or minimal modifications. However, there are some differences to be aware of.

Installation

# macOS (Homebrew)
brew install octave

# Ubuntu/Debian
sudo apt install octave

# Fedora
sudo dnf install octave

# Windows
# Download installer from https://octave.org/download

Running Octave

# Interactive mode
octave

# Run script
octave script.m
octave --eval "disp('Hello')"

# GUI mode
octave --gui

# Command-line only (no graphics)
octave --no-gui
octave-cli

Syntax Differences

Comments

% MATLAB style (works in both)
% This is a comment

# Octave style (Octave only)
# This is also a comment in Octave

% For compatibility, always use %

String Quotes

% MATLAB: Single quotes only (char arrays)
str = 'Hello';              % char array
str = "Hello";              % string (R2017a+)

% Octave: Both work, but different behavior
str1 = 'Hello';             % char array, no escape sequences
str2 = "Hello\
";           % Interprets \
 as newline

% For compatibility, use single quotes for char arrays
% Avoid double quotes with escape sequences

Line Continuation

% MATLAB style (works in both)
x = 1 + 2 + 3 + ...
    4 + 5;

% Octave also accepts backslash
x = 1 + 2 + 3 + \\
    4 + 5;

% For compatibility, use ...

Block Terminators

% MATLAB style (works in both)
if condition
    % code
end

for i = 1:10
    % code
end

% Octave also accepts specific terminators
if condition
    # code
endif

for i = 1:10
    # code
endfor

while condition
    # code
endwhile

% For compatibility, always use 'end'

Function Definitions

% MATLAB requires function in file with same name
% Octave allows command-line function definitions

% Octave command-line function
function y = f(x)
    y = x^2;
endfunction

% For compatibility, define functions in .m files

Operator Differences

Increment/Decrement Operators

% Octave has C-style operators (MATLAB does not)
x++;                        % x = x + 1
x--;                        % x = x - 1
++x;                        % Pre-increment
--x;                        % Pre-decrement

% For compatibility, use explicit assignment
x = x + 1;
x = x - 1;

Compound Assignment

% Octave supports (MATLAB does not)
x += 5;                     % x = x + 5
x -= 3;                     % x = x - 3
x *= 2;                     % x = x * 2
x /= 4;                     % x = x / 4
x ^= 2;                     % x = x ^ 2

% Element-wise versions
x .+= y;
x .-= y;
x .*= y;
x ./= y;
x .^= y;

% For compatibility, use explicit assignment
x = x + 5;
x = x .* y;

Logical Operators

% Both support
& | ~ && ||

% Short-circuit behavior difference:
% MATLAB: & and | short-circuit in if/while conditions
% Octave: Only && and || short-circuit

% For predictable behavior, use:
% && || for scalar short-circuit logic
% & | for element-wise operations

Indexing After Expression

% Octave allows indexing immediately after expression
result = sin(x)(1:10);      % First 10 elements of sin(x)
value = func(arg).field;    % Access field of returned struct

% MATLAB requires intermediate variable
temp = sin(x);
result = temp(1:10);

temp = func(arg);
value = temp.field;

% For compatibility, use intermediate variables

Function Differences

Built-in Functions

Most basic functions are compatible. Some differences:

% Function name differences
% MATLAB          Octave Alternative
% ------          ------------------
% inputname       (not available)
% inputParser     (partial support)
% validateattributes  (partial support)

% Behavior differences in edge cases
% Check documentation for specific functions

Random Number Generation

% Both use Mersenne Twister by default
% Seed setting is similar
rng(42);                    % MATLAB
rand('seed', 42);           % Octave (also accepts rng syntax)

% For compatibility
rng(42);                    % Works in modern Octave

Graphics

% Basic plotting is compatible
plot(x, y);
xlabel('X'); ylabel('Y');
title('Title');
legend('Data');

% Some advanced features differ
% - Octave uses gnuplot or Qt graphics
% - Some property names may differ
% - Animation/GUI features vary

% Test graphics code in both environments

File I/O

% Basic I/O is compatible
save('file.mat', 'x', 'y');
load('file.mat');
dlmread('file.txt');
dlmwrite('file.txt', data);

% MAT-file versions
save('file.mat', '-v7');    % Compatible format
save('file.mat', '-v7.3');  % HDF5 format (partial Octave support)

% For compatibility, use -v7 or -v6

Features Unique to Octave

do-until Loop

% Octave only
do
    x = x + 1;
until (x > 10)

% Equivalent MATLAB/compatible code
x = x + 1;
while x <= 10
    x = x + 1;
end

unwind_protect

% Octave only - guaranteed cleanup
unwind_protect
    % code that might error
    result = risky_operation();
unwind_protect_cleanup
    % always executed (like finally)
    cleanup();
end_unwind_protect

% MATLAB equivalent
try
    result = risky_operation();
catch
end
cleanup();  % Not guaranteed if error not caught

Built-in Documentation

% Octave supports Texinfo documentation in functions
function y = myfunction(x)
    %% -*- texinfo -*-
    %% @deftypefn {Function File} {@var{y} =} myfunction (@var{x})
    %% Description of myfunction.
    %% @end deftypefn
    y = x.^2;
endfunction

Package System

% Octave Forge packages
pkg install -forge control
pkg load control

% List installed packages
pkg list

% For MATLAB compatibility, use equivalent toolboxes
% or include package functionality directly

Features Missing in Octave

Simulink

% No Octave equivalent
% Simulink models (.slx, .mdl) cannot run in Octave

MATLAB Toolboxes

% Many toolbox functions not available
% Some have Octave Forge equivalents:

% MATLAB Toolbox        Octave Forge Package
% ---------------       --------------------
% Control System        control
% Signal Processing     signal
% Image Processing      image
% Statistics            statistics
% Optimization          optim

% Check pkg list for available packages

App Designer / GUIDE

% MATLAB GUI tools not available in Octave
% Octave has basic UI functions:
uicontrol, uimenu, figure properties

% For cross-platform GUIs, consider:
% - Web-based interfaces
% - Qt (via Octave's Qt graphics)

Object-Oriented Programming

% Octave has partial classdef support
% Some features missing or behave differently:
% - Handle class events
% - Property validation
% - Some access modifiers

% For compatibility, use simpler OOP patterns
% or struct-based approaches

Live Scripts

% .mlx files are MATLAB-only
% Use regular .m scripts for compatibility

Writing Compatible Code

Detection

function tf = isOctave()
    tf = exist('OCTAVE_VERSION', 'builtin') ~= 0;
end

% Use for conditional code
if isOctave()
    % Octave-specific code
else
    % MATLAB-specific code
end

Best Practices

% 1. Use % for comments, not #
% Good
% This is a comment

% Avoid
# This is a comment (Octave only)

% 2. Use ... for line continuation
% Good
x = 1 + 2 + 3 + ...
    4 + 5;

% Avoid
x = 1 + 2 + 3 + \\
    4 + 5;

% 3. Use 'end' for all blocks
% Good
if condition
    code
end

% Avoid
if condition
    code
endif

% 4. Avoid compound operators
% Good
x = x + 1;

% Avoid
x++;
x += 1;

% 5. Use single quotes for strings
% Good
str = 'Hello World';

% Avoid (escape sequence issues)
str = "Hello\
World";

% 6. Use intermediate variables for indexing
% Good
temp = func(arg);
result = temp(1:10);

% Avoid (Octave only)
result = func(arg)(1:10);

% 7. Save MAT-files in compatible format
save('data.mat', 'x', 'y', '-v7');

Testing Compatibility

# Test in both environments
matlab -nodisplay -nosplash -r "run('test_script.m'); exit;"
octave --no-gui test_script.m

# Create test script
# test_script.m:
# try
#     main_function();
#     disp('Test passed');
# catch ME
#     disp(['Test failed: ' ME.message]);
# end

Octave Packages

Installing Packages

% Install from Octave Forge
pkg install -forge package_name

% Install from file
pkg install package_file.tar.gz

% Install from URL
pkg install 'http://example.com/package.tar.gz'

% Uninstall
pkg uninstall package_name

Using Packages

% Load package (required before use)
pkg load control
pkg load signal
pkg load image

% Load at startup (add to .octaverc)
pkg load control

% List loaded packages
pkg list

% Unload package
pkg unload control

Common Packages

Package Management

% Update all packages
pkg update

% Get package description
pkg describe package_name

% Check for updates
pkg list  % Compare with Octave Forge website