Using Octave
First, follow the installation instructions for:
or consult the GNU Octave manual to install GNU Octave on your system.
Then, start the GNU Octave by clicking the icon in the programs menu or launch the interactive prompt by typing octave in a terminal.
See the manual page on running Octave.
Variable Assignment
Assign values to variables with = (Note: assignment is pass-by-value).
Read more about variables.
a = 1;
Comments
# or % start a comment line, that continues to the end of the line.
Read more about comments.
% Função e derivada f = @(x) 1 - x^3; % f(x) = 1 - x^3 f_prime = @(x) -3*x^2; % f'(x) = -3*x^2
% Condição inicial e tolerância x0 = 2; % ponto inicial tolerancia = 1e-6; % tolerância max_iter = 100; % máximo de iterações para evitar loop infinito iter = 0; % contador de iterações erro = inf; % inicializando o erro como infinito
% Método de Newton-Raphson while erro > tolerancia && iter < max_iter
iter = iter + 1; % incrementar contador de iteração x1 = x0 - f(x0) / f_prime(x0); % fórmula de Newton-Raphson erro = abs(x1 - x0); % erro absoluto entre iterações x0 = x1; % atualizar x0 para o novo valor
end
% Resultado disp(['Raiz aproximada: ', num2str(x1)]); disp(['Número de iterações: ', num2str(iter)]);
Elementary math
Many mathematical operators are available in addition to the standard arithmetic. Operations are floating-point. Read more about elementary math.
x = 3/4 * pi;
y = sin (x)
y = 0.70711
Matrices
Arrays in Octave are called matrices. One-dimensional matrices are referred
to as vectors. Use a space or a comma , to separate elements in a row and
semicolon ; to start a new row. Read more
about matrices.
rowVec = [8 6 4]
rowVec =
8 6 4
columnVec = [8; 6; 4]
columnVec =
8
6
4
mat = [8 6 4; 2 0 -2]
mat =
8 6 4
2 0 -2
size(mat)
ans =
2 3
length(rowVec)
ans = 3
Linear Algebra
Many common linear algebra operations are simple to program using Octave’s matrix syntax. Read more about linear algebra.
columnVec * rowVec
ans =
64 48 32
48 36 24
32 24 16
rowVec * columnVec
ans = 116
columnVec'
ans =
8 6 4
Accessing Elements
Octave is 1-indexed. Matrix elements are accessed as
matrix(rowNum, columnNum). Read more
about accessing elements.
mat(2,3)
ans = -2
Control flow with loops
Octave supports for and while loops, as well as other control flow
structures. Read more
about control flow.
x = zeros (50,1);
for i = 1:2:100 # iterate from 1 to 100 with step size 2
x(i) = i^2;
endfor
y = zeros (50,1);
k = 1;
step = 2;
while (k <= 100)
y(k) = k^2;
k = k + step;
endwhile
Vectorization
For-loops can often be replaced or simplified using vector syntax. The
operators *, /, and ^ all support element-wise operations writing
a dot . before the operators. Many other functions operate element-wise
by default (sin, +, -, etc.). Read more
about vectorization.
i = 1:2:100; # create an array with 50-elements
x = i.^2; # each element is squared
y = x + 9; # add 9 to each element
z = y./i; # divide each element in y by the corresponding value in i
w = sin (i / 10); # take the sine of each element divided by 10
Plotting
The function plot can be called with vector arguments to create 2D line and scatter plots. Read more about plotting.
plot (i / 10, w);
title ('w = sin (i / 10)');
xlabel ('i / 10');
ylabel ('w');
Strings
Strings are simply arrays of characters. Strings can be composed using C-style formatting with sprintf or fprintf. Read more about strings.
firstString = "hello world";
secondString = "!";
[firstString, secondString] # concatenate both strings
ans = hello world!
fprintf ("%s %.10f \n", "The number is:", 10)
The number is: 10.0000000000
If-else
Conditional statements can be used to create branching logic in your code. Read more in the manual.
# Print 'Foo' if divisible by 7,
# 'Fizz' if divisible by 3,
# 'Buzz' if divisible by 5,
# 'FizzBuzz' if divisible by 3 and 5
for i = 1:1:20
outputString = "";
if (rem (i, 3) == 0) # rem is the remainder function
outputString = [outputString, "Fizz"];
endif
if (rem (i, 5) == 0)
outputString = [outputString, "Buzz"];
elseif (rem(i,7) == 0)
outputString = "Foo";
else
outputString = outputString;
endif
fprintf("i=%g: %s \n", i, outputString);
endfor
i=1:
i=2:
i=3: Fizz
i=4:
i=5: Buzz
i=6: Fizz
i=7: Foo
i=8:
i=9: Fizz
i=10: Buzz
i=11:
i=12: Fizz
i=13:
i=14: Foo
i=15: FizzBuzz
i=16:
i=17:
i=18: Fizz
i=19:
i=20: Buzz
Getting Help
The help and doc commands can be invoked at the Octave prompt to print documentation for any function.
help plot
doc plot
Octave packages
Community-developed packages can be added from the Octave Packages website to extend the functionality of Octave’s core library. (Matlab users: Packages act similarly to Matlab’s toolboxes.) The pkg command is used to manage these packages. For example, to use the image processing library visit its page on Octave Packages, copy the install command and run it in octave
pkg install "https://downloads.sourceforge.net/project/octave/Octave%20Forge%20Packages/Individual%20Package%20Releases/image-2.14.0.tar.gz" # install package
pkg load image # load new functions into workspace
Octave User Codes
There are also User Codes available for GNU Octave which are not part of the core program or any of the packages.