Introdução ao GNU/Octave

Breve, muito breve mesmo, introdução ao GNU/Octave

Última actualização: 2008-05-03 [16:50]

Primeiras impressões

octave:1> help help
help is a built-in command
 -- Command: help
Octave's `help' command can be used to print brief usage-style
messages, or to display information directly from an on-line
version of the printed manual, using the GNU Info browser.  If
invoked without any arguments, `help' prints a list of all the
available operators, functions, and built-in variables.  If the
first argument is `-i', the `help' command searches the index of
the on-line version of this manual for the given topics.
For example, the command `help help' prints a short message
describing the `help' command, and `help -i help' starts the GNU
Info browser at this node in the on-line version of the manual.
Once the GNU Info browser is running, help for using it is
available using the command `C-h'.
Overloaded function
dispatch_help(string,...)
Additional help for built-in functions, operators, and variables
is available in the on-line version of the manual.  Use the command
`help -i <topic>' to search the manual index.
Help and information about Octave is also available on the WWW
at http://www.octave.org
and via the help@octave.org
mailing list.

Help

octave:2> help exit
exit is a built-in function
-- Built-in Function:  exit (STATUS)
-- Built-in Function:  quit (STATUS)
Exit the current Octave session.  If the optional integer value
STATUS is supplied, pass that value to the operating system as the
Octave's exit status.
Additional help for built-in functions, operators, and variables
is available in the on-line version of the manual.  Use the command
`help -i <topic>' to search the manual index.
Help and information about Octave is also available on the WWW
at http://www.octave.org
and via the help@octave.org
mailing list.

octave:3> exit

Recomeçando

octave:1> help diary

diary is a built-in command
--Command: diary options
Create a list of all commands and the output they produce, mixed
together just as you see them on your terminal.  Valid options are:
`on' Start recording your session in a file called `diary' in your
current working directory.
`off' Stop recording your session in the diary file.
`FILE'
Record your session in the file named FILE.
Without any arguments, `diary' toggles the current diary state.
Additional help for built-in functions, operators, and variables
is available in the on-line version of the manual.  Use the command
`help -i <topic>' to search the manual index.
Help and information about Octave is also available on the WWW
at http://www.octave.org and via the help@octave.org
mailing list.

octave:2> diary

Operações básicas

Aritmérica

octave:3> 2+3
ans = 5
octave:4> 2*3
ans = 6
octave:5> 2/3
ans = 0.66667
octave:6> 1-4
ans = -3
octave:7>

Ocultar o resultado

octave:8> 1+2;
octave:9>

Constantes

octave:9> e
e = 2.7183
octave:10> pi
pi = 3.1416
octave:11> Inf
Inf = Inf
octave:12> true
true = 1
octave:13> false
false = 0
octave:14>

Formato numérico

Formato longo (format long)

octave:14> pi
pi = 3.1416
octave:15> format long
octave:16> pi
pi = 3.14159265358979
octave:17>

Formato curto (format short) ou simplesmente (format)

octave:17> format short
octave:18> pi
pi = 3.1416
octave:19>

Mostrar o o valor de x sem o "x=0"

octave:19> x=1
x = 1
octave:20> disp(x)
1
octave:21>

ou ainda

octave:21> disp("O valor de pi  igual a "),disp(pi)
O valor de pi  igual a
3.1416
octave:22>

Matrizes

Um vector linha

octave:22> A=[1 2 3]
A =
  1  2  3
octave:23>

um vector coluna

octave:23> B=[1;2;3]
B =
1
2
3
octave:24>

uma matriz

octave:24> C=[1 2 3;4 5 6; 7 8 9]
C =
1  2  3
4  5  6
7  8  9
octave:25>

Operações sobre matrizes

Matriz transposta A'

octave:25> A'
ans =
1
2
3
octave:26>

Produto por um escalar

octave:26> m=2*A
m =
  2  4  6
octave:27>

produto interno

octave:27> A*B
ans = 14
octave:28>

produtos de matrizes

octave:28> A*C
ans =
  30  36  42
octave:29> C*B
ans =
14
32
50
octave:30>

octave:30> A^2
error: for A^b, A must be square
error: evaluating binary operator `^' near line 30, column 2
octave:30>

Operações sobre os elementos de uma matriz

octave:30> A.^2
ans =
  1  4  9
octave:31>

Resolução numérica de equações diferenciais ordinárias

1D e. d. o.

Comece-se por definir a e. d. o. a estudar:

octave:1> function xdot = f(x,t)
> xdot = -x;
> endfunction

Calcular uma solução usando o algoritmo LSODE (Hindmarsh) no intervalo [0,1] com 50 pontos:

octave:2> x=lsode("f",2,(t=linspace(0,1,50)'));

e fazer o plot da solução

octave:3> plot(t,x)

2D e. d. o.

Definindo o sistema:

octave:4> function xdot = f (x,t)
> a=1;
> b=1.4;
> xdot(1) = x(2);
> xdot(2) = -a*x(1)-b*x(2);
> endfunction

resolvendo com condições iniciais x(1)=1 e x(2)=2 no intervalo [0,50] com 200 pontos

octave:5>  x = lsode ("f", [1; 2], (t = linspace (0, 50, 200)'));

e fazer o plot

octave:6> plot(x,t)