Using Octave

Revision as of 10:25, 13 January 2023 by 217.158.171.38 (talk) (Updated the package section to use Octave Packages instead of Forge (as forge is being deprecated)
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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.

The GNU Octave graphical user interface (GUI).

Variable AssignmentEdit

Assign values to variables with = (Note: assignment is pass-by-value). Read more about variables.

a = 1;

CommentsEdit

# or % start a comment line, that continues to the end of the line. Read more about comments.

Command evaluationEdit

The output of every command is printed to the console unless terminated with a semicolon ;. The disp command can be used to print output anywhere. Use exit or quit to quit the console. Read more about command evaluation.

t = 99 + 1  # prints 't = 100'
t =  100
t = 99 + 1; # nothing is printed
disp(t);
 100

Elementary mathEdit

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



MatricesEdit

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 AlgebraEdit

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 ElementsEdit

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 loopsEdit

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

VectorizationEdit

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


PlottingEdit

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');

StringsEdit

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-elseEdit

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 HelpEdit

The help and doc commands can be invoked at the Octave prompt to print documentation for any function.

help plot
doc plot


Octave packagesEdit

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

Read more about packages.

Octave User CodesEdit

There are also User Codes available for GNU Octave which are not part of the core program or any of the packages.

See Category User Codes.