Using Octave

First, follow the installation guide to install GNU Octave on your system. Then, launch the interactive prompt by typing octave in a terminal or by clicking the icon in the programs menu. For further guidance, 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.

Command evaluation

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 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 wih 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-step))
  y(i) = 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');

<img src="octave_basics-1.png" alt="octave_basics-1.png">

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 forge packages

Community-developed packages can be added from the Octave Forge website to extend the functionality of Octave’s core library. (Matlab users: Forge packages act similarly to Matlab’s toolboxes.) The pkg command is used to manage these packages. For example, to use the image processing library from the Forge, use:

pkg install -forge image # install package
pkg load image           # load new functions into workspace

Read more about packages.