# 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;

# 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');

## 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