Editing Using Octave

First, follow the [https://www.gnu.org/software/octave/doc/interpreter/Installation.html installation guide]
to install GNU Octave on your system.  Then, launch the interactive prompt by
typing <code>octave</code> in a terminal or by clicking the icon in the programs menu.
For further guidance, see the manual page on
[https://www.gnu.org/software/octave/doc/interpreter/Running-Octave.html Running Octave].



= Variable Assignment =

Assign values to variables with <code>=</code> (Note: assignment is ''pass-by-value'').
Read more [https://www.gnu.org/software/octave/doc/interpreter/Variables.html about variables].

<syntaxhighlight lang="octave">a = 1;</syntaxhighlight>

= Comments =

<code>#</code> or <code>%</code> start a comment line, that continues to the end of the line.
Read more [https://www.gnu.org/software/octave/doc/interpreter/Comments.html about comments].

= Command evaluation =

The output of every command is printed to the console unless terminated with
a semicolon <code>;</code>.  The [https://www.gnu.org/software/octave/doc/interpreter/XREFdisp.html disp] command can be used to print output
anywhere.  Use [https://www.gnu.org/software/octave/doc/interpreter/XREFquit.html exit] or [https://www.gnu.org/software/octave/doc/interpreter/XREFquit.html quit] to quit the console.
Read more [https://www.gnu.org/software/octave/doc/interpreter/Simple-Examples.html about command evaluation].

<syntaxhighlight lang="octave">t = 99 + 1  # prints 't = 100'</syntaxhighlight>
<syntaxhighlight>t =  100</syntaxhighlight>

<syntaxhighlight lang="octave">t = 99 + 1; # nothing is printed
disp(t);</syntaxhighlight>
<syntaxhighlight> 100</syntaxhighlight>

= Elementary math =

Many mathematical operators are available in addition to the standard
arithmetic.  Operations are floating-point.  Read more
[https://www.gnu.org/software/octave/doc/interpreter/Arithmetic.html about elementary math].

<syntaxhighlight lang="octave">x = 3/4 * pi;
y = sin (x)</syntaxhighlight>
<syntaxhighlight>y =  0.70711</syntaxhighlight>




= Matrices =

Arrays in Octave are called matrices.  One-dimensional matrices are referred
to as vectors.  Use a space or a comma <code>,</code> to separate elements in a row and
semicolon <code>;</code> to start a new row.  Read more
[https://www.gnu.org/software/octave/doc/interpreter/Linear-Algebra.html about matrices].
<syntaxhighlight lang="octave">rowVec = [8 6 4]</syntaxhighlight>
<syntaxhighlight>rowVec =
   8   6   4
</syntaxhighlight>

<syntaxhighlight lang="octave">columnVec = [8; 6; 4]</syntaxhighlight>
<syntaxhighlight>columnVec =
   8
   6
   4
</syntaxhighlight>

<syntaxhighlight lang="octave">mat = [8 6 4; 2 0 -2]</syntaxhighlight>
<syntaxhighlight>mat =
   8   6   4
   2   0  -2
</syntaxhighlight>

<syntaxhighlight lang="octave">size(mat)</syntaxhighlight>
<syntaxhighlight>ans =
   2   3
</syntaxhighlight>

<syntaxhighlight lang="octave">length(rowVec)</syntaxhighlight>
<syntaxhighlight>ans =  3</syntaxhighlight>



= Linear Algebra =

Many common linear algebra operations are simple to program using Octave’s
matrix syntax.  Read more
[https://www.gnu.org/software/octave/doc/interpreter/Linear-Algebra.html about linear algebra].

<syntaxhighlight lang="octave">columnVec * rowVec</syntaxhighlight>
<syntaxhighlight>ans =
   64   48   32
   48   36   24
   32   24   16
</syntaxhighlight>

<syntaxhighlight lang="octave">rowVec * columnVec</syntaxhighlight>
<syntaxhighlight>ans =  116</syntaxhighlight>

<syntaxhighlight lang="octave">columnVec'</pre>
<syntaxhighlight>ans =
   8   6   4
</syntaxhighlight>



= Accessing Elements =

Octave is 1-indexed.  Matrix elements are accessed as
<code>matrix(rowNum, columnNum)</code>.  Read more
[https://www.gnu.org/software/octave/doc/interpreter/Index-Expressions.html about accessing elements].

<syntaxhighlight lang="octave">mat(2,3)</syntaxhighlight>
<syntaxhighlight>ans = -2</syntaxhighlight>



= Control flow wih loops =

Octave supports <code>for</code> and <code>while</code> loops, as well as other control flow
structures.  Read more
[https://www.gnu.org/software/octave/doc/interpreter/Statements.html about control flow].

<syntaxhighlight lang="octave">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</syntaxhighlight>



= Vectorization =

For-loops can often be replaced or simplified using vector syntax.  The
operators <code>*</code>, <code>/</code>, and <code>^</code> all support element-wise operations writing
a dot <code>.</code> before the operators.  Many other functions operate element-wise
by default ([https://www.gnu.org/software/octave/doc/interpreter/XREFsin.html sin], <code>+</code>, <code>-</code>, etc.).  Read more
[https://www.gnu.org/software/octave/doc/interpreter/Vectorization-and-Faster-Code-Execution.html about vectorization].

<syntaxhighlight lang="octave">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
</syntaxhighlight>



= Plotting =

The function [https://www.gnu.org/software/octave/doc/interpreter/XREFplot.html plot] can be called with vector arguments to
create 2D line and scatter plots.  Read more
[https://www.gnu.org/software/octave/doc/interpreter/Two_002dDimensional-Plots.html about plotting].

<syntaxhighlight lang="octave">plot (i/10, w);
title ('w = sin(i/10)');
xlabel ('i ÷ 10');
ylabel ('w');</syntaxhighlight>

<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 [https://www.gnu.org/software/octave/doc/interpreter/XREFsprintf.html sprintf] or
[https://www.gnu.org/software/octave/doc/interpreter/XREFfprintf.html fprintf].  Read more
[https://www.gnu.org/software/octave/doc/interpreter/Strings.html about strings].

<syntaxhighlight lang="octave">firstString = "hello world";
secondString = "!";
[firstString, secondString] # concatenate both strings
</syntaxhighlight>
<syntaxhighlight>ans = hello world!</syntaxhighlight>

<syntaxhighlight lang="octave">fprintf ("%s %.10f \n", "The number is:", 10)</syntaxhighlight>
<syntaxhighlight>The number is: 10.0000000000</syntaxhighlight>



= If-else =

Conditional statements can be used to create branching logic in your code.
Read more [https://www.gnu.org/software/octave/doc/interpreter/The-if-Statement.html in the manual].

<syntaxhighlight lang="octave"># 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</syntaxhighlight>

<syntaxhighlight>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 
</syntaxhighlight>



= Getting Help =

The [https://www.gnu.org/software/octave/doc/interpreter/XREFhelp.html help] and [https://www.gnu.org/software/octave/doc/interpreter/XREFdoc.html doc] commands can be invoked at the
Octave prompt to print documentation for any function.

<syntaxhighlight lang="octave">help plot
doc plot</syntaxhighlight>



= Octave forge packages =

Community-developed packages can be added from the
[http://octave.sourceforge.net/index.html Octave Forge] website to extend
the functionality of Octave’s core library.  (Matlab users: Forge packages
act similarly to Matlab’s toolboxes.)  The [https://www.gnu.org/software/octave/doc/interpreter/XREFpkg.html pkg] command is used
to manage these packages.  For example, to use the image processing library
from the Forge, use:

<syntaxhighlight lang="octave">pkg install -forge image # install package
pkg load image           # load new functions into workspace
</syntaxhighlight>

[https://www.gnu.org/software/octave/doc/interpreter/Packages.html Read more about packages].

[[Category:Tutorials]]