# Using Octave

(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)

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

`a = 1;`

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

# Command evaluation

The output of every command is printed to the console unless terminated with a semicolon `;`. The <a href="https://www.gnu.org/software/octave/doc/interpreter/XREFdisp.html">disp</a> command can be used to print output anywhere. Use <a href="https://www.gnu.org/software/octave/doc/interpreter/XREFquit.html">exit</a> or <a href="https://www.gnu.org/software/octave/doc/interpreter/XREFquit.html">quit</a> to quit the console. Read more

```t = 99 + 1  <span class="comment"># prints 't = 100'</span>
t = 99 + 1; <span class="comment"># nothing is printed</span>
disp(t);```
```t =  100
100

```

# Elementary math

Many mathematical operators are available in addition to the standard arithmetic. Operations are floating-point. Read more

```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

`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

`columnVec * rowVec`
```ans =
64   48   32
48   36   24
32   24   16

```
`rowVec * columnVec`
```ans =  116
```
`columnVec<span class="string">'</span>`
```ans =
8   6   4

```

# Accessing Elements

Octave is 1-indexed. Matrix elements are accessed as `matrix(rowNum, columnNum)`. Read more

`mat(2,3)`
```ans = -2
```

# Control flow wih loops

Octave supports `for` and `while` loops, as well as other control flow structures. Read more

```x = zeros (50,1);
<span class="keyword">for</span> i = 1:2:100 <span class="comment"># iterate from 1 to 100 with step size 2</span>
x(i) = i^2;
<span class="keyword">endfor</span>

y = zeros (50,1);
k = 1;
step = 2;
<span class="keyword">while</span> (k <= (100-step))
y(i) = k^2;
k = k + step;
<span class="keyword">endwhile</span>```

# 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 (<a href="https://www.gnu.org/software/octave/doc/interpreter/XREFsin.html">sin</a>, `+`, `-`, etc.). Read more

```i = 1:2:100;      <span class="comment"># create an array with 50-elements</span>
x = i.^2;         <span class="comment"># each element is squared</span>
y = x + 9;        <span class="comment"># add 9 to each element</span>
z = y./i;         <span class="comment"># divide each element in y by the corresponding value in i</span>
w = sin (i / 10); <span class="comment"># take the sine of each element divided by 10</span>
```

# Plotting

The function <a href="https://www.gnu.org/software/octave/doc/interpreter/XREFplot.html">plot</a> can be called with vector arguments to create 2D line and scatter plots. Read more

```plot (i/10, w);
title (<span class="string">'w = sin(i/10)'</span>);
xlabel (<span class="string">'i ÷ 10'</span>);
ylabel (<span class="string">'w'</span>);```

<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 <a href="https://www.gnu.org/software/octave/doc/interpreter/XREFsprintf.html">sprintf</a> or <a href="https://www.gnu.org/software/octave/doc/interpreter/XREFfprintf.html">fprintf</a>. Read more

```firstString = <span class="string">"hello world"</span>;
secondString = <span class="string">"!"</span>;
[firstString, secondString] <span class="comment"># concatenate both strings</span>
```
```ans = hello world!
```
`fprintf (<span class="string">"%s %.10f \n"</span>, <span class="string">"The number is:"</span>, 10)`
```The number is: 10.0000000000
```

# If-else

Conditional statements can be used to create branching logic in your code. Read more

<a href="https://www.gnu.org/software/octave/doc/interpreter/The-if-Statement.html">in the manual</a>.

```<span class="comment"># Print 'Foo'      if divisible by 7,</span>
<span class="comment">#       'Fizz'     if divisible by 3,</span>
<span class="comment">#       'Buzz'     if divisible by 5,</span>
<span class="comment">#       'FizzBuzz' if divisible by 3 and 5</span>
<span class="keyword">for</span> i = 1:1:20
outputString = <span class="string">""</span>;
<span class="keyword">if</span> (rem (i, 3) == 0)  <span class="comment"># rem is the remainder function</span>
outputString = [outputString, <span class="string">"Fizz"</span>];
<span class="keyword">endif</span>
<span class="keyword">if</span> (rem (i, 5) == 0)
outputString = [outputString, <span class="string">"Buzz"</span>];
<span class="keyword">elseif</span> (rem(i,7) == 0)
outputString = <span class="string">"Foo"</span>;
<span class="keyword">else</span>
outputString = outputString;
<span class="keyword">endif</span>
fprintf(<span class="string">"i=%g: %s \n"</span>, i, outputString);
<span class="keyword">endfor</span>```
```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 <a href="https://www.gnu.org/software/octave/doc/interpreter/XREFhelp.html">help</a> and <a href="https://www.gnu.org/software/octave/doc/interpreter/XREFdoc.html">doc</a> 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 <a href="http://octave.sourceforge.net/index.html">Octave Forge</a> website to extend the functionality of Octave’s core library. (Matlab users: Forge packages act similarly to Matlab’s toolboxes.) The <a href="https://www.gnu.org/software/octave/doc/interpreter/XREFpkg.html">pkg</a> command is used to manage these packages. For example, to use the image processing library

from the Forge, use:

```pkg install -forge image <span class="comment"># install package</span>