# 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.

## Contents

# 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 with 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
pkg load image # load new functions into workspace
```