matlab-reference.rst

Matlab Reference

Note that this is a very brief introduction to everything you can do with Matlab. The online reference is very detailed (https://www.mathworks.com/help/matlab/)

Variables and assignment

Variables store data, such as numbers, vectors, matrices, strings. You assign values to variables by the = operator:

myVar = [1 2 3]

You can reassign variables:

myVar = 'I love Matlab'

You can use variables (and their values) by simply using their names:

disp(myVar)

ans

The special variable ans contains the value of the last evaluated expression. You can reference it as any other variable, but you cannot assign values to it.

Semicolon

If you put a semicolon at the end of a line, the result of the expression is hidden:

a = 4;  % doesn't display anything
b = 3   % prints 3

Vectors and matrices

You can build matrices by the following syntax:

m = [1 2 3 4
5 6 7 8
9 10 11 12]

This creates a 3 \times 4 matrix.

Note that in other languages, such as C, whitespace (such as newlines) don't matter. In Matlab, it does make a difference. For example, the following command creates a 1 \times 12 matrix (a vector), which is different from the one above:

m2 = [1 2 3 4 5 6 7 8 9 10 11 12]

A matrix which has only one row or one column is called a vector. You can create row and column vectors with the following syntax:

vRow = [1 2 3]
vColumn = [1
2
3]
vColumn2 = [1; 2; 3]  % this also creates a column vector

You can access elements by specifying the row and column indices, or ranges:

vRow(2)    % prints 2
vRow(2:3)  % prints [2 3]
m(2, 3)    % prints 7 (2nd row, 3rd column)
m(2, 3:4)  % prints [7 8]
m(2, :)    % prints [5 6 7 8]

Strings

You can create string literals using single quotes:

'This is a character array'
"This is a string"
myString = "A string"
myCharArray = 'My char array'

Please note that there is actually a difference between a string and a character array! Most of the time it doesn't matter, but sometimes its worth switching which is used if Matlab is throwing an error.

Command history

Use the up and down arrows to browse and edit your command history. This way you don't have to always retype all commands.

Terminating scripts

If you're running a script or writing a command and you want to exit it, hit Ctrl+C on the keyboard.

Equality operator

To compare two values for equality, we use the == operator. For example:

if a == b
  disp('Equal!')
else
  disp('Not equal')
end

Note that == is different from =. The former tests for equality; the latter is the assignment operator.

You can also test for inequality using the ~= operator.

Comments

Comments are part of a Matlab program that are completely ignored by the Matlab interpreter. However, they are indispensable for explaining and documenting your code.

You can add comments using the % sign:

% This is a comment and it doesn't affect the program flow
a = [1 2 3]  % I can even add comments to the ends of lines

Conditionals (ifs)

You can alter the control flow of your program based on certain conditions. For example:

if mod(x, 2) == 0
  disp(['The number', num2str(x), 'is even'])
else
  disp(['The number', num2str(x), 'is odd'])
end

In this case, only one of the two display lines will be executed, depending on whether x is odd or even.

You don't have to specify an else branch for your conditional:

if a < 0
  disp('a cannot be negative!')
end

For loops

You can use a for loop to iterate through a series of values. For example, the following piece of code displays the numbers from 1 to 100:

for i = 1:100
  disp(i)
end

What happens is that the variable i takes on the values 1 to 100 in turn, and then the body of the loop (the disp(i) command) is executed with each value.

If you specifically want to iterate over an array, you can use the following syntax:

for element = my_list
  % do something with the element!
end

Functions

Functions are shorthands to use more complex computations. You can define your own functions, but there are also lots and lots of built-in ones in Matlab.

In the below, we explain functions using their signatures. For example, the size(matrix) function returns the size of a matrix. size is the name of the function, while matrix is its only argument. You can use functions by specifying their names and supplying appropriate arguments; for example, size([1 2 3]).

disp(value)

You can print values using the disp command. It will attempt to convert whatever value you give it to a string:

disp(3)
disp('Lego robot')
disp(['My favourite number is ', num2str(favNumber)])

mod(number, divisor)

The mod function returns the remainder of a number for a given divisor. For example:

mod(10, 4)  % this will print 2

randi(range, rows, columns)

Generates a matrix of size rows x columns filled with integers uniformly distributed between 1 and range. For example:

randi(10, 3, 4)

size(matrix)

Returns the size of a matrix (or vector).

plot(vector)

Plots a graph using the values in the vector.

The plot command provides lots of ways to customise your graph. Search online for the Matlab plot command for more details.

Also check out the xlabel(label), ylabel(label) and grid on commands.

tic, toc

tic resets Matlab's internal stopwatch. toc returns its current value.

Making you own function

Making your own function is simple. The syntax to create a function is:

function [y1, ..., yM] = myfun(x1, ..., xM)
  % do some things
end

Where the function takes input parameters x1, ..., xM and returns the values of y1, ... yM. To call this function, you need to call myfun(arguments).

If you dont need to return or input anything to the function, you can use the following syntax:

function myfun()
  % do some things
end

** Note that functions must be saved in a file consisting only of functions. Furthermore, the name of the file must match the name of the first function in the file ** e.g. for above, the file should be called myfun.m.