Desktop Basics

For convenience, we will work with Desktop@UCL Anywhere which gives you access to the same environment in the classroom and on your own computers. You should be able to keep working on the same files if you save them on the N: drive which is the same as your UCL OneDrive. See installation instructions.

To start Matlab in a UCL terminal, go to the UCL Applications Store. and type matlab in the search bar. Select Matlab 2024a. When you start MATLAB®, the desktop appears in its default layout.

The desktop includes these areas:

• Files panel — Access your files.
• Workspace panel — Explore data that you create or import from files.
• Command Window — Enter commands at the command line, indicated by the prompt (>>).
• Sidebars — Access tools docked in the desktop and additional panels.

Some things to know before doing anything

How to navigate within a folder:

• >> pwd … present working directory
• >> dir (or >> ls) … what folders (or files) are in this directory
• >> cd .. … move up one directory from here
• >> mkdir Data … make a new folder named “Data” in the current directory
• >> mkdir Students … make a new folder named “Students” in the current directory
• >> cd Data … change directory into the folder “Data” within this directory
• >> cd ../Students … move up one directory then into “Students” within that directory

Tip:
You can also use the path box above the command window to navigate to a different folder. Copy the path to the folder you want to navigate to, paste it into the path box, and press Enter. This will change the current working directory to the specified folder.

Variables

• Their names can be a mix of letters and numbers but they begin with a letter
• Their names are case sensitive
• Their names must be no longer than 31 characters in length

• They can be:

  • A string
  • A scalar
  • A vector
  • A matrix…

• Their ‘index’ starts at 1

Create a string, a scalar, and a vector as follows:

MyString = 'I am learning Matlab';
MyScalar = 6;
MyVector = [1,2,3];
MyMatrix = [1,2;3,4];

Arrays

In Matlab, the variable we referred to as a vector can also be simply a list of numbers, known as an array.

Create an array with ‘elements’ increasing from -4 to +4 in increments of 0.01.

MyArray = -4:0.01:4;

How many elements do you think there are in this array? Note: Each element can be ‘indexed’ by entering its name followed by an index in round brackets (e.g MyArray(1)). By using indexing, what is the 230th element of MyArray? Use Matlab to find out the dimensions of your three vectors and this new array using the function size (type help size if you want to know how).

Functions

The concept of a function describes how one quantity (the input) determines the value of another (the output) e.g., The quantity y is a function of the quantity x. This concept is used all the time in Matlab. We are going to use a function from the library to create several data sets and plot them on the same graph.

Multiply your array by π to create a new array with numbers running from -4π to 4π.

MyInput = pi*MyArray;

This array will help us plot two complete periods either side of zero with a period 2π. The first function is simply a sine function:

sinY0 = sin(MyInput);

The second is a cosine:

cosY0 = cos(MyInput);

A third is a sine function that is π/6 radians out of phase with the first:

sinY0a = sin(MyInput - pi/6);

A fourth is a sine function that has twice the frequency of the first – 1st harmonic!

sinY1 = sin(2*MyInput);

You can recall previous commands by pressing the up- and down-arrow keys, ↑ and ↓. Press the arrow keys either at an empty command line or after you type the first few characters of a command. For example, to recall the command MyInput = pi*MyArray, type M, and then press the up-arrow key.

How to get some help

• >> help <functionName> … gives a brief description of a function called functionName
• >> doc <functionName> … returns a complete description of functionName

Get help about the function plot and document this in your lab-book.

Plotting

The plot function can be used in quite a sophisticated fashion to plot several data sets on the same axis in different colours and different line-types. Axes can be labelled and the graph can be titled – experiment with this because it can make your thesis or papers look great!

Use the help function to find out more about the plot function and decide on the best way to represent the data you’ve created (e.g., hold on). Create some further data with different frequencies and phase differences and add some together. Comment in your workbook on the relative phase of each of these and how that affects the summation.

Editor

The Matlab editor is a powerful tool for writing and debugging scripts and functions. You can open the editor by by selecting New Script from the Home tab in the toolstrip or by switching to the Editor tab and clicking on New and selecting Script from the dropdown menu.

• Create a new script and copy all the commands above into it.
• Save the script with a meaningful name, such as my_first_script.m.

Running and debugging code

• To run your script you can type its name (without the .m extension) in the Command Window and press Enter. For that to work, you should ensure that the current folder is the one where your script is saved (use cd command to change folder if necessary). It is possible to call scripts and functions from other folders when they are on the Matlab path. More on that later.
• Another way to run your script is to click the Run button (green triangle) in the Editor toolbar.

If your code does not work as expected, you can use the debugging tools in the editor to find and fix errors. You can set breakpoints by clicking in the left margin next to a line of code (a red dot should appear). When you run the script, execution will pause at the breakpoint, allowing you to inspect variables and step through the code line by line, using Step button located next to the Run button in the toolbar. To exit debugging mode, click the Continue button (green triangle with a line) or the Stop button (red square).

Some other useful debugging commands are:

• dbstop if error … this command will cause Matlab to enter debug mode if an error occurs, allowing you to inspect the state of your program at the point of the error.
• dbclear all … this command will clear all breakpoints that you have set in your code.
• dbquit … this command will exit debug mode and return to normal execution.

Comments

Any line starting with % is a comment and will not be executed. Comments are useful for explaining what your code does or for leaving notes for yourself or others who may read your code later.

• Add comments to your script to explain what each section of code does.

Cells

You can divide your script into sections called cells. Each cell can be executed independently, which is useful for debugging and testing parts of your code without running the entire script. To create a cell, add a line with %% at the beginning. For example:

%% Create an array
MyArray = -4:0.01:4;

You can run a cell by clicking the Run Section button in the editor toolbar or by pressing Ctrl + Enter.