Matlab Class Chapter 1.pdf

Chapter 1 - 1 - 4/18/2007

Chapter 1

Topic 1 – What is programming?

Programming is telling a computer what to do. There are only 2 tricky things

1) Computers are very stupid – you have to tell them exactly what to do

2) Computers don’t speak English. Any programming language is a compromise between

the computer’s native language (0010001) and your native language (English). High level

programming languages are closer to English, low level languages are closer to

computer-ese. The closer a language is to English, the easier it tends to be to program,

but the slower it is for the computer to interpret it and the more constrained it is on what

it can do. Low level languages tend to be hard to program but very fast to run, and less

constrained. Matlab is a mid- to high level language.

Like any language, programming languages have grammar. Like real languages some

things are easier to say in one language rather than another (Italian is the language of love

etc. etc.). Some languages are better for computations, others for graphics. Unlike people

who speak real languages (with the exception of the French) computers are very fussy

about grammatical errors. Like learning a real language, at first it will be hard to say the

simplest thing, but it will get easier, fast.

Hardware

Hardware is the physical presence of the computer. The monitor, the hard drive, the CPU

(central processing unit, i.e. the “brain”). Hardware is anything you can damage by

poking it with a screwdriver.

Software

Software = programs. Programs are instructions to your computer to behave in a

particular way. So a software program like Microsoft Office gets all machines to behave

in a particular way – the instructions are slightly different for different computers

(different hardware), but the program makes all computers behave (almost) the same.

All software is written in a programming language. Some programs, (like Matlab) are

there to help you write new programs. Matlab will run on Macs, PC, and UNIX. But

some of the commands only run on some computers.

NOTE – these days it is almost impossible to damage your computer by writing a

program. The computer usually makes it very hard for you to do anything that will

damage it. In this book you won’t be using any commands that can do any permanent

damage. So crash your computer hard. Have fun!

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Getting Started

You need:

1. A PC running Windows or a Mac computer running OSX.

WARNING: This book is not designed to be compatible with Macs running Mac OS9 or

earlier. Most of the book will work with Mac OS9 anyway, but if you are using Mac OS9

then where we differentiate in the code between PCs and Mac, pretend you are a PC.

2. You need to install Matlab on your computer. The student version is fine.

You may run into license manager problems installing Matlab Mac OSX since it’s really

confusing and badly designed – just call Mathworks’ technical support number and they

will sort you out.

Now, start Matlab. A window will appear that’s divided into a number of sub-windows.

Close all the sub-windows except the one that called the “command window” which has a

little prompt >>. Later you may find these other windows helpful.

OK – so where there’s a line with a >> that means I want you to type the text at the

prompt in the command

window. Where there isn’t the

>> then the text should match

what the command window is

spitting out at you.

Text that looks like this is stuff

that is happening in the command

window. Either stuff you are typing

in at the prompt, or stuff that the

command window is spitting back out.

>>str1=’I have no clue

what I am doing’

You just told the computer to create a list

of letters ’I have no idea what I’m doing’

and to name that list of letters str1. str1 is

a variable - The single quotes tell the

computer that str1 is a list of letters (not

numbers, more on that later). Any list of

letters is called a string . Now type:

>>who

This command asks your computer to give you a list of all the variables you have. At the moment the only variable you

have is str1.

Text that looks like this is me telling you what’s happening in Matlab

for a particular command.

Text that looks like this is me giving you a general

overview.

Typing the name of a variable asks your computer to tell you what is contained within that variable.

>>str1

The computer should show you what’s in str1

str1 =

I have no clue what I am doing

Using disp also displays what a variable is, but only shows the contents, instead of repeating the name of the variable

>>disp(str1)

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So all the computer does is display the contents of the variable as follows:

I have no clue what I am doing

>>str2=’Is it all going to be boring?’;

>>str2=’Is it all going to be boring?’

The first time you typed this you added a semi-colon at the end. The second time you didn’t. The semi-colon tells the

computer whether or not you want it to display the output of each command.

>>who

Now you have both str1 and str2

>>str1=’I still have no clue what I am doing’

Now you are re-defining str1 by making it represent a slightly different list of letters

>>str1

See - the list of letters contained within str1 has changed

>>str1(3)

You’ve asked the computer to display the third letter in str1. This is called indexing or subscripting . 3 is an index (or

subscript) into the third character in str1. Now try:

>>str(6)

You can see from this that the computer is counting spaces

>>mixstr=str1;

Now you’ve created a new variable called mixstr. You’ve told the computer to make mixstr the same as str1

>>mixstr

See, they are exactly the same

>>str

>>mixstr(3)=str1(1);

Now you are telling the computer to make the 3 rd letter in mixstr the same as the 1 st letter in str1

>>mixstr

>>mixstr(1)=str1(3);

Now make the 1st letter in mixstr the same as the 3rd letter in str1

>>mixstr

You should now have:

’h Iave no idea what I am doing’

You can also create lists of numbers. These are called arrays or vectors . Here’s four different ways of creating a vector

list that goes from 2 to 9 in steps of 1. A variable is a generic term that can be used to describe a character (a single

letter), a string (a string of characters), a double (a single number, more on that later), a vector , or a matrix (a two or

more dimensional set of numbers, more on that later too), as well as some other funky things we’ll get to near the end

of this book.

>>array1=[2 3 4 5 6 7 8 9]

>>array1=linspace(2, 9, 8)

Here you are saying you want a list of 8 numbers that are evenly spaced between 2 and 9. You can imagine that this

command would be useful if you had collected 8 pieces of data evenly spaced between two and 9 seconds.

>>array1=2:1:9

Here you are saying that you want a list of numbers that goes from 2 to 9 with a step-size between each number of 1.

You can imagine that this command would be useful if you collected data that went from 2 seconds, to 9 seconds, and

had collected data every second.

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>>array1=2:9

Matlab assumes a default step-size of 1, so you can simply skip it for this particular way of creating vectors.

Here are the three ways of creating a list of numbers that goes from 1 to 17 in steps of two.

>>array2=[1 3 5 7 9 11 13 15 17]

>>array2=linspace(1, 17, 9)

>>array2=1:2:17

>>array3=5:10

Remember, there is a convention that if you don’t specify the step size, then the list of numbers goes up in steps of 1

You can also index vectors. 2 indexes the second integer in array3

>>array3(2)

You can also index more than one number in an array or string.

>>array3(2:4)

>>disp(array3)

disp can also be used to display numbers

By now you should be getting a little irritated with having to type in every command one

at a time. We are therefore going to create a program – a program is simply a document

containing a sequence of commands.

In Matlab programs are written in documents called m-files . So now we are going to put

the commands you just did in a m-file.

Creating an m-file

Make sure the command window is at the front. Now go to the menu bar and choose

File->New->M-file.

You’ll get a blank document in a new editor window.

Every program begins with a few lines of documentation. This is called a header . Good

headers contain the following information

1) The name of the program

2) A description of what it does

3) Who wrote it, and when

% MixStrings.m

% % Replaces ‘this is gibberish’ with a

% string of xxxxx’s

% written by IF & GMB 4/2005

% because there is a empty line with no % above it,

% this line isn’t part of the header

OK, type this header into your m-file.

Make sure every new line begins with a %. The % tells the computer to ignore that line – these comments aren’t for the

computer, they’re for you. Commented text will probably show up as being green.

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Headers are important. You may think that you will remember the programs you write – but trust me, you won’t!

Getting in the habit of having good up-to-date headers is like flossing – it’s boring but it will save you a lot of pain in

the long run.

Now we need to save the file. Make a folder called “MatlabClass” somewhere. Create a

subfolder called “Misc”. Don’t put these folders inside the Matlab application folder or

you will lose everything if you reinstall Matlab.

Save the file as MixStrings.m (the same as the header) in the “Misc” folder.

Now go back to the command window and type

>>help MixStrings

You will almost certainly get an error message:

MixStrings.m not found.

When the computer says that a file is “not found” that means the computer can’t find an

m-file that has that particular name. The reason the computer can’t find the file even

though you saved it in the folder ‘Misc’ is because the computer is only allowed to look

for files in certain places.

One place that the computer always looks for files is the current directory or working

directory . In fact this is the first place that the computer looks. When Matlab opens it

automatically links to a particular folder (the default setting is made by Matlab). If you

don’t tell it otherwise it will save files to that folder. You can see what folder Matlab

thinks is the current directory using the print working directory command:

>>pwd

The other places that the computer can find files are in folders that are in Matlab’s search

path . This path is a simply a list of folders that the computer is allowed to look in

whenever it is trying to find a file. Again Matlab comes with a default set of paths. You

can get a list of the current folders in the path very easily:

>>path

To tell the computer where to look, you set the path …

Setting the path via the menu bar

Make sure the command window is at the front, and go to:

File->Set Path in the menu bar. A pop-up window will appear

Choose “Add With Subfolders”, choose the “Misc” folder, and click

OK. Then choose Save and Close.

Now type

>> help MixStrings

You should see the following information.

MixStrings.m

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