Introduction to Programming
using Matlab
Session 2
P Duffour
Jan 2008
To try not to overload the system, you will be
working in pairs for this session
• Before starting Matlab, try Task 1 (you can use
the first session power point presentation to help
you with this task).
Writing a program in Matlab
• For more complex tasks than simple calculations
or plots, writing commands line by line at the
command prompt becomes quickly tedious.
• You can ‘feed’ Matlab with a whole sequence of
commands at a time by writing a Matlab
programme.
• Programmes written for Matlab are called m-files
because they take the extension .m
• Because Matlab is a scripting language, m-files
are also sometimes scripts.
Matlab editor
• Like any other source code, Matlab programmes
(m-files) are simply text files so you can use any
text editor to read and edit them.
• However the Matlab environment includes its own
editor which offers useful capabilities that other
non-dedicated editors do not.
• 3 ways to open a new m-file using Matlab editor
– Type >> edit myprogram.m at the command prompt
– Click on the blank page on Matlab main window toolbar
– Click File → New → M-file
M-files
• Once the file is open with the editor, you can write
matlab commands in it. For instance the three
lines:
x=0:0.1:4*pi;
y=sin(x);
plot(x,y);
• To execute them, save and name the file then type
the name of the file at the command prompt (you
don’t need to type the extension). For instance:
>>myprogram or
>>myprogram.m
M-files (cont)
• This will only work if you have saved your m-file in
the ‘working directory’ – the directory where
Matlab first looks to execute programs.
• You can check the working directory in the current
directory frame (shared with Workspace) or by
typing >>pwd
• Programs can also be executed directly from the
editor by clicking the buttom showing an arrow
down next to a page on the editor toolbar.
■ Now change the range in the previous script so
that it goes up to 6π and re-run the script.
Commenting
• Comments can be added to a program in Matlab
using the sign ‘%’. Matlab ignores whatever comes
after this sign on the same line. For example:
x=0:0.1:2*pi %Defines a vector from 0 to 2pi by 0.1 steps
• You can write whatever you like after %.
• Commenting using the command line does really
make sense but it is important to comment m-files.
• Commenting is very important and is essential to
any programming. From now on, all your m-files
should be commented.
■
Now try task 2…
Saving/Loading variables and data
• In session 1, it was said that variables in the
workspace only last the time Matlab is running.
• Sometimes it is useful to save some results from a
long calculation for future use or to use data
obtained by some other means (e.g. some
experiment).
• Data can be saved with Matlab using the keyword
save. For example:
>> V=rand(200)
(check the help for rand)
>> save V
Saving/Loading (cont.)
• Check in the working directory (it might need
refreshing) and a new file V.mat should have
been created.
• The extension .mat is used for Matlab data files.
• You can clear the variable V from the workspace
using clear V. This wipes the variable but the
file V.mat will remain unless you delete it like any
other file.
• Mat-files can be read using the keyword load e.g:
>> load V.mat will bring the variable V in
memory.
■ Now try task 3
User keyboard input
• When Matlab runs an m-file, it is not as easy to
interact with the flow of instructions as it was when
typing them line by line at the command prompt.
• Loading/Saving data-files is one way to get your
program to interact but sometimes it is useful to
ask the user for an input as you did with the
ConvertTime.exe program last time.
• In Matlab, the keyword for the user to input some
information via the keyboard during the execution
is input.
Variable = input(‘question to the user?’);
• The line above shows how to use the keyword
input.
• Such a statement will cause Matlab to display
‘question to the user?’ at the command line (or
whatever you chose to write between the quote
signs)
• It will wait for an answer.
• Once the return key is pressed by the user, it will
store the answer into Variable.
Now try task 4
Programming Constructs
• Programming constructs are logical
elementary operations that allow you to
carry out more complex tasks in
programming.
• All programming languages have them in
one form or another.
• They fall in two main categories:
– Conditional statements
(do that if this happens)
– Loops (repetition of a task)
Conditional Statements:
• Matlab has two main types of conditional
statements:
– if… then (else)… end
– switch….case… end
If…then… end
• The syntax for the simplest if-statement is:
if <condition is true>
do action1;
do action2;
…
end;
• Note: there is no need to write ‘then’ in Matlab
although it is understood.
• <condition is true> is a Boolean test
Boolean expressions
• Boolean expressions are simple logical
statements which can be either true or false (there
is no in-between in computing).
• Examples:
4<5
1==0
This is true
This is false. The double == means that
this is an equality test and not a variable
assignment which is the other type of statement
that uses the equal sign =.
• If you type these statements at the command
prompt, you will get the answer 1 for the first and
0 for the second. In computing 1 means true and 0
means false.
Boolean operators in Matlab
Symbol
==
~=
>
>=
<
<=
&
¦
~
Meaning
equal
not equal
strictly greater than
greater than or equal to
strictly smaller than
smaller than or equal to
AND
OR
NOT
Example
if x==1
if x~=0
if x>y
if x>=c
if x<18*s
if x<=11
if (x==1)&(y>3)
if (x==1)|(y>3)
if x~=y
►The operators AND, OR, NOT allow you to combine simpler tests
Simple example:
a=1;b=0;
if a>b
c=3;
disp(‘hello, the logical test is
true’)
end;
■ What would be happen if this sequence was
executed? What if a=1 and b=2 ?
if… then…else… end
• There is a straightforward extension to the
simplest if…then…end.
• It works as follows:
if u==a
disp(‘u is equal to a’);
else
disp(‘u is not equal to a’);
end;
• if-constructs can be interleaved:
if a==b
if b==0
disp(‘b=0’);
end;
else
disp(‘b is not equal to a’);
end;
Now try Task 5…
Switch-case Selection
• Switch conditional structures is particularly useful when
dealing with some kind of menu where a variable can take
a set of values and various actions must take place
accordingly.
• The syntax is:
switch variable
case value1
do this
case value2
do that
…
end;
Example of using switch
day=4;
%This corresponds to Thursday=4th day
%after Monday
switch day
case 1 %this effectively means if day==1
disp(‘Today is Monday);
case 2 % i. e. if day==2
disp(‘Today is Tuesday’);
…
otherwise
disp(‘The number should be between 1 and 7’)
end;
► Note: The otherwise statement is optional. If it isn’t there and the variable doesn’t
take any of the case values tested then Matlab reaches end and nothing has
happened.
Now try task 6…