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Introduction to Programming using Matlab

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Introduction to Programming using Matlab Powered By Docstoc
					    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      This is true
  1==0     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                            Example
 ==       equal                              if x==1
 ~=       not equal                          if x~=0
 >        strictly greater than              if x>y
 >=       greater than or equal to           if x>=c
 <        strictly smaller than              if x<18*s
 <=       smaller than or equal to           if x<=11
 &        AND                                if (x==1)&(y>3)
 ¦        OR                                 if (x==1)|(y>3)
 ~        NOT                                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…

				
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