Introduction to Matlab Part 2 Deniz Savas and Mike Griffiths Corporate Information and Computing Services The University of Sheffield 2012 [email protected] [email protected]

Part 2: Topics Covered • • • •

Relational Operations Flow Control statements Using the Toolboxes Debugging and Profiling Matlab Scripts

Relational & Logical Operations RELATIONAL OPERATIONS • Comparing scalars, vectors or matrices. < less than greater than >= greater than or equal to = = equal to ~ = not equal to Result is a scalar, vector or matrix of of same size and shape whose each element contain only 1 ( to imply TRUE) or 0 (to imply FALSE ) .

RELATIONAL OPERATIONS CONTINUED • Example 1: A = magic(6) ; A is 6x6 matrix of magic numbers. P = ( rem( A,3) = = 0 ) ; Elements of P divisible by 3 are set to 1 while others will be 0.

• Example 2: Find all elements of an array which are greater than 0.7 and set them to 0.5. A = rand(12,1) ; ---> a vector of random no.s p = a >= 0.7 ; ----> p will be 1’s and 0’s . b = A(find(p>0))=0.5; ----> only the elements of A which are >=0.7 will be set to 0.5.

Logical Operations • •

These are : & (meaning AND) | (meaning OR) ~ (meaning NOT) Operands are treated as if they contain logical variables by assuming that 0=FALSE NON-ZERO=TRUE Example : a = [ 0 1 2 0 ] ; b = [ 0 c=a&b will result in c = [ 0 1 0 0 ] d=a|b will result in d= [ 0 1 1 1 ] e=~a

will result in e= [ 1 0

-1 0 8 ] ;

0 1]

There is one other function to complement these operations which is xor meaning “Exclusive-OR”. Therefore; f = xor( a , b ) will return f= [ 0 0 1 1 ] ( Note: any non-zero is treated as 1 when logical operation is applied)

Exercises involving Logical Operations & Find function Type in the following lines to investigate logical operations and also use of the find function when accessing arrays via an indices vector. A = magic(4) A>7 ind = find (A>7) A(ind) A(ind)= 0

Summary of Program Control Statements • Conditional control: • if , else , elseif statements • switch statement

• Loop control : • • • •

for loops while loops break statement continue statement

• Error Control: try … catch … end • return statement

if statements if logical_expression statement(s) end or if logical_expression statement(s) else statement(s) end or ...

if statements continued... if logical_expression statement(s) elseif logical_expression statement(s) else statement(s) end

if statement examples if a < 0.0 disp( 'Negative numbers not allowed '); disp(' setting the value to 0.0 ' ); a = 0.0 ; elseif a > 100.0 disp(' Number too large' ) ; disp(' setting the value to 100.0 ' ); a = 100; end NOTES: In the above example if ‘a’ was an array or matrix then the condition will be satisfied if and only if all elements evaluated to TRUE

Conditional Control Logical operations can be freely used in conditional statements. Example: if (attendance >= 0.90) & (grade_average >= 50) passed = 1; else failed = 1; end;

To check the existence of a variable use function exist. if ~exist( ‘scalevar’ ) scalevar = 3 end

for loops •

this is the looping control (similar to repeat, do or for constructs in other programming languages): SYNTAX: for v = expression statement(s) end where expression is an array or matrix. If it is an array expression then each element is assigned one by one to v and loop repeated. If matrix expression then each column is assigned to v and the loop repeated. •

for loops can be nested within each other

for loops continued ... • EXAMPLES: sum = 0.0 for v = 1:5 sum = sum + 1.0/v end below example evaluates the loop with v set to 1,3,5, …, 11 for v = 1:2:11 statement(s) end

for loops examples w = [ 1 5 2 4.5 6 ] % here a will take values 1 ,5 ,2 so on.. for a = w statement(s) end A= rand( 4,10) % A is 4 rows and 10 columns. % Below the for loop will be executed 10 times ( once for each column) and during each iteration v will be a column vector of length 4 rows. for v = A statement(s) end

Practice Session Perform 5.Exercises on Matlab Programming on the exercises sheet.

while loops • while loops repeat a group of statements under the control of a logical condition. • SYNTAX: while expression : statement(s) : end

while loops continued .. • Example: n=1 while prod(1:n) < 1E100 n= n + 1 end • The expression controlling the while loop can be an array or even a matrix expression in which case the while loop is repeated until all the elements of the array or matrix becomes zero.

Practice Session

Perform Exercises 5b on the Exercises Sheet

break statement • break out of the while and for control structures. Note that there are no go to or jump statements in Matlab ( goto_less programming )

continue statement This statement passes control to the next iteration of the for or while loop in which it appears, skipping any remaining statements in the body of the loop.

break statement example This loop will repeat until a suitable value of b is entered. a=5; c=3; while 1 b = input ( ‘ Enter a value for b: ‘ ); if b*b < 4*a*c disp (‘ There are no real roots ‘); break end end

switch statement Execute only one block from a selection of blocks of code according to the value of a controlling expression. Example: method = 'Bilinear'; % note: lower converts string to lower-case. switch lower(method) case {'linear','bilinear'} disp('Method is linear') case 'cubic' disp('Method is cubic') case 'nearest' disp('Method is nearest') otherwise disp('Unknown method.') end Note: only the first matching case executes, ‘i.e. control does not drop through’.

return statement • This statement terminates the current sequence of commands and returns the control to the calling function (or keyboard it was called from top level) • There is no need for a return statement at the end of a function although it is perfectly legal to use one. When the programcontrol reaches the end of a function the control is automatically passed to the calling program. • return may be inserted anywhere in a function or script to cause an early exit from it.

try … catch construct This is Matlab’s version of error trapping mechanism. Syntax: try, statement, catch, statement, end When an error occurs within the try-catch block the control transfers to the catch-end block. The function lasterr can than be invoked to see what the error was.

Handling text •

• •

•

Characters & text strings can be entered into Matlab by surrounding them within single quotes. Example: height=‘h’ ; s = ‘my title’ ; Each character is stored into a single ‘two-byte’ element. The string is treated as an array. Therefore can be accessed manipulated by using array syntax. For example: s(4:8) will return ‘title’ . Various ways of conversion to/from character to numeric data is possible; Examples: title=‘my title’ ; double(title) > will display : 109 121 116 105 116 108 101 grav = 9.81; gtitle1 = num2str(grav) > will create string gtitle1=‘9.81’ gtitle2 = sprintf(‘%f ’ , grav) > will create string gtitle2=‘9.8100’ A= [65:75] ; char(A) > will display ABCDEFGHIJK. Now try : A= A+8 ; char(A) What do you see ?

Keyboard User Interactions • Prompting for input and reading it can be achieved by the use of the input function: • Example: n = input ( ‘Enter a number:’ ); will prompt for a number and read it into the variable named n .

Other useful commands to control or monitor execution • echo ------> display each line of command as it executes. echo on or echo off • keyboard ------> take keyboard control while executing a Matlab Script.

• pause ----- > pause execution until a key presses or a number of seconds passed. pause or pause (n) for n-seconds pause

Practice Session Add course/root into the Matlab Path Run plotroot command to investigate its behaviour Use the built-in Matlab Editor to edit plotroot and comment out the pause statement. Set a break-point within the program and use the debugger to investigate its progress. Hint: F12 can be used for setting break-points

Matlab Profiler This facility was enhanced at version 6.x, which enables us to investigate the time taken to run a matlab task or a script. The profiling reports help us to improve the efficiency of the code by pin-pointing the parts of the code that takes most of the time. Type profile on to start profiling, Run the code or scripts you want to time, Type profile viewer to stop profiling and view the profile report file that has been automatically generated. You must also use profile clear if you wish to clear the previous statistics. Alternatively click on the profiler icon on the toolbar, In the run this code field enter the name of your script file, or keep that field clear and just click on Start profiling followed by running your task as usual from the Matlab Command Window and when finish click on the stop profiling icon. Clicking on the profiling summary icon will display a profile summary which can be expanded by clicking on the routine names on the list.

Extending Matlab’s capabilities Toolboxes & Matlab-Central Toolboxes are collections of functions for tackling particular classes of problems. They can be free or only commercially available. They may have to be purchased separately. See: http://www.sheffield.ac.uk/wrgrid/software/matlab Use the ver command to list the toolboxes that are currently installed and also their version numbers. Also the path command will give an indication of the installed toolboxes. By joining the Matlab-Users community you can share your Matlab code with others and download their code. See: http://www.mathworks.com/matlabcentral/

Current list of Toolboxes we have at Sheffield • • • • • • • • • • • •

SIMULINK Control Systems Signal Processing M-u analysis Fuzzy Logic Neural Networks Optimisation Statistics Symbolic maths Image Processing Mapping Wavelet

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