MatLAB Lesson 8 : M-file Programming II

MatLABLesson 8 : M-file Programming II Edward Cheung email: icec@polyu.edu.hk Room W311g 2008

Switch & Case • Structure for pick one condition out of several cases • Anything that you can do with switch & case could be done with if/else/elseif but the programme is more readable • The criterion can be either a scalar or a string • Once the truth case has been hit, the rest of the cases will be ignored • Structure switch expression/variable case option1 statements case option2 statements otherwise statements end

Example on Switch & Case %Determine the sign of an input number n=input('Please input a number'); x=sign(n); switch x case -1 disp('The input number is negative') case 0 disp('The input number is zero') case 1 disp('The input number is positive') end

Example on flow control with strings % Example: filename ex_switch.m % An algorithm to check if a user has enter a correct % module code code='IC3003'; course=input('Training Module Number for MatLab=','s'); switch course case code disp('Correct') otherwise disp('Wrong!') if code(1:2)==course(1:2) %first 2 char equal? disp('but the department code is correct.') end end

Loops • For loop • Use when the number of passes are known ahead of time • While loop • Use when the loop must terminate when a specific condition is met. • Automatic indentation is available on the editor/debugger when pressing the enter key until an end statement is reached.

Example – define a new matrix using for loop >> for k=1:5 a(k)=k^2 end a = 1 a = 1 4 a = 1 4 9 a = 1 4 9 16 a = 1 4 9 16 25

Another example using for loop • The for loop will execute once for every element of the index matrix k >> for K=[1 3 5] A(K)=K end A = 1 A = 1 0 3 A = 1 0 3 0 5 • For zero is inserted in a(2) and a(4) when a(3) and a(5) are appended to a

While Loop • Problems that can be solved with a while loop can also be solved with a for loop • Use while loop if you don’t care the number of looping • Remember to update the control variable on every pass through the loop to avoid infinite looping

Example % create a table of degrees to radians % in 10 degree intervals with while loop clear,clc k=1; while k<=36 degree(k)=k*10; radians(k)=degree(k)*pi/180; k=k+1; end % create a table table=[degree;radians] disp(' Conversion Table') disp(' Degree Radian') fprintf('%8.0f %8.2f \n',table)

Efficiency of Loops • In MatLab array operations are more effective than Loops (for & while) • Minimize the amount of I/O when using a loop. Printing output to screen will increase execution time • Example • Create a large matrix, perform a scalar multiplication calculation and measure time elapsed using for loop and array operation • We measure the elapsed time take for execution instead of the actual time for programme execution because we are running the application on a multi-task environment

Example – programme elapsed time - array % Create a 200 row square matrix A % Scalar multiplication with pi % Measure elapsed time using tic/toc % filename: ex_ttime_array.m clear A=ones(200); tic B=A*pi; t=toc; disp('The elapsed time using array operation is') disp(t)

Example – programme elapsed time - loop % Create a 200 row square matrix A % Scalar multiplication with pi % Measure elapsed time using tic/toc % filename: ex_ttime_loop.m clear A=ones(200); tic for k=1:length(A(:)) B(k)=A(k)*pi; end t=toc; disp('The elapsed time using loop is') disp(t)

Example using etime & clock % Create a 200 row square matrix A % Scalar multiplication with pi % Measure elapsed time using etime % filename: ex_etime_loop.m clear A=ones(200); starttime=clock; for k=1:length(A(:)) B(k)=A(k)*pi; end elapsedtime=etime(clock,starttime); disp('elapsed time using loop is') disp(elapsedtime)

Profiling • Reports time spent in each line of m-file • Identify problematic lines >> profile on >> sphere(300) >> profile report >> profile off

Structure of Programme • The first comment line in a M-file is searchable by the lookfor command. It make sense to put keywords in the first line. • Typical structure • Comments Section • Name of programme, keywords in first line • Creation Date, Programmer’s Names • Definitions of all input & output variables in subsections with units of measurement • Definitions of variables used in the calculation in another section • Name of every user-defined function used in the programme

Try Catch Block • When MatLab detects an error while executing code in the try block, it will pass the control to the catch block immediately for error handling. • Example:- Clear try variable_never_exist catch fprintf('ERROR=%s \n Programme terminated gracefully. \n',lasterr) end

Structure of Programme (cont.) • Input Section • Input data and/or the input function for data entry to this programme (comments where appropriate) • Calculation Section • Output Section • Function for display, plotting, I/O device, etc. • Good practice • Make your programme readable • Check existence of variables and functions • Generate test cases

The Difference Function • Y = diff(X) calculates differences between adjacent elements of X. • If X is a vector, then diff(X) returns a vector, one element shorter than X, of differences between adjacent elements: • [X(2)-X(1) X(3)-X(2) ... X(n)-X(n-1)] • If X is a matrix, then diff(X) returns a matrix of row differences: • [X(2:m,:)-X(1:m-1,:)] • Y = diff(X,n) applies diff recursively n times, resulting in the nth difference; • diff(X,2) == diff(diff(X))

Examples on diff >> x=[1 3 4 7 4] diff(x) ans = 2 1 3 -3 >> diff(x,2) ans = -1 2 -6 >> diff(x,3) ans = 3 -8 >> diff(x,4) ans = -11 >> x=[1 3 5;7 9 10;11 13 15]; x = 1 3 5 7 9 10 11 13 15 >> diff(x) ans = 6 6 5 4 4 5

MatLAB Lesson 8 : M-file Programming II