Matlab 1: Get Started

Transcription

1 Matlab 1: Get Started 1

2 Starting/Existing Matlab 2/2 Run matlab in command line: In Terminal/console i.change to you work directory ii.type: matlab -nodesktop iii.to close it, type exit 3

3 Keep track of your work Comment your code: % Save your command history: diary Display current variables: who Clear variables: clear >> diary MatlabHistory.txt; %Start to record you command history >> a=1; >> who; Your variables are: a >> clear; >> who; >> diary off; %End of recording you command history 4

4 Get Help Type Help, your will get a bounch of help topics. Select the topic your are looking for. Or you can type helpdesk to start the help window. Use help toolbar. If you know your function. Type help -Function_Name 5

5 More learning material Get a book from library by searching keyword: matlab Some toturials from internet: Quick Reference: Examples: 6

6 Mathematical Functions >> a=1; b=10; c=-2; >> d = a*b + c d =8 >> d = b/c d = -5 More math funtions and predefined constant in - Search the page by: Mathematics to find the math functions Constants and Test Matrices for predefined constant 7

7 Matlab 2: Plotting 8

8 Simple Plot Sample Code: >> x = 0:pi/100:2*pi; >> % x is a sequence from 0 to 2*pi, step by pi/100 >> y = sin(x); >> sineplot = figure; %open new plot window >> plot(x,y); % plot sin(x) >> xlabel('x:0 : 2\pi'); ylabel('sin(x)'); >> title('sine function'); %add labels and title >> saveas(sineplot,'sineplot','jpg'); >> % save plot to sineplot.jpg 9

9 1 Sine function sin(x) x:0 : 2 10

10 Multiple data sets in one plot >> x = 0:pi/100:2*pi; y1 = sin(x); y2=2*sin(x); >> plot(x,y1,'r-'); % plot sin(x) >> hold >> plot(x,y2,'g+'); % plot 2*sin(x) to the same figure >> hold off >> % or we can use >> %plot(x,y1,'r-',x,y2,'g+'); >> xlabel('x:0 : 2\pi'); ylabel('sin functions'); >> title('sin functions'); >> legend('sin(x)','2*sin(x)');% add legend 11

11 12

12 Subplots >> x = 0:pi/100:2*pi; y1 = sin(x); y2=2*sin(x); >> subplot(1,2,1); >> % plot with one row, two columns >> plot(x,y1,'r-'); >> title('sin(x)')% plot sin(x) >> subplot(1,2,2); >> plot(x,y2,'g+'); >> title('2*sin(x)');% plot 2*sin(x) 13

13 14

14 Matlab 3: Matrices

15 Construct Vectors and Matrices 1/3 Format: Vectors and matrices are enbraced by squares braces ([]). For example: vector1 = [ ]; Row elment separated by spaces or commas (,) Column vector separated by semicolon (;) Special Matrix: Check the functions to generate special matrices, for example, eye(n) to generate identity matrix. 16

16 Construct Vectors and Matrices 2/3 Preallocate your matrix make programs run faster. For example, you can allocate a matrix x before you use it by : x = zeros(1, 10^6); And then you can fill the matrix with values later. Check for more technics to improve your program: 17

17 Construct Vectors and Matrices 3/3 Sample Codes: >> vector1 = [ ] vector1 = >> >> matrix1 = [1 2 3; 4 5 6; 7 8 0] matrix1 =

18 Matrix Operation 1/3 Transpose: by adding singl quote (') >> matrix1 matrix1 = >> matrix1' ans =

19 >> a = [1 2; 3 4]; >> b = [2 2; 2 2]; >>c= a*b c = >> a.*b ans = Matrix Operation 2/3 The dimensiones of two matrices must be agree to do matrix operation Matrix operation is defined by * / ^ If you want to do element-by-element operation, add. in front of the matrix operation. 21

20 Matrix Operation 3/3 Inverse of matrix: inv(matrix) >> inv(a)*c ans = >> a\c ans =

21 Matlab 4: Control flow

22 If structure 1/2 if expression statements elseif expression statements else statements end Note: No semicolon(;) is needed at the end of line Indentation is not needed but good for reading Non't forget end at last 24

23 If structure 2/2 a = rand(1); %generate random number from uniform(0,1) if a>0.5 fprintf('a = %f is larger than 0.5',a); elseif a==0.5 fprintf('a = %f equales 0.5',a); else fprintf('a = %f is less than 0.5', a); end 25

24 for/while Loop for index = values statements end while expression is true statements end Note: Try to avoid loop if possible. Matrix operation is much faster. 26

25 >> t = cputime;%start to record cpu time a = ones(10^3); b = ones(10^3,10^3); for i = [1:1:10^3]%for loop to do a*b abi = 0; for j = [1:1:10^3] abi = abi + a(j)*b(i,j); end end Cputime-t %print cpu time to do a*b by loop ans = >> t = cputime; a*b; cputime-t %print cpu time to do a*b by matrix operation ans =

26 Matlab 4: Functions

27 Define Function Format: function return_value = function_name( parameters ) Statement End Example: Create m file as average.m (The same name as your function), put in your matlab path. Type the following content in average.m function y = average(x) y = sum(x)/length(x); end Then, call the fucntion by: >> z = 1:49; >> average(z) ans = 25' 29

28 Matlab Built-in Functions Check the built-in functions. They are always more efficient. >> help elfun % display the elementary functions >> help specfun % display special functions There are many free toolboxs for specific fields. Check the following links (Please don't use them in this course if you are required to write functions by yourself.)