Lab of COMP 406 Introduction of Matlab (III) Programming and Scripts

Transcription

1 Lab of COMP 406 Introduction of Matlab (III) Programming and Scripts Teaching Assistant: Pei-Yuan Zhou Contact: Lab 3: 26 Sep.,

2 Open Matlab 2012a Find the Matlab under the folder 1. Y:\Win32\Matlab\R2012a 2. Double click it and open Matlab 2

3 Review 2-D Plots >> plot(x, y) >> xlabel( x ); ylabel( y ) >> legend( x, y ) ; title ( title ) >> bar(x,y); stairs(x,y);errorbar(x,y,e);stem(x,y) 3-D Plots >> meshgrid; mesh(x,y,z);surf(x,y,z) >> peaks(n); colormap(hsv) Subplots >> subplot(row,col,num); 3

4 Lecture and Lab Notes mp406/main.html 4

5 Outline Programming and Scripts Character String Script Functions 5

6 Outline Programming and Scripts Character String Script Functions 6

7 Character String A character string is a sequence of any number of characters enclosed in single quotes. You can assign a string to a variable. Text1 = 'Hello, world'; If the text includes a single quote, use two single quotes within the definition. >>Text2 = 'You''re right' Text2 = You're right Text1 and Text2 are arrays, like all MATLAB variables. Their class or data type is char, which is short for character. (whos) 7

8 Character String You can concatenate strings with square brackets, just as you concatenate numeric arrays. >>Text3 = [Text1,' - ', Text2] Text3 = Hello, world - You're right To convert numeric values to strings, use functions, such as num2str or int2str. f = 71; %fahrenheit degree c = (f-32)/1.8; % centi degree temptext = ['Temperature is ',num2str(c),'c'] temptext = Temperature is C 8

9 Outline Programming and Scripts Character String Script Functions 9

10 Script There are two types of *.m file, Scripts and Functions The simplest type of MATLAB program is called a script. A script is a file with a.m extension that contains multiple sequential lines of MATLAB commands and function calls. 10

11 Script - Creation To create a script, use the edit command. This opens a blank file named newscript.m. >>edit newscript Go to File->New->Script Give a name to script 11

12 Script - Building Typing the sentences in newscript.m Print the value of each elements in the vector x and shows whether they are positive or negative. 12

13 Script - Saving For saving the script, just go to File->Save 13

14 Script - Run If you want to run it, typing >> newscript Then you can see the output in command window like right figure shows. >> newscript x(1) = 1 is positive x(2) = 4 is positive x(3) = -2 is negative or zero x(4) = 3 is positive x(5) = -1 is negative or zero x(6) = -5 is negative or zero 14

15 Script - Run If you want to run it, typing >> newscript And all generated variables are listed in workspace >> newscript x(1) = 1 is positive x(2) = 4 is positive x(3) = -2 is negative or zero x(4) = 3 is positive x(5) = -1 is negative or zero x(6) = -5 is negative or zero 15

16 Script - Comments Whenever you write code, it is a good practice to add comments that describe the code. Comments allow others to understand your code, and can refresh your memory when you return to it later. Add comments using the percentage (%) symbol. n = 50; % 50 data points r = rand(n,1); plot(r) % Draw a line from (0,m) to (n,m) m = mean(r); hold on plot([0,n],[m,m]) hold off title('mean of Random Uniform Data') 16

17 Ex1 Create a script, named as myscript.m The script can draw the plot of function y=sin(x) when x is in [0,2pi] Save your script Run the script 17

18 Ex1 >>edit myscript.m The content of script Save myscript.m myscript x=0:0.1:2*pi; y = sin(x); plot(x,y); 18

19 Script Advantage: Better for Simple but High Repeatability code All generated variables can be stored in workspace easy for viewing Disadvantage: Not support Input/ Output Arguments The variables stored in workspace, which may be covered with each other Function can improve it 19

20 Outline Programming and Scripts Character String Script Functions 20

21 Function A function is a black box that gets some input and produces some output. We do not care about the inner workings of a function. Functions provide reusable code. Functions simplify debugging. Functions have private workspaces: The only variables in the calling program that can be seen by the function are those in the input list. The only variables in the function that can be seen by the calling program are those in the output list. 21

22 Function- Black Box View Param 1 Param 2... Result Param n The black box consists of two parts: the definition of the interface by which the user passes data items to and from the function; and the code block that produces the results required by that interface. 22

23 Function- Black Box View Param 1 Param 2... Result Param n A function definition consists of the following components: A name that follows the same syntactic rules as a variable name; A set of 0 or more parameters provided to the function; Zero or more results to be returned to the called of the function. 23

24 Function Template function <return info> <function name> (<parameters>) <documentation> <code body> % must return the results 1. The <return info> section for most functions involves providing the name(s) of the results returned followed by an = sign. If more than one result is to be returned, they are defined in a vector like container. If nothing is to be returned from this function, both the result list and the = sign are omitted. 2. The <function name> is a name with the same syntactic rules as a variable name, and will be used to invoke the code body. 25

25 Function Template function <return info> <function name> (<parameters>) <documentation> <code body> % must return the results 3. The <parameters> section is a comma-separated list of the names of the data to be provided to the function. 4. The <documentation> section is one or more lines of comments that describe what the function does and how to call it All the documentation lines up to the first non-document line are printed in the command window when you type the following: >> help <function name> The first line is listed next to the file name in the current directory listing 26

26 Function An Example 1. function volume = cylinder(height, radius) 2. % function to compute the volume of a cylinder 3. % volume = cylinder(height, radius) 4. base = pi * radius^2; 5. volume = base * height; Line1: the Matlab function definition is introduced by the key word function, followed by the name of the return variable (if any) and the = sign. Line2: all comments written immediately after the function header are available to the command window when you enter: >> help cylinder 27

27 Function An Example 1. function volume = cylinder(height, radius) 2. % function to compute the volume of a cylinder 3. % volume = cylinder(height, radius) 4. base = pi * radius^2; 5. volume = base * height; Line3: it is a good idea to include in the comments a copy of the function header line to remind a user exactly how to use this function. Line5: you must make at least one assignment to the result variable. The function definition needs no end statement. The code body terminates automatically at the end of the file. 28

28 Function An Example function p = factorial(n) %FACTORIAL Factorial function. % FACTORIAL(N) is the product of all the name integers of function from 1 to N, % i.e. prod(1:n). Since double precision numbers only have about % 15 digits, the answer is only accurate for N <= 21. For larger N, % the answer will have the right magnitude, input and argument is accurate for % the first 15 digits. % % See also PROD. % Copyright The MathWorks, Inc. % $Revision: 1.5 $ if (length(n)~=1) (fix(n) ~= n) (n < 0) error('n must be a positive integer'); end p = prod(1:n); other comment lines output argument H1 comment line executable code 29

29 Function Create A Function 1. >> edit function1 % create a function1.m file 2. type the code in the file function average = function1(vector) average = sum(vector)/length(vector); 3. save file function1.m 4. View your function code : >>type function1.m 5. call the function: >>vector = [1 5 3] >>ave = function1(vector) 30

30 Function Create A Function 6. Adding Help & Comments for function function average = function1(vector) % FUNCTION1: A simple function with a single help line. % % Usage of this function: % output = function1(input) % "output" is the average of the input vector "input". % Pei-Yuan, 2014/09/26. average = sum(vector)/length(vector); % calculate average 31

31 Function Create A Function After creating function, the comments (starting from percent (%) symbol) are the content of help Type help + function name, you can see the help documentation you added >> help function1 FUNCTION1: A simple function with a single help line. Usage of this function: output = function1(input) "output" is the average of the input vector "input". 32

32 Function *Maximum identifier Length 1. The maximum identifier length is 63. len = namelengthmax => returns the maximum length allowed for MATLAB identifiers. The maximum identifier length is different for different version of MATLAB 2. The name of function can be different from name of M-file. (It is better to use the same name) The name of M-file is used for calling function 33

33 Function Input and Output A function can have multiple inputs and outputs The following func2 has two inputs and two outputs function [ave1, ave2] = func2(vector1, vector2); ave1 = sum(vector1)/length(vector1); ave2 = sum(vector2)/length(vector2); Run func3.m >> [a,b] = func3 ([1 2 3], [ ]) 34

34 Function Input and Output nargin and nargout can be used to control the number of inputs and outputs. nargin: returns the number of input arguments that were used to call the function. nargout: returns the number of output arguments that were used to call the function 35

35 Function Input and Output Then you can change the func2.m into func3.m function [ave1, ave2] = func4(vector1, vector2) if nargin == 1, % only one input ave1 = sum(vector1)/length(vector1); end if nargout == 2, % two outputs ave1 = sum(vector1)/length(vector1); ave2 = sum(vector2)/length(vector2); end 36

36 Function Input and Output Then you can change the func2.m into func3.m >> [a, b] = func4([1 2 3], [ ]) a = 2 b = 6 >> c = func4([ ]) c = 5 The output is different based on different calling method 37

37 Function Nested Function function out = func4(x) r = reciproc(x); out = sum(r); >> func4([1 2 3]) ans = % Definition for subfunctions function output = reciproc(input) output = 1./input; Calculate the sum of reciprocal of all elements in vector 38

38 Ex1 Problem: write a function called strsearch that takes a string s and a character c, and returns the number of occurrences of c in s and the index of the first occurrence. Hint & Pseudo code: Function [cnt, pos] = strsearch( s, c) For each character of s in reverse order If character is equal to c increment the counter save the index 39

39 Ex1 - answer function [ cnt, pos ] = strsearch( s, c ) %STRSEARCH find the number of occurrences of a character in a string pos = 0; cnt = 0; n = length(s); for i = n:-1:1, % from n to 1, and minus 1 for each loop if ( s(i) == c ), cnt = cnt + 1; pos = i; end end 40

40 Ex1 - answer [ a, b ] = strsearch( 'abccdecfac', 'c' ) a = 4 b = 3 a = strsearch( 'abccdecfac', 'c' ) a = 4 strsearch( 'abccdecfac', 'c' ) ans = 4 41

41 Ex2 Nested Function Problem: write a function called mystats that find mean and median with internal functions. Hint: Creation: function [avg, med] = mystats(u) The functions you can use: avg = mean(u,n) % u is the vector, n is the length of u med = median (u,n) 42

42 Ex2 - answer function [avg, med] = mystats(u) %MYSTATS Find mean and median with internal functions. n = length(u); avg = mean(u,n); med = median(u,n); function a = mean(v,n) % Subfunction to calculate average. a = sum(v)/n; function m = median(v,n) % Subfunction to calculate median. w = sort(v); if rem(n,2) == 1 m = w((n+1)/2); else m = (w(n/2)+w(n/2+1))/2; end 43

43 Ex2 - answer >> [a,m]=newscript([ ]) a = m = >> [a,m]=newscript([ ]) a = m =

44 Summary Both scripts and functions are saved as m-files. Functions are special m-files that receive data through input arguments and return results through output arguments. Scripts are just a collection of MATLAB statements. Functions are defined by the function statement in the first line. Scripts use the global workspace but functions have their own local independent workspaces. 45

45 Contact: Lab 3: 26 Sep.,