Lecture 6 MATLAB programming (4) Dr.Qi Ying

Transcription

1 Lecture 6 MATLAB programming (4) Dr.Qi Ying

2 Objectives User-defined Functions Anonymous Functions Function name as an input argument Subfunctions and nested functions Arguments persistent variable

3 Anonymous Functions You can create a simple function without creating an M-file file_handle=@(argument list) expression For example, define an anonymous function f1: >> f1=@(x,y) x^2+y^2; >> f1(3,4) ans = 25

4 Anonymous Functions Anonymous function can include variables exist in the workspace where it is created: >> a=4;b=2; >> a*x^b; >> f2(3) ans = 36 Note that if you changed the variables a and b later, their values in the anonymous function does not change.

5 Function name as an input argument (function functions) Function functions: are functions whose input arguments include the names of other functions. Example: matlab contains a function function called fzero, which locates a zero of the function that is passed to it. Try: means get the handle of the matlab build-in function cos

6 Anonymous function as an input argument Don t need to get the handle of the function if you are using anonymous function as input. For example, using the matlab function fplot to make a plot of an anonymous function 20 >>f1=@(x) x^2-x-1; >>fplot(f1,[0,5]);

7 Example A simple plotter function quickplot( fun, xlim ) %QUICKPLOT Generate a quick plot of a function % generate x for plotting n_steps=100; x=linspace(xlim(1),xlim(2),n_steps); % linspace is a matlab build-in function, it is equivalent % to the following code: %step_size=(xlim(2)-xlim(1))/n_steps; %x=xlim(1):step_size:xlim(2); % evaluate the function y=fun(x); %plot plot(x,y);

8 Example A simple plotter >> quickplot(f1, [0, 5])??? Error using ==> mpower Matrix must be square. Error in Error in ==> quickplot at 13 y=fun(x); So, it looks like we need to have a vector version of the function f1: >> f1=@(x) x.^2-x-1; >> quickplot(f1, [0, 5])

9 Evaluate a string as a function eval: evaluate a character string as if it has been typed from the command window try: y=eval( sin(pi/4) ) x=0:10; y=eval( x.*x+3.*x+1 )

10 Another simple plotter function quickplot( funstr, xlim ) %QUICKPLOT Generate a quick plot of a function % funstr: is a string to evaluate n_steps=100; x=linspace(xlim(1),xlim(2),n_steps); % evaluate the function y=eval(funstr); plot(x,y);

11 Test the plotter It allows input from the user: Code for testplot.m fun=input('enter the expression you want to plot:','s'); xmin=input('enter the lower limit xmin:'); xmax=input('enter the upper limit xmax:'); quickplot(fun,[xmin xmax]); 5 4 >> testplot Enter the expression you want to plot:sin(x)*5 Enter the lower limit xmin:-pi Enter the upper limit xmax:pi

12 Subfunctions It is possible to place more than one function in a single file. The first function (which has the same name as the file itself) is the primary function. The other functions below it are subfunctions. Used as utility functions to implement the primary function. Only accessible to other functions in the same file.

13 Subfunctions my_function.m function my_function(x) a=subfunc1(x); b=subfunc2(x); function y=subfunc1(x) function y=subfunc2(x) This is the primary function These are subfunctions.

14 Nested functions Nested functions are functions that are defined entirely in the body of another function, called the host function. The are only visible to the host function and the same level nested functions. Host variables are visible to the nested function. Variables in the nested function is not visible to the host function.

15 Nested functions my_function.m function my_function(x) a=nestfunc1(x); b=nestfunc2(x); function y=nestfunc1(x) function y=nestfunc2(x)

16 Example Variable in the host function is visible to the nested function. function testnest( x ) nestsub1; fprintf('in testnest: x=%f\n',x); function nestsub1 fprintf('in sub1: x=%f\n',x); x=x+1; >> testnest(3) in sub1: x= in testnest: x=

17 Example A variable in the nested function with the same name as a variable in the host function: function testnest( x ) nestsub1(x); fprintf('in testnest: x=%f\n',x); function nestsub1(x) fprintf('in sub1: x=%f\n',x); x=x+1; >> testnest(3) in sub1: x= in testnest: x=

18 Argument related MATLAB functions nargin: returns the number of actual input arguments nargout: returns the number of actual output arguments

19 Example function [ r, theta ] = polar_value( x, y ) %POLAR_VALUE Converts (x,y) to (r, theta) if nargin ~= 2 msg='two input arguments are required.'; error(msg); Stop program execution and print out an error message if nargout ~=2 msg='two output arguments are preferred.'; warning(msg); Print out a warning message, program continues if x==0 & y==0 msg='both x and y are zero: angle is meaning less.'; warning(msg); % calculate the magnitude and angle r=sqrt(x.^2+y.^2); theta=atan2(y,x)*180/pi; % results in degrees % of function polar_value

20 Persistent variable Sometimes it is desirable to keep the value of a local variable in a function function y = testpersistent( x ) persistent n; if isempty(n) n=0; % initialize n n=n+x; fprintf('n=%f\n',n); A persistent variable is empty when it is first defined by the persistent keyword. It needs to be initialized before participating in any operations.