MATLAB Writing Your Own Functions 3-minute exercise % whatsitdo1.m Nt=300; xmax=10; x=linspace(0,xmax,Nt); y=0.95+0.05*sin(2*pi*x); % plot for it=1:2:Nt plot(x(1:it),y(1:it),... x(it),y(it),'ro'); axis([0 xmax 0 1.2]); grid on drawnow end EG112 2 What does plot look like? 3-minute exercise % whatsitdo2.m Nt=300; xmax=10; x=linspace(0,xmax,Nt); y=0.95+0.05*sin(2*pi*x); % plot tailLength=35; for it=1:2:Nt itail=max([1,it-tailLength]); plot(x(itail:it),y(itail:it),... x(it),y(it),'ro'); axis([0 xmax 0 1.2]); grid on drawnow end EG112 3 What does plot look like? Function syntax mypoly.m function y=mypoly(x) a=1; b=2; c=3; y=a*x.^2+b*x+c; EG112 4 Defining a function: First line must start with the word “function” and the declaration. File name must be the same as the function name. Function must calculate value to be returned. Function can be used like other MATLAB function. z=mypoly(1.2); Function input and output EG112 5 mypoly.m function y=mypoly(x) a=1; b=2; c=3; y=a*x.^2+b*x+c; name input return variable function must assign a value to the return Functions “pass by value” >> a=101; >> z=2; >> f=mypoly(z); >> disp(f) 11 >> disp(z) 2 >> disp(a) 101 EG112 6 mypoly.m function y=mypoly(x) a=1; b=2; c=3; y=a*x.^2+b*x+c; The value stored in the external variable z is passed to the function by storing it in the local variable named x. Similarly the value stored in the local variable y is copied to the external variable f. Functions “pass by value” >> a=101; >> z=2; >> f=mypoly(z); >> disp(f) 11 >> disp(z) 2 >> disp(a) 101 EG112 6 mypoly.m function y=mypoly(x) a=1; b=2; c=3; y=a*x.^2+b*x+c; The value stored in the external variable z is passed to the function by storing it in the local variable named x. Similarly the value stored in the local variable y is copied to the external variable f. Function input and output EG112 7 a=101; z=2; f=mypoly(z); function y=mypoly(x) 2 2 z x 1 a 2 b 3 c 11 y f 11 101 a function y=mypoly(x) a=1; b=2; c=3; y=a*x.^2+b*x+c; local variables: x, a, b, c, and y Function input and output EG112 7 a=101; z=2; f=mypoly(z); function y=mypoly(x) 2 2 z x 1 a 2 b 3 c 11 y f 11 101 a function y=mypoly(x) a=1; b=2; c=3; y=a*x.^2+b*x+c; local variables: x, a, b, c, and y Function input and output parameters Input parameters being “passed by value,” means that the function cannot alter the value of a variable in the calling statement. The function can only directly alter the value of variables through its output. (A function can do other things, e.g., make plots, sounds, etc.) EG112 8 a=101; z=2; f=mypoly(z); Local variables Variables created inside the function are local—not accessible outside the function. Local variables can have the same name as variables outside the function without conflict. This enables one to write a function without concern about variables named outside the function. Each function has its own private workspace. Local variables are temporary. They are created when the function runs and disappear afterwards. EG112 9 Use of MATLAB functions Use functions to: Extend the language by adding new functions and commands. Break down large programs into simpler tasks. Develop a reusable library. Function Prime Directive: Each function should do one thing and do it well. EG112 10 Functions with multiple returns Syntax: [var1, var2, …]=funcName(x1, x2, x3,…); After var1, other returned variables are optional, but they must occur in the prescribed order. Each input or output variable can be any MATLAB data structure: double, string, vector, matrix, other Specify in header comments what each input and output represent. >>help myfun prints header comments EG112 11 Debugging MATLAB functions Cannot run function without argument by pressing green save-and-run button. Often helpful to have a test main program Set run configuration file (next to Save-and-run) EG112 12