To days Outline

1. To days Outline • Functions • Strings • Sparse Arrays • Cell Arrays • Structures • Exercises on this days topics lecture 3

2. Functions files • Runs in its own independent workspace • Receives input data through an • Input argument list • Returns data through an • Output argument list lecture 3

3. Functions files • The first word written in the file must bethe word function • function [outarg1,…] = fname(inarg1,…) %Comment lines … (Executable code) … (return) lecture 3

4. Functions files • Variable passing • When a function calls occurs. • MATLAB makes a copy of the arguments • If the function modifies the input argument it will not affect the original data in the caller. • This apply both scalars and arrays. lecture 3

5. Example • Create a function that adds a value to the input argument and returns the new value. lecture 3

6. Example In command window >> A=[1 2]; >> B=myfun(A); Input argument before modifying: [1 2] Input argument after modifying: [11 12] >> disp(A) 1 2 >> disp(B) 11 12 lecture 3

7. Functions files • Optional Arguments • Many function supports optional input and output arguments • We have used the plot command with several different Input argument lists. • The max command accepts different output arguments • How do MATLAB functions know how many arguments are present and how do they adjust their behavior? lecture 3

8. Functions files • MATLAB offers some special functions to get information about the arguments and to report errors • nargin Returns the actual number of input arguments used to call the function. • nargout Returns the actual number of output arguments used to call the function. • error(msg) Display error message and abort the function producing the error. • warning(msg) Display warning message Execution can continue • nargchk Returns a standard error message if a function is called with to few or to many arguments lecture 3

9. Example The location of a point in the Cartesian plane can be represented either in rectangular (x,y) or polar coordinates (r,θ). • Write a function rect2polar that converts rectangular coordinates to polar coordinates. • The function is design to support two input arguments (x,y) • However if only one input argument is supplied the functions should suppose that y is equal to zero. • The function normally returns both magnitude and angle but if only one output argument is present, it returns only the magnitude lecture 3

10. Test the function: In command window: >> [mag,angle]=rect2polar ??? Error using ==> rect2polar Not enough input arguments. >> [mag,angle]=rect2polar(1,-1,2) ??? Error using ==> rect2polar Too many input arguments. >> [mag,angle]=rect2polar(1) mag = 1 angle = 0 >> [mag,angle]=rect2polar(1,1) mag = 1.4142 angle = 45 >> [mag]=rect2polar(1,1) mag = 1.4142

11. Functions files • Global Memory • Provides a way to share data between functions • A global memory is declared with the global statement. • global var1 var2 … lecture 3

12. Functions files • Preserving data • When a function finishing executing the special workspace will be destroyed. • Persistent memory • Only accessed within the function • A persistent variable is declared using the persistent statement • persistent var1 var2 … lecture 3

13. Functions files • Subfunctions or Internal functions • More than one function in a single file • Top function is the normal function • The ones below are subfunctions or Internal functions lecture 3

14. Strings • In MATLAB a string is an array of type char. • str=’This Is a string’ • Creating two-dimensional Character arrays • Each row must have the same length • MATLAB offers a number of string operations • strcmp(str1,str2) Compares two strings Returns 1 if true. • See MATLAB help for more string operations lecture 3

15. Sparse Arrays • Sparse arrays are special arrays in which memory is only allocated for the nonzero elements. • using sparse arrays saves memory and can make a program run faster. • sparse(A) Converts A to a sparse array lecture 3

16. Cell Arrays • A cell array is a special array that can contain different data: • Strings • Complex numbers • vectors • Cell array uses braces {} instead of parentheses () for selecting cells • To view a cell array graphically use cellplot(A) lecture 3

17. Structures • Structures can be created in two ways • A field at a time using assignment statements. >>student.name='Carl'; >>student.addr='Borlänge'; >>student.phone=778858; student = name: 'Carl' addr: 'Borlänge' phone: 778858 lecture 3

18. Structures • A second student can be added to the structure by using a subscript >>student(2).name=’John’; • The other field will automatically be initialized as an empty array. • Accessing field in the structure using parentheses lecture 3

19. Structures • All at once using the struct function • str_array=struct(field1,val1,field2,val2) • Example: student=struct('Name',{'Carl','John'},'Addr',... 'Borlänge','Falun'},'Phone',{778858,123456}) • This will construct a 1x2 structure. lecture 3

20. Exercises on this days topics • 5.1 , 5.2, 5.10(8) , 5.16(13) , 5.19(16), 5.24(20) • 6.1 , 6.20(18) • 7.6 • The exercise numbers are referring to the book MATLAB Programming for engineers, third edition 20(second edition). lecture 3