More on Functions. Dmitry Adamskiy 07 Dec 2011

Transcription

1 More on Functions Dmitry Adamskiy 07 Dec

2 Other Types of Functions Subfunctions Anonymous functions Inline functions 2

3 Subfunctions We can place additional functions within the file with a script or a function They are only visible to the primary function / script or to other subfunctions in the same file. 3

4 Example function [avg, med] = newstats(u) % Primary function % NEWSTATS Find mean and median with internal functions. n = length(u); avg = mean(u, n); med = median(u, n); function a = mean(v, n) % Calculate average. a = sum(v)/n; function m = median(v, n) % 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 4 % Subfunction % Subfunction

5 Anonymous Functions You can create simple functions without having to create M-files each time. The syntax: fhandle arg2) expr fhandle is a function handle You can construct an anonymous function either at the prompt line or in any M-file function or script. 5

6 Example (1) The statement below creates an anonymous function that finds the square of a number: >> sqr x.^2; sqr is a function handle and can be used as a usual function >> whos sqr >> a = sqr(5) a = 25 6

7 Anonymous Functions (2) Because sqr is a function handle, you can pass it as an argument to other functions. The code shown here passes the sqr anonymous function to the MATLAB quad function to compute its integral from zero to one: >> quad(sqr, 0, 1) >> ans =

8 Function Handles If you want to pass an existing function as an argument to another function, you have to pass its handle. To obtain a handle, operator in front of the name of the function 8

9 Example (2) >> 0, 1) ans = >> b >> whos b >> quad(b, 0, 1) ans =

10 Example (3) This function uses two input arguments, x and y. >> A = 3; >> B = 10; >> sumaxby y) (A*x + B*y); >> res = sumaxby(5, 7) 10

11 Inline Functions (1) inline(expr) constructs an inline function object from the MATLAB expression contained in the string expr. Example: >> f = inline( 3*sin(2*x.^2) ) f = Inline function: f(x) = 3*sin(2*x.^2) >> whos f 11

12 Inline Functions (2) The input arguments are automatically determined by searching expr for variable names other than 'i', 'j', 'pi', 'inf', 'nan', 'eps' and common functions After you defined the inline function object, you can use it as a usual function >> f_res1 = f(0); >> f_res2 = f(pi); 12

13 Inline Functions (3) argnames(fun) returns the names of the input arguments of the inline object fun as a cell array of strings. formula(fun) returns the formula for the inline object fun in a character array. char(fun) is identical to formula(fun). vectorize(fun) inserts a. before any ^, * or / in the formula for fun. The result is a vectorised version of the inline function 13

14 Example (1) >> argnames(f) ans = x >> formula(f) ans = 3*sin(2*x.^2) 14

15 Example (2) >> g = inline('sin(alpha*x)') g = Inline function: g(alpha,x) = sin(alpha*x) >> argnames(g) ans = 'alpha' 'x' >> formula(g) ans = sin(alpha*x) 15

16 Example (3) >> g_res = g(2,pi/2); >> h = vectorize(g); h = Inline function: h(alpha,x) = sin(alpha.*x) >> h_res = h(2,0:pi/10:pi); 16

17 Inline Arguments If inline does not return the desired function variables or if the function variables are in the wrong order, you can specify the desired variables explicitly with the inline argument list. inline(expr,arg1,arg2, )constructs an inline function whose input arguments are specified by the strings arg1, arg2,... Example: >> g2 = inline('sin(alpha*x)','x','alpha') 17

18 Writing to and Reading Data Files 18

19 Transferring Data The easiest way to transfer your data between any programs is to use tab delimited text. This is a basic text file where you have a matrix of data in the format: number TAB number TAB... NEWLINE number TAB number TAB... NEWLINE This format is compatible with practically any programme, including Excel and SPSS 19

20 Saving and Reading in Data Option 1: save and load functions >> save file_name matrix_name ascii double -tabs >> load file_name 20

21 dlmwrite dlmwrite(file_name, matrix_name, \t ) write in ASCII tab-delimited file Example: >> dlmwrite('mydata.txt',data,'\t') dlmwrite(file_name, matrix_name)- uses the default delimiter (,) to separate matrix elements dlmwrite(file_name, matrix_name, 'D') uses delimiter D to separate matrix elements 21

22 dlmread M = dlmread(file_name, \t ) read from the ASCII tab-delimited file to output matrix M Example: >> M = dlmwrite('mydata.txt','\t') M = dlmread(file_name)- infers the delimiter from the formatting of the file. M = dlmread(file_name, 'D') uses delimiter D matrix elements 22

23 Complicated File Formats Programmes tend to give you back data in a much messier form, where each line of your text file has a mixture of words and numbers. In this case, you will have to write a custom script to read your file. It s not easy, but once you have it working, you will be able to process your data very quickly! 23

24 Helpful String Manipulation Function num2str, str2num: convert between numbers and strings isspace: finds white space characters isletter: finds letters strmatch, strfind, strcmp and findstr are all variants on finding a string within a line Type help strfun for more information on text functions 24

25 Help on Input / Output Type help iofun for a complete list of input / output functions (i.e. functions to save and load data) 25

26 26 Handling Errors

27 error Command displays a message and aborts a function error ('message') 27

28 Example function word = digit2word(digit) switch digit case 0 word = zero ; case 1 word = one ; case 9 word = nine ; otherwise error( Invalid argument ) end 28

29 try catch It is used for catching errors and preventing them from crashing your script. You can ask MATLAB to try to do something, and if it generates an error, to catch the error with another instruction. This is useful when you are doing things like reading in text files or processing data where a few trials might be anomalous. 29

30 Example try load( data.mat ) catch disp( Unable to read the file. ) end 30

31 System commands The system commands dir, ls, pwd and cd work directly to show you the files in the current directory and let you change directory, just like at the Unix or Dos command prompt. You can also use the command system (for Windows) or Unix (for Unix) to send a command to system. Syntax: system( command ) 31

32 Example >> system('copy data.txt processed_data.txt') copies the data file data.txt to the file processed_data.txt. You can use these commands in FOR loops to move your data from one directory to another or rename files as you process them 32

33 Acknowledgements In the preparation of the material for this handout use has been made of the following sources: MATLAB manuals ers/ahamilton/matlab.htm 33