Introduction. Like other programming languages, MATLAB has means for modifying the flow of a program

Transcription

1 Flow control 1

2 Introduction Like other programming languages, MATLAB has means for modying the flow of a program All common constructs are implemented in MATLAB: for while then else switch try 2

3 FOR loops. If you must use a FOR loop, it is very advisable to preallocate memory for the results: for for n=1:10 x=zeros(1,10); n=1:10 x=zeros(1,10); % preallocation preallocation x(n)=sin(n*pi/10); for for n=10:-1:1 n=10:-1:1 x(n)=sin(n*pi/10); >> >> x x = Columns Columns 1 through through Columns 8 through 10 When Columns large arrays 8 through are involved, 10 preallocation saves the overhead which is incurred by allocating memory in each iteration of the loop

4 FOR loops. On the other hand, using a column vector: The loop executes only once, since the array has only one column! A FOR loop cannot be terminated by reassigning the loop variable within it An array example: i=0; i=0; for for n=(1:10) n=(1:10) i=i+1; i=i+1; End End >> >> i i = 1 i=1; i=1; for for x=rand(4,5) x=rand(4,5) y(i)=sum(x); y(i)=sum(x); i=i+1; i=i+1; >> >> y % each each element element of of y is is the the sum sum of of a column column in in x y =

5 FOR loops. FOR loops can be nested: BUT: FOR loops can often be avoided altogether by efficient programming, using vectors Vectorized code runs faster! The code below gives the same result : n=1:5; n=1:5; m=1:5; m=1:5; [nn,mm]=meshgrid(n,m); A=nn.^2+mm.^2; A=nn.^2+mm.^2; for for n=1:5 n=1:5 for for m=1:5 m=1:5 A(n,m)=n^2+m^2; A(n,m)=n^2+m^2; >> >> A A = Have a look at the meshgrid we ll discuss it later in detail 5

6 WHILE loops A group of commands is evaluated an indefinite number of times: while while expression expression (commands) (commands) The (commands) part is executed as long as ALL elements in expression are true Usually expression is a scalar, but an array is also valid e.g. computing the smallest number we can add to 1 so that the result is larger than 1: If for an array expression we wish the loop to continue when ANY element is true, use: while while any(array any(array expression) expression) (commands) (commands) num=0; num=0; EPS=1; EPS=1; while(1+eps)>1 while(1+eps)>1 EPS=EPS/2; EPS=EPS/2; num=num+1; num=num+1; EPS=EPS*2; EPS=EPS*2; >> >> num num num num = >> >> EPS EPS EPS EPS = e e-016 6

7 IF-ELSE-END constructions Conditional evaluation: The (commands) part is executed as long as ALL elements in expression are true Several more complicated constructs: expression expression (commands) (commands) expression expression (commands (commands evaluated evaluated True) True) expression1 expression1 else else (commands (commands evaluated evaluated expression1 expression1 is is True) True) (commands (commands evaluated evaluated False) False) else else expression2 expression2 (commands (commands evaluated evaluated expression2 expression2 is is True) True) else else expression3 expression3 (commands (commands evaluated evaluated expression3 expression3 is is True) True) else else (commands (commands evaluated evaluated no no expression expression is is True) True) 7

8 Example: BREAK and CONTINUE Two useful statements that can be used with conditional evaluation are break and continue break stops the FOR loop: apples=10; apples=10; cost=apples*25; cost=apples*25; apples>5 apples>5 cost=(1-20/200)*cost; continue jumps to the statement: EPS=1; EPS=1; for for num= num= 1:1000; 1:1000; EPS=EPS/2; EPS=EPS/2; (1+EPS)<=1 (1+EPS)<=1 EPS=EPS*2; EPS=EPS*2; break break EPS EPS EPS EPS = = e e-016 EPS=1; EPS=1; for for num= num= 1:1000; 1:1000; EPS=EPS/2; EPS=EPS/2; (1+EPS)>1 (1+EPS)>1 continue continue EPS=EPS*2; EPS=EPS*2; break break EPS EPS EPS EPS = = e e-016 8

9 SWITCH-CASE constructions For repeated comparison with the same variable: switch switch expression expression case case test_expression1 test_expression1 (commands1) (commands1) case case {test_expression2,test_expression3,test_expression4} (commands2) (commands2) otherwise otherwise (commands3) (commands3) expression must be a scalar or a string Scalar: it is compared to each test_expression String: compared with strcmp to each test_expression In the case of a cell array as above comparison is carried out to each of the elements When the comparison is True, the appropriate (commands) are executed 9

10 SWITCH-CASE constructions At most only one command group will be executed! x=2.7; x=2.7; units= units= 'm' 'm' ; ; switch switch units units case case {'inch','in'} {'inch','in'} y=x*2.54; y=x*2.54; case case {'feet','ft'} {'feet','ft'} y=x*2.54*12; y=x*2.54*12; case case {'meter','m'} {'meter','m'} y=x*100; y=x*100; case case {'millimeter','mm'} {'millimeter','mm'} y=x/10; y=x/10; otherwise otherwise disp(['unknown disp(['unknown Units: Units: ',units]) ',units]) y=nan; y=nan; Result: y=270 10

11 TRY-CATCH constructions For error trapping : try try (commands1) (commands1) catch catch (commands2) (commands2) All expressions in (commands1) are executed If no errors are generated control passes to If an error appears then (commands2) are executed Variable lasterr in the catch block contains the string generated by the error 11

12 TRY-CATCH constructions Example: >> >> x=ones(1,3); x=ones(1,3); >> >> y=zeros(1,4); y=zeros(1,4); try try z=x*y; z=x*y; catch catch z=nan; z=nan; disp('nooo! disp('nooo! ') ') nooo! nooo! >> >> z z = NaN NaN Last error: >> >> lasterr lasterr ans ans = Error Error using using ==> ==> * Inner Inner matrix matrix dimensions dimensions must must agree. agree. 12

13 Function M-files 13

14 Introduction: Template of calling syntax code Script M-files enable us to run a set of MATLAB commands without the need to type them all every time They interact with all the variables in the workspace Function M-files are like Matlab s: they receive arguments and return results Intermediate variables created in a are erased when the terminates Example: Usage info for help command res=dosomework(a,b) %DOSOMEWORK %DOSOMEWORK An An example example of of a that that does does something something % dosomework(a,b) dosomework(a,b) takes takes a or or b randomly randomly % % Example: Example: who who needs needs examples? examples? % See See also also DONOWORK, DONOWORK, WORKALITTLE WORKALITTLE ind=round(rand(1,1)); res=[a,b]; res=[a,b]; res=res(ind+1); res=res(ind+1); 14

15 Some notes: The first line starts with the word The rest of the line defines the input and output variables Function files have the same extension as a script file:.m There is no return statement at the of a The continuous sequence of comment lines are the text displayed in response to help dosomework or helpwin dosomework The first help line, called the H1 line is the line search by the lookfor command 15

16 Running the example : We call the dosomework like it was any Matlab Internal Variables ind and res are not recognized after the has terminated res=dosomework(a,b) res=dosomework(a,b) %DOSOMEWORK %DOSOMEWORK An An example example of of a a % dosomework(a,b) takes a or b randomly % dosomework(a,b) takes a or b randomly % % % % Example: Example: who who needs needs examples? examples? % See also DONOWORK, WORKALITTLE % See also DONOWORK, WORKALITTLE ind=round(rand(1,1)); ind=round(rand(1,1)); res=[a,b]; res=[a,b]; res=res(ind+1); res=res(ind+1); The result of the can be assigned to a variable: The variable a in the main workspace has no connection to the variable a in the 16

17 Input and output arguments: Function M-files can have zero input and zero output arguments Functions can be called with fewer input and output arguments then those that appear in the definition line but NOT more The number of input and output arguments used in a call can be determined by calling nargin and nargout: y=mmdigit(x,n,b,t) y=mmdigit(x,n,b,t) %MMDIGIT Round Values to Given Signicant Digits. (MM) %MMDIGIT Round Values to Given Signicant Digits. (MM) % MMDIGIT(X,N,B) rounds array X to N signicant places in base B. % MMDIGIT(X,N,B) rounds array X to N signicant places in base B. % If B is not given, B=10 is assumed. % If B is not given, B=10 is assumed. % If X is complex the real and imaginary parts are rounded separately. % If X is complex the real and imaginary parts are rounded separately. % MMDIGIT(X,N,B,'fix') uses FIX instead of ROUND. % MMDIGIT(X,N,B,'fix') uses FIX instead of ROUND. % MMDIGIT(X,N,B,'ceil') uses CEIL instead of ROUND. % MMDIGIT(X,N,B,'ceil') uses CEIL instead of ROUND. % MMDIGIT(X,N,B,'floor') uses FLOOR instead of ROUND. % MMDIGIT(X,N,B,'floor') uses FLOOR instead of ROUND. nargin<2 nargin<2 error('not enough input arguments.') error('not enough input arguments.') else nargin==2 else nargin==2 b=10; b=10; t='round'; t='round'; else nargin==3 else nargin==3 t='round'; t='round'; 17

18 Input and output arguments- cont. Input variables are not copied into the workspace, except they are modied If nothing is assigned to output variables no output occurs. Trying to assign the result to a variable will give an error. For advanced programmers: A variable, unlimited number of input arguments can be passed using varargin, which becomes a cell array. It can be mixed with other inputs, but must be the last: y=myfunc(x,n,b,t,varargin) y=myfunc(x,n,b,t,varargin) nargin counts each cell as a dferent input variable. A variable, unlimited number of output arguments can be passed using varargout, which must be a cell array [a,b,varagout]y=myfunc(x) [a,b,varagout]y=myfunc(x) 18

19 Function workspaces All variables created in a are hidden from the MATLAB base workspace These variables reside in a temporary workspace (the workspace) created with each call and deleted when the terminates If a is stopped temporarily, e.g. by the keyboard command the workspace is active, not the base workspace Certain techniques are available for communication among the workspaces and the base workspace 19

20 Linking the and base workspaces basic method 1: 1) GLOBAL variables Functions can share global variables with other s and the MATLAB base workspace These are declared by: global varname. In order to be recognized in the base workspace, they must be declared global there also. Example: timing a set of commands with tic and toc tic tic Global Global TICTOC TICTOC TICTOC=clock; TICTOC=clock; y=toc y=toc Global Global TICTOC TICTOC y=etime(clock,tictoc); y=etime(clock,tictoc); Calling syntax: >> >> tic,(command),toc tic,(command),toc Usage: 20

21 Linking the and base workspaces basic method 2: 2) Persistent variables Functions can share variables with repeated or recursive calls to themselves These are declared by: persistent varname Example: persisty persisty persistent persistent threshold threshold isempty(threshold) isempty(threshold) threshold=1 threshold=1 else else threshold=threshold*.5 threshold=threshold*.5 Note: persisent variables are initially empty 21

22 Linking the and base workspaces advanced methods 1) Evalin An expression can be evaluated in another workspace: either in the calling workspace or the base workspace, using: a=evalin( caller, expression ) a=evalin( base, expression ) a=evalin( caller, try, catch ) a=evalin( base, try, catch ) Note: catch is evaluated in the current workspace try creates an error in the base or caller workspace 22

23 Linking the and base workspaces advanced methods 2) assignin Assigning can also be performed in another workspace using: assignin( workspace, vname,x) where workspace is either caller or base. This assigns the content of variable X to a variable named vname in the base or caller workspace. 3) inputname With this you can determine the variable names that were used when the was called. After a call: >> y=my(xdot,time,sqrt(2)) >> y=my(xdot,time,sqrt(2)) inputname (1) inside the gives xdot as a character string, but inputname(3) gives an empty array because the third argument is an expression, not a name. 23

24 M-file construction rules: The name of the M-file should be identical to the name in the first line of the Function M-file names can have up to 31 characters Case sensitivity deps on platform it is recommed to use only lowercase letters. Function names must begin with a letter. Any combination of letters, numbers and underscores can appear afterwards. The first line of a is called the -declaration line and must contain the word followed by the calling syntax. The input and output variables are local to the A terminates after the last line OR whenever a return statement is encountered Function M-files can contain calls to script files. Scripts are evaluated in the s workspace, not the base workspace 24

25 M-file construction rules cont.: A can abort execution by calling the error. This can be used for flagging improper usage as in: length(val)>1 length(val)>1 error( VAL error( VAL must must be be a scalar. ) scalar. ) A can report a warning and continue by calling the warning( a message ). Note: Warnings can be turned on or off globally with warning on or warning off 25

26 Subs and private s Multiple s can appear in a single M-file. Additional subs or local s are apped to the primary and follow construction rules. Subs can be called by the primary in the M-file as well as other subs in the M-file. Help is obtained for subs by typing >> help func>subfunc M-files can also call private M-files which are standard M-files that reside in a subdirectory of the calling titled private. 26

27 Command- duality In addition to M-files, it is possible to create MATLAB commands such as clear, dir, ver, etc. Commands are similar to s, except: Commands do not have output arguments Input arguments are not enclosed in parentheses In fact any can be called as a command and any command can be called as a! Calling a MATLAB command as a : >> >> dir dir *.mat *.mat noamdata.mat noamdata.mat test.mat test.mat >> >> a=dir('*.mat') a=dir('*.mat') a a = = 2x1 2x1 struct struct array array with with fields: fields: name name date date bytes bytes isdir isdir 27

28 Command- duality cont. Rules for calling a MATLAB command as a : A MATLAB command with any number of arguments can be called as a with each argument in quotes The result, it exists can be assigned to a variable Assigning can be performed ONLY in the call form Calling a user defined as a command: Any user defined can be called as a command All arguments are passed to the as strings This can be misleading! y=test(a,b) y=test(a,b) y=a+b; y=a+b; >> >> test(21,22) test(21,22) ans ans = = >> >> test test ans ans = = In the example on the right: The numbers were interpreted as strings, and their ASCII values were summed! 28

29 Function evaluation with feval Sometimes a character string of a is passed to a for evaluation (e.g. in solving dferential equations) The can then be evaluated in a number of ways: >> >> [a,b]=feval( my,x,y,z,t) >> >> [a,b]=eval( my(x,y,z,y) ) eval calls the entire MATLAB interpreter, thus it is much less efficient than feval feval can be further speed up by passing it a handle instead of a string: >> fhan=@humps >> fhan=@humps >> >> [a,b]=feval(fhan,x,y,z,t) Function handles can also be arrays: >> >> fhan(1)=@humps fhan(1)=@humps >> >> fhan(2)=@cos fhan(2)=@cos 29

30 feval The content of a file handle can be examined in detail with the s command: >> >> fhan=@humps fhan=@humps >> >> s(fhan) s(fhan) ans ans = : : 'humps' 'humps' type: type: 'simple' 'simple' file: file: 'c:\matlab6p5\toolbox\matlab\demos\humps.m' A string to be evaluated can also be converted into an inline and assigned a handle: The old way: and >> >> myfun='x^2+y^2'; myfun='x^2+y^2'; >> >> x=2;y=3;eval(myfun) ans ans =

31 feval The new way: >> >> myfuni=inline(myfun,'x','y') myfuni myfuni = Inline Inline : : myfuni(x,y) myfuni(x,y) = x^2+y^2 x^2+y^2 >> >> a=feval(myfuni,2,3) a = Queries into the inline : For more info on how to deal with all these look also into fcnchk >> >> argnames(myfuni) argnames(myfuni) ans ans = 'x' 'x' 'y' 'y' >> >> formula(myfuni) formula(myfuni) ans ans = x^2+y^2 x^2+y^2 31