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 modifying the flow of a program All common constructs are implemented in MATLAB: for while if 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 n=1:10 >> x x = x(n)=sin(n*pi/10); Columns 1 through Columns 8 through x=zeros(1,10); % preallocation for n=10:-1:1 x(n)=sin(n*pi/10); When large arrays are involved, preallocation saves the overhead which is incurred by allocating memory in each iteration of the loop 3

4 FOR loops. i=0; 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: for n=(1:10) i=i+1; End >> i i = 1 i=1; for x=rand(4,5) y(i)=sum(x); i=i+1; >> y % each element of y is the sum of a column 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; m=1:5; [nn,mm]=meshgrid(n,m); A=nn.^2+mm.^2; for n=1:5 for m=1:5 A(n,m)=n^2+m^2; >> A A = Have a look at the meshgrid function we ll discuss it later in detail 5

6 WHILE loops A group of commands is evaluated an indefinite number of times: while expression (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 any(array expression) (commands) num=0; EPS=1; while(1+eps)>1 EPS=EPS/2; num=num+1; EPS=EPS*2; >> num num = 53 >> EPS EPS = e-016 6

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

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

9 SWITCH-CASE constructions For repeated comparison with the same variable: switch expression case test_expression1 (commands1) case {test_expression2,test_expression3,test_expression4} (commands2) otherwise (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; units= 'm' ; switch units case {'inch','in'} y=x*2.54; case {'feet','ft'} y=x*2.54*12; case {'meter','m'} y=x*100; case {'millimeter','mm'} y=x/10; otherwise disp(['unknown Units: ',units]) y=nan; Result: y=270 10

11 TRY-CATCH constructions For error trapping : try (commands1) catch (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); >> y=zeros(1,4); try z=x*y; catch z=nan; disp('nooo! ') nooo! >> z z = NaN Last error: >> lasterr ans = Error using ==> * Inner matrix dimensions must agree. 12