why my conditional function is not what I expected?

I have this question to solve:

this is my program:

close all
clear all
x = -3:0.5:3;
y = -3:0.5:3;
[X,Y] = meshgrid(x,y);
[m,n]=size(X);
f=zeros(m,n);
if X>=0 & X.^2+Y.^2<4
    f=ones(m,n);
    elseif (-2 < X) & (X < 0) & abs(Y)<2
        f=1/2*ones(m,n);
else
    f=zeros(m,n);;
end
surfl(X,Y,f)       

I don't exactly which part of my program is wrong that I can't see that 1/2 in my function in this plot.

Answer 1

[David Hill]

[x,y] = meshgrid(-3:0.1:3);
f=zeros(size(x));
f(x>=0&(x.^2+y.^2)<4)=1;
f(x>-2&x<0&abs(y)<2)=.5;
surfl(x,y,f)

[2NOR_Kh]

Thank you David!

Answer 2

[Steven Lord]

if X>=0 & X^2+Y^2<4

From the documentation for the if keyword: "if expression, statements, end evaluates an expression, and executes a group of statements when the expression is true. An expression is true when its result is nonempty and contains only nonzero elements (logical or real numeric)."

Since X and Y are not scalars, the expression for this if statement is true only if all the elements of X are greater than or equal to 0 and all the elements of X^2+Y^2 are less than 4. If even one pair of elements of X and Y doesn't satisfy that condition, the if statement is not satisfied.

You probably want to use logical indexing and the array power operator .^ rather than the matrix power operator ^ which will give a different answer.

Answer 3

[Mathieu NOE]

hello

this works better :)

close all
clear all
x = -3:0.1:3;
y = -3:0.1:3;
[X,Y] = meshgrid(x,y);
[m,n]=size(X);
f=zeros(m,n);

% condition 1 
ind1 = (X>=0) & (X.^2+Y.^2<4); % NB : dots !! .^ 
f(ind1) = 1 ;

% condition 2 
ind2 = (-2 < X) & (X < 0) & abs(Y)<2 ;  
f(ind2) = 1/2 ;
surfl(X,Y,f)  
xlabel('X');
ylabel('Y');