Assigning values in a shape in an array

I have made an initial array, 161 x 161, of zeros:

x = (-80:80);
box  = numel(x);
[X,Y] = meshgrid(x,x);
B = zeros([box, box]);

Then, I could make a circle in the array with value 10:

a = 10;
B(X.^2 + Y.^2 <= 100) = a;

However, I would like to make a semicircle of 'a' in the array, rather than the circle.

My code:

B(Y.^2<=abs(sqrt(100-X.^2))

However, this isn't giving me the correct shape.

What kind of things do I have to keep in mind to get the shapes I want?

Thanks.

[Walter Roberson]

sqrt never returns a negative so the abs() only helps if 100-x^2 is negative in which case you are dealing with complex absolute value and the result comes out like sqrt(abs(100-x^2)). This is nothing like a semicircle.

You are also comparing the sqrt to y^2, so definitely not semicircle.

How do you want the semicircle to be oriented?

(Y>=0) & (X.^2 + Y.^2 <= 100)

For example.

[Shashvat Verma]

Oh I see, this makes a lot more sense.

Thank you :)

Answer 1

[Shashvat Verma]

From Walter Robinson:

sqrt never returns a negative so the abs() only helps if 100-x^2 is negative in which case you are dealing with complex absolute value and the result comes out like sqrt(abs(100-x^2)). This is nothing like a semicircle.

You are also comparing the sqrt to y^2, so definitely not semicircle.

How do you want the semicircle to be oriented?

(Y>=0) & (X.^2 + Y.^2 <= 100)

For example.