Construct quarter circle of data in matrix

Hello everyone,

I have a 5000x1 vector P. I would like to make a 5000x5000 matrix where the values of P fill the matrix as if P is dragged clockwise, as shown below:

(As you can see my MS paint skills exceed my matlab skills.)

I don't know how to deal with the circular shape. I see how it is not possible to fit a perfect circle into a matrix, but an approximation will do. How would one go about making a good approximation of the desired matrix?

Answer 1

[sloppydisk]

I would do it like this:

n = 5000;
P = [randi(n, n, 1); zeros(n, 1)];
[X, Y] = meshgrid(1:n, flip(1:n));
r = round(sqrt(X.^2 + Y.^2));
A = reshape(P(r(:)), n, n);

First I wrote

A = arrayfun(@(x) P(x), r);

but that's stupidly slow of course.

[Roel]

Thank you!