Check matrix element

Question

I need a (rho,theta) meshgrid and to do that first I defined the meshgrid in Cartesian coordinates and then convert it to polar coordinates:

[X,Y] = meshgrid(x,y);
R = sqrt(X.^2+Y.^2);
PHI = atan2(Y,X);

Now what I get is a mesh in polar coordinates, but since it is a squared mesh, I get this thing

I say that the values greater than R are wrong and therefore I set them to zero. I did it in this way

for i = 1:1:length(R)
    for j = 1:1:length(R)
        if R(i,j) > a
            R(i,j) = 0;
        else
            R(i,j);
        end
    end
end

How can I do this less convoluted?

Adriaan · Accepted Answer · 2018-11-19T21:55:26.460

3

For direct Cartesian to polar conversion of coordinates: use cart2pol.

[theta,rho] = cart2pol(x,y)

Then a simple logical check will do:

tmp = rho>R;
rho(tmp)=0; %=[] to delete completely
theta(tmp)=0;

For what it's worth: you can of course create a direct polar grid:

[theta,rho] = meshgrid(0:dtheta:theta,0:dR:R)

Finally: i and j denote the imaginary unit in MATLAB, and I'd argue against using them as regular variables for reasons mentioned in this post, but that's my opinion.

edited Nov 19 '18 at 21:55

answered Nov 19 '18 at 21:47

Adriaan

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I thought to create a polar grid directly but I was having issues to plot the results using `quiver` due to the different dimension of the matrix `theta` and `rho`. With the solution that I choose (cartesian-polar-cartesian again) I have a squared mesh, easier to manage – Shika93 Nov 19 '18 at 22:26
@Shika93 you could just make your `theta` and `rho` vectors which you feed to `meshgrid` the same length, and then `quiver` is fine with it. Anyway the `cart2pol` does basically the same as what you did by hand, just wrapped in a handy function. – Adriaan Nov 20 '18 at 09:34

Cedric Zoppolo · Answer 2 · 2018-11-19T22:05:23.877

1

If we say that a is the limit you want to use you can use below code instead of the for loop:

R = (R<=a).*R

Or you can use as well:

R(R>a) = 0

edited Nov 19 '18 at 22:05

answered Nov 19 '18 at 21:59

Cedric Zoppolo

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Just notice that in your code you used `R` to denote radius and `a` to denote the limit. Whereas in the figure you use `R` as the limit and `rho` for radius. I used the notation you stated in your code. – Cedric Zoppolo Nov 19 '18 at 22:07
I like the second option. Oh yes I didn't follow the notation that I used in my code inside the image, sorry. It was just to clarify what I needed. Thanks – Shika93 Nov 19 '18 at 22:21

Check matrix element

2 Answers2