Attempting a simple matrix value replacement. Could i get some help?

im trying to write a script that says basically plots circles, but for radius less than sqrt(2) it plots lines. it seems like a simple task; where the vale of z(i,j) doesnt meet a condition, replace it with the corresponding value y(i,j)

(eg if z(3,2) is < 2, then z(3,2)=y(3,2))

currently i have:

    clear all

    X = 0:0.1:4;
    Y = 0:0.1:4;

    [x,y]=meshgrid(X,Y);

    i = 1 : length(X);
    j = 1 : length(Y);

      z(i,j) = x(i,j).^2+y(i,j).^2;

        z(z(i,j)<2)=y(i,j); %ie for values in z that are <2, they will be replaced by the corresponding values from the matrix y

    contour(X,Y,z,5);
    grid on;

however the problem is that it always returns the error:

> In an assignment A(I) = B, the number of elements in B and I must be the same.

any help please? the mathworks link on arrays doesnt help me

Answer 1

[Andrei Bobrov]

X = 0:0.1:4;
Y = 0:0.1:4;
[x,y]=meshgrid(X,Y);
z = x.^2+y.^2;
t = z<2;
z(t)=y(t);
grid on;
contour(X,Y,z,5);

[Angus Keane]

you bloody ripper!

ive spent all day on this!

any idea what it falls under so i can read more about it?

[Andrei Bobrov]

Firstly, please read Getting Started with MATLAB from MATLAB - Documentation

Answer 2

[Björn]

That code with a two for-loops would have worked. In my code below I have taken the first z-equation outside the for-loop because it's faster.

    clear all
    X = 0:0.1:4;
    Y = 0:0.1:4;
    [x,y]=meshgrid(X,Y);

    z=x.^2+y.^2;

    for i = 1 : length(X);
    for j = 1 : length(Y);
        z(z(i,j)<2)=y(i,j); %ie for values in z that are <2, they will be replaced by the corresponding values from the matrix y
    end
    end
    contour(X,Y,z,5);
    grid on;

I'm sure there is a better way (maybe with the function find(z<2)) than doing this replacement with a for-loop, but I couldn't figure it out right now.