Index of points which satisfy a given equation

Hellow everyone,

I am working on a problem in which I have defined following variables:

NodesAlongOneDirection = 10
x1 = 0; x2 = 1; Nx = NodesAlongOneDirection; x = linspace(x1, x2, Nx); dx = x(2) - x(1);
y1 = 0; y2 = 1; Ny = NodesAlongOneDirection; y = linspace(y1, y2, Ny); dy = y(2) - y(1);
[X, Y] = meshgrid(x, y); X = X(:); Y = Y(:); N = length(X);

Now I want to find index of points (index of X, Y) which satisfy any given equation, say, Y = -X +1, which is equation of "secondary diagonal" of square domain.

I have tried following lines:

A = find(Y == -X +1);

But it doesn't contain all the points on secondary diagonal.

Regards,

Ali Baig

[Walter Roberson]

You should not be using floating point equality as it demands bit for bit exact comparisons. You should compare with a tolerance.

[Ali Baig]

Dear Walter Roberson

Thank you for reply. Keeping what you have suggested, I have written the following lines:

Y_Low  = -X + 1 - 0.25 * sqrt(dx^2 + dy^2);
Y_High = -X + 1 + 0.25 * sqrt(dx^2 + dy^2);
A = find(Y > Y_Low & Y < Y_High);
figure()
plot(X(A), Y(A), '*')

This works fine at least for points on secondary diagonal. The "0.25 * sqrt(dx^2 + dy^2)" is used as a tolerance value.

Once again, thank you so much for your help.

Regards,

Ali Baig