no arrows appear when plotting direction field with quiver()

I have an equation y' = x - (1/y) and I need to make a direction field for it. Here is the code I have tried:

if true
  [x,y] = meshgrid(-10:1:10 -10:1:10);
  dy = x - (1./y);
  dx = ones(size(y));
  quiver(x,y,dx,dy)
end

When I input this I only end up with a grid of points, no vectors. Although when I place a Datatip in the figure window it does display the componets of the vectors that are supposed to be there and correctly. Still i see no reason for the arrows not to appear.

What might I be doing wrong?

[John]

I just realized something that may or may not be important, at some points dy is equal to Inf. Would that mess it up or would it just draw a straight line up?

Answer 1

[Matt Fig]

The infs are messing with you.

[x,y] = meshgrid(-10:1:10,-10:1:10);
F = x - (1./y);
[dx,dy] = gradient(F,1,1);
dy(isinf(dy)) = nan;
quiver(x,y,dx,dy);

[John]

I see I've worked it out by editing my domain and range.

BTW, does Matlab scale the vectors relative to each other?

Because if one of my slopes get too steep it seems that the others just get reduced to dots.

Thank you for the tip, but could you explain the

dy(isinf(dy)) = nan;

part?

[Walter Roberson]

isinf(dy) is true exactly at the infinities. dy(isinf(dy)) is logical indexing, selecting the dy locations that have the infinities. Those are then assigned NaN. MATLAB graphics know that NaN are not to be plotted.

[Matt Fig]

That line just sets any inf values in dy to nan. When plotting, nans don't show up.

dy = [ 3 4 5 inf 2 3 inf 1 inf];
dy(isinf(dy)) = nan;

MATLAB scales the arrows to fit within the grid, then stretches them by the optional input scale factor. To see them without scaling, use 0 for the scale value.