MATLAB: Conditionally removing rows dependent on values of other rows

boundary matrix restrictions limits

I have a set of points in a matrix that will be plotted as points in a x,y such it ends up looking like:
A:
   2 0
   3 0
   4 0
   2 1
   3 1
   4 1
   ...
   2 8
   3 8
   4 8
   ...
(except with much more data)
giving a plot resembling:
 y
 | x x x x x x x x x x x
 | x x x x x x x x x x x
 | x x x x x x x x x x x
 | x x x x x x x x x x x 
 | x x x x x x x x x x x
 | x x x x x x x x x x x
 | x x x x x x x x x x x
 ------------------------- x
I am trying to set boundary conditions such that I eliminate all the values outside a-x < y < a+x, where a is a constant, so that I can end up with something like this:
 y
 |                 x x x
 |               x x x x
 |             x x x x 
 |           x x x x  
 |         x x x x 
 |       x x x x  
 |     x x x x  
 ------------------------- x
I would also like to restrict the values that x can be in order to isolate a specific region of points, then count how many I have left while being able to see it visually on a plot.
I have tried writing a few conditions, such as A(A(:,1)>A(:,2))=[], and even putting it in a loop, but it doesn't seem to work how I would like it to.
Any suggestions would be appreciated. I am still very green in terms of programming.

Best Answer

[x,y]=ndgrid(1:10);
A=[x(:) y(:)];
a=3;
mask = ((x-a)<y) & (y<(a+x));
plot(A(mask,1),A(mask,2),'x','MarkerSize',15,'LineWidth',2)
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