MATLAB: For loop of distance equation returning a confusing output

distancefor loopif statement

Hello,
I have two matrices of coordinate points for two surfaces, one inside the other, and I am trying to create a third surface by cutting the outer surface down to a certain size (get rid of excess noise). To do this I am using two for loops, for each matrix and an if statement with the Euclidean distance equation to see if the points are within a predetermined distance. As shown below:
A = [];
for pointr = boundred
    for pointb = blue
        if sqrt((pointb(1)-pointr(1))^2 + (pointb(2)-pointr(2))^2 + (pointb(3)-pointr(3))^2) < 100
            A = pointb;
        end
    end
end
The output of A is just the Z coordinates of surface blue, which confuses me. To my understanding the
for pointb = blue
iterates the loop over all of the rows, so pointb should be a row of the matrix blue. I don't understand why it only outputs pointb(3), the Z coordinate. If someone could explain why my code outputs only a part of what I am hoping to get, I would greatly appreciate it!

Best Answer

When something is going wrong, it's a good idea to look at the intermediate results. You could use the debugger, or you could just make some things print out as you go. As a crude check of what's going on, try the following code:
boundred = 0.3*[1 1 1; 2 2 2].'
blue = 0.2*[1 1 1; 2 2 2].'
A = [];
for pointr = boundred
  pointr
  for pointb = blue
      pointb
      if sqrt((pointb(1)-pointr(1))^2 + (pointb(2)-pointr(2))^2 + (pointb(3)-pointr(3))^2) < 100
          A = pointb
      end
  end
end
A few things should be evident:
  1. The for statement is taking one column, not row, at a time.
  2. The value of A keeps changing.
If you want to collect the values of A, you'll need to change the assignment line to
A = [A pointb];
But if you do that, you'll find that there are repeated values of pointb in A. In essence, you're keeping each point of blue if it is within a sphere of radius 10 around any point in boundred. Is that what you want?
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