Iterative plot, stop when lines intersect.

So I am trying to create a simulation that plots a path for one object up to another, stationary, object. The idea being that it uses the for loop below to plot each point in the path, checks whether it has intersected with the stationary line, then keeps plotting the original path until it does intersect, at which point it would stop.

I'm torn currently as to whether: it is working, but the distance between each plotted point isn't fine enough for it to ever match; or whether it doesn't actually work.

I've included in the code below the relevant variables, but haven't included any of the initial conditions that would give each variable its value. If the extra bits would help I'm happy to provide. "%Side1" is the object to be found, and "X1 and Y1" are the x and y of the line moving round (the "searching line"). Any help/clarification would be appreciated.

%Side 1
Side1x = [S1x S2x];
Side1y = [S1y S2y];
m1 = (S2y - S1y)/(S2x - S1x); %Gradient
c1  = S1y - S1x*m1; %Y-Intercept

      %while loop to find object
    Foundblock = false;
    while Foundblock == false;

      figure();
      ylim([-1.5,1.5]);           %Sets y range based on...
      xlim([-1.5,1.5]);           %Sets x range based on...
      grid on;                    %Create a grid in the background of plot
      hold on
      plot(Side1x,Side1y,'b',Side2x,Side2y,'b',Side3x,Side3y,'b',Side4x,Side4y,'b');
      i=1;
      for i = 1:length(theta);
          X1 = [x1o(i) x1i(i)];
          Y1 = [y1o(i) y1i(i)];
          h1o = plot(X1, Y1,'k','linewidth',0.1);
          pause(0.0001);
          delete(h1o);

            b1x = x1i(i);
                b1y = m1*b1x+c1;
                if b1y == y1i(i);
                    Foundblock = true;
                else
                end
            b1x = x1o(i);
                b1y = m1*b1x+c1;
                if b1y == y1o(i);
                    Foundblock = true;
                else
                end
       end
  end
  hold off

Answer 1

[Jos (10584)]

Comparing two numbers is always tricky. I suggest to use a tolerance, so instead of

if x==y, ...

use

if (abs(x-y) < tol), ...

where tol is a value that you can define, based on the expected or needed precision of your data.

Furthermore, in your case you might want to check for a change in sign

signStart = x(1) - y
for i=2 ...
   if sign(x(i)-y) ≈ signStart, ... % ha, a change in sign!

[Ethan Murray]

Hi there,

just to say thanks for your answer, you helped solve it. Final code now looks like:

              if b1x = x1i(i);
                  b1y = m1*b1x+c1;
                  if abs(b1y - y1i(i)) < tol;
                      Foundblock = true;
                      break
                  else
                  end
               end

Where 'tol' is, as you've said, a tolerance value. and it works perfectly.