Matlab keeps saying this matrix is undefined?

function [Crash,D_min] = crash_func (t, x, P_S, P_N, N_S, N_N)

% D_min: minimum distance between NS and E cars
% x is position vector for a single E car
% P_S and P_N are position vectors for S and N cars
% N_S and N_N are number of S and N cars

      [dangerT,dangerCol] = find(x>= -5 & x<= 5);

      for i = 1:N_S-1
          for j = 1:numel(dangerT)
              d_ES(i,j) = find_d(x(j),0,-2, P_S(j,i));
          end
      end

      for i = 1:N_N-1
          for j = 1:numel(dangerT)
              d_EN(i,j) = find_d(x(j),0,2, P_N(j,i));
          end       
      end

      d_ES = d_ES';
      d_EN = d_EN';

      mins = [min(d_ES),min(d_EN)];
      D_min = min(mins);

      % if minimum distance between cars is less than 2*radius
      Crash = false;
      if D_min < 3
          Crash = true;
      end

end

function [d] = find_d (x1,y1,x2,y2)
    d = sqrt((y2-y1)^2+(x2-x1)^2);
end

I keep getting an error that says d_EN and d_ES are undefined when the lines d_ES = d_ES' and d_EN = d_EN' are run

[Roger Stafford]

I can only guess at the trouble, but if ‘find_d’ is attempting to find certain elements in P_S and P_N, and if it fails to do so or if it finds multiple copies, then d_ES and d_EN will not be properly defined everywhere.

Answer 1

[Walter Roberson]

dangerT might be empty so numel(dangerT) might be 0, so the "for j" loops might never be executed.

[MJE]

This seems to have fixed it. Since not every car entered the danger zone, dangerT was indeed empty at some points in the code. Thanks!