Plotting a circle and finding intersections

I am trying to plot a circle using existing data I have and find the points where the circle intersect grid lines. I randomly picked a set of coordinates from the data to be used as the center point of the circle. However, the code I have isn't displaying the circle. When I run the code, it says there is an error in the line "plot(Q(1,:),Q(2,:),'.b')". Can anyone solve this problem? Thanks in advance!

 nc = 1; % number of circle 
 r = 10; % radius
 th = linspace(0,2*pi);
 iwant = cell(nc,1);

 for i = i:nc
    % get center of circle
    x = center(:,2); % x coordinate  
    y = center(:,3); % y coordinate 

    % circle coordinates
    cx = x+r*cos(th);
    cy = y+r*sin(th);
    plot(cx,cy,'k');

    % get intersection 
    Q = cell(1,[]) ;
      count = 0 ;

     % loop along lines parallel to x-axes
     for j = 1:N 
          P = InterX([cx ;cy],[X(j,:) ; Y(j,:)]) ;
          if ~isempty(P)
              count = count+1 ;
              Q{count} = P ;
          end
      end
      % loop along lines parallel to y-axes
      for k = 1:N
          P = InterX([cx ;cy],[X(:,k)' ; Y(:,k)']) ;
          if ~isempty(P)
              count = count+1 ;
              Q{count} = P ;           
          end
      end
      Q = cell2mat(Q) ;
      plot(Q(1,:),Q(2,:),'.b')
      iwant{i} = Q ;

 end

[KSSV]

Here is complete code which is working https://in.mathworks.com/matlabcentral/answers/340636-how-to-randomly-plot-circles-on-grid-lines-and-determine-coordinates-of-intersection. YOu have missed many variables in the above code.

[Atiqah Zakirah]

Thank you for the help! I made some changes to the code. I have troubling integrating my code with the code you provided. Instead of getting random center of the circle, I'm getting random coordinates from my data set to be the center of my circle this time. When I make this change to the code, it presents me with an error.

Index exceeds matrix dimensions error: P = InterX([cx ;cy],[X(:,k)' ; Y(:,k)']) ; Error: plot(Q(1,:),Q(2,:),'g');

 %%create grid 
 N = 30;
xgrid = linspace(103.6,104,N);
ygrid = linspace(1.25,1.5,N);
[X,Y] = meshgrid(xgrid,ygrid);
figure 
hold on
plot(X,Y,'r');
plot(X',Y','r');
xlim([103.6 104])
ylim([1.25 1.5])

 %%plot points
plot(coordinates(:,2),coordinates(:,3),'.');
plot (train(:,1),train(:,2),'x');

 %%obtaining random coordinates
row = randi(size(coordinates,1),1); % generates random row number 
% obtain x and y coordinates from selected row 
center = coordinates(row,:); 

 %%create a circle using center 
nc = 1; % number of circle 
R = 30; % radius
th = linspace(0,2*pi);
iwant = cell(nc,1); 

 for i = i:nc
    % get center of circle
    x = center(:,2); % x coordinate  
    y = center(:,3); % y coordinate 

    % circle coordinates
    cx = x+R*cos(th);
    cy = y+R*sin(th);
    plot(cx,cy,'k');

    % get intersection 
    Q = cell(1,[]) ;
      count = 0 ;

     % loop along lines parallel to x-axes
     for j = 1:N 
          P = InterX([cx ;cy],[X(j,:) ; Y(j,:)]) ;
          if ~isempty(P)
              count = count+1 ;
              Q{count} = P ;
          end
      end
      % loop along lines parallel to y-axes
      for k = 1:N
          P = InterX([cx ;cy],[X(:,k)' ; Y(:,k)']) ;
          if ~isempty(P)
              count = count+1 ;
              Q{count} = P ;           
          end
      end
      Q = cell2mat(Q) ;
      plot(Q(1,:),Q(2,:),'g');
      iwant{i} = Q ;

 end

[KSSV]

coordinates and train are missing..