Creating an array within a for loop

I have a plotting function which gets the points to plot from a for loop, i.e.

 for i=1:length(rat2ind)
       ind = A(:,1)==A(rat2ind(i),1);  % pick all the rat locs for rat i
       plot(A(ind,2),A(ind,3),randcolor);axis([-0.5 7.5, -0.5 7.5])  % plot rat i trajectory 
       hold on;
    end

but instead of plotting the points, I'd like to save the points in an (n,2) array. I tried this:

   for i=1:length(rat2ind)
          ind = A(:,1)==A(rat2ind(i),1);
          H=vertcat(H,[A(ind,1),A(ind,2),A(ind,3)]);
      end

but it gave me a variable which is definitely not what I needed as its twice as large as my original data (the data, and my actual goal isn't really relevant here)

Thanks for any help

[Cedric]

So in some sense you would like to order A by whatever key is stored in A(:,1), in a "key" order that is given by rat2ind ..?

[Matt J]

the data, and my actual goal isn't really relevant here

The data's size might be relevant when choosing an approach for you

[Cedric]

This line is suspicious to me:

 ind = A(:,1)==A(rat2ind(i),1);

are you sure that it is not:

 ind = A(:,1) == rat2ind(i) ;

[Timothy Russell]

changing this gives me exactly the same wrong answer...!

Answer 1

[Timothy Russell]

Thanks for the help, turns out that this function is exactly what I wanted:

http://www.mathworks.co.uk/matlabcentral/fileexchange/29692-repeated-values#comments

Answer 2

[Matt J]

   N=size(A,1);
   map=double(bsxfun(@eq,sparse(A(:,1)),A(ratind,1).'));

   ind=nonzeros( bsxfun(@times, (1:N).', map )   );

   H=A(ind,1:3);

[Timothy Russell]

This gives me the exact same array as my original code...which is very odd.

[Matt J]

Yeah, I see that now. My guess is that it's because, as Cedric pointed out, you probably really want to be comparing with ratind instead of A(ratind,1).

[Timothy Russell]

Yeah you're right, sorry. The confusion came from trying to use exactly the same method in my plotting function, and forgetting what the variables actually represented. Thanks alot for the help, i'm actually starting to enjoy using matlab!_