Creating an array within a for loop

Question

Timothy Russell 2013-1-27

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https://ww2.mathworks.cn/matlabcentral/answers/60036-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

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Cedric 2013-1-27

在 MATLAB Online 中打开

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 2013-1-28

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

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Answer 1

Timothy Russell 2013-1-28

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https://ww2.mathworks.cn/matlabcentral/answers/60036-creating-an-array-within-a-for-loop#answer_72698

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

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Answer 2

Matt J 2013-1-27

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/60036-creating-an-array-within-a-for-loop#answer_72587

编辑：Matt J 2013-1-27

在 MATLAB Online 中打开

   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);

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Matt J 2013-1-29

编辑：Matt J 2013-1-29

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 2013-2-2

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!_

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