Matrix Dimensions not agreeing

2015 11 13
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更新时间：2015 11 15
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Hi
I am working with the following:
phi = @(ep,r1) exp(-(ep*r1).^2);
ep = 1;
r1=0.1;
S=linspace(0,1);
t=linspace(0,1);
% Randomly Select 10 points from each of S and t.
S_rand = S(sort(randperm(numel(S), 10)));
t_rand = t(sort(randperm(numel(t), 10)));
[S_,t_]=meshgrid(S_rand,t_rand);
X=[S_(:) t_(:)];
D=phi(ep, distm(X,X));
U=bsf(S_,t_,1,1,0.25,0.05);
c=D\U;
I need D to be a 10 x 10 matrix (like U) and not a 100 x 100 matrix as this code is generating. I can't seem to see why this is happening. Any ideas?
Many thanks
Joe

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Star Strider
Star Strider 2015-11-13

1 个投票

Two ideas:
First, it’s best to always completely vectorise your functions unless you intend matrix operations:
phi = @(ep,r1) exp(-(ep.*r1).^2);
Second, we don’t have ‘distm’. I would break it out into its own variable for troubleshooting purposes to be certain it is not creating your (100x100) matrix:
X=[S_(:) t_(:)];
Q1 = distm(X,X) % See What ‘distm’ Returns
D=phi(ep, distm(X,X));

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Joe Bannen
Joe Bannen 2015-11-14
Hi
Thanks for your response. I have split off the variable as you suggest and vectorised the phi function.
In terms of:
X=[S_(:) t_(:)];
D=phi(ep, distm(X,X));
Is this not taking my new values of S_ and t_ as rows and columns so that the first point is (0th,0th) etc.? Where is the 100*100 coming from?
Sorry if I am not seeing your point!
Thanks again
Joe
Star Strider
Star Strider 2015-11-14
My pleasure.
I am hypothesising that your ‘distm’ function (that we still have not seen) is creating the (100x100) matrix. That is the reason you need to insert the ‘Q1’ assignment between those lines:
X=[S_(:) t_(:)];
Q1 = distm(X,X) % See What ‘distm’ Returns
D=phi(ep, distm(X,X));
to see what ‘distm’ is returning.
Joe Bannen
Joe Bannen 2015-11-14
Hi
Oops! Here it is:
function D=distm(X,Y)
% distm is the distance matrix
% distm(X,Y) is the matrix of the pairwise Euclidean distance
% between the points in the rows of X and Y
[M,s]=size(X);
[N,s]=size(Y);
D=repmat(sum(X.*X,2),1,N)-2*X*Y'+repmat((sum(Y.*Y,2))',M,1);
D=sqrt(D);
Cheers
Joe
Star Strider
Star Strider 2015-11-14
Having the ‘distm’ code solved the problem (and absolved it of blame, since it is doing exactly what you asked it to do). The problem is here:
X=[S_(:) t_(:)];
Your ‘X’ is now a (100x2) matrix, and since ‘distm’ creates pairwise distances between the rows of the argument matrices, the size of the output matrix will be (NrowsX x NrowsY), giving you the (100x100) matrix.
Consider using ‘S_rand’ and ‘t_rand’ to create ‘X’ instead, if that fits the logic of your code.
Joe Bannen
Joe Bannen 2015-11-14
Thanks, that is absolutely fantastic. This is such a great place to learn more about this awesome computer program!
Cheers
Joe
Star Strider
Star Strider 2015-11-15
As always, my pleasure! I appreciate your compliment.
It definitely is! MATLAB Answers is where I learned a lot about MATLAB, both by reading other Answers and by helping to solve problems I would never have encountered otherwise.

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