I want to save the data of the rows of U that are not empty. However, since this is inside a loop, I keep deleting the data from the previous loop. How can I keep saving the data without deleting the previous one?

 if isempty(U)==0    
 u = U(any(U,2),:);
 U=[];
 end

On the first iteration, it works how it should, but on the second iteration it will overwrite the previous data saved in "u". How can I fix this? It should keep saving on the next row, not deleting the previous data.

[Adam]

Please give the full relevant code, including the loop. Just the code inside the loop is not very helpful.

It isn't obvious what U is because a numeric array cannot have empty elements in it other than being empty itself.

[Eduardo Rocha]

while n<size(Preos2013S1,1)/20 || N<100
  X=zeros(50,7);
  for k=1:size(X,2)
    for j=1:size(X,1)
      X(j,k)=perce_min(1,k)+rand(1,1)*(perce_max(1,k)-perce_min(1,k));
    end
  end
  D=pdist2(X,classesalgimunS1);
  T=zeros(size(D,1),1);
  for k=1:size(D,2)
    A(k)=min(D(:,k));
  end
  for k=1:size(D,1)
    for j=1:size(D,2)
      if  D(k,j)<0.4 && D(k,j)==A(j) && b(j)==0
        T(k)=T(k)+1;
      end
    end
    if T(k)>(size(classesalgimunS1,1)/size(X,1))*3
      U(k,:)=X(k,:);   
    end
  end
  if isempty(U)==0    
    u = U(any(U,2),:);
    U=[];
  end
  nclasse=max(b);
  for k=1:size(u,1)
    for j=1:size(A,2)
      if pdist2(u(k,:), classesalgimunS1(j,:))==A(j) && pdist2(u(k,:), classesalgimunS1(j,:))<0.4 && b(j)==0
        b(j)=nclasse+k;
        n=n+1;
      end
    end
  end
  N=N+1;
end

Answer 1

[dpb]

In most rudimentary to make work, add

u=[];  % initialize

before beginning the outer loop and then

if ~isempty(U)
  u = [u; U(any(U,2),:)];  % accrete new U into u
  U=[];
end

This has the issue of dynamic reallocation of u on each pass, but if isn't terribly long the runtime penalty shouldn't be too great. If it does bog down, then preallocate a very large array and populate it explicitly by keeping track of number of elements added each pass by

nU=size(U,1);

and a running total for the next insertion location into the array. And, of course, have to check don't overrun it and reallocate more room, etc., etc., if do. But, for all but the extreme cases the first solution should be just fine; wouldn't worry about the other details until after have shown that is a real performance issue on real data.

[Eduardo Rocha]

Well it takes a long time, but it works :) thank you!

[dpb]

Do you foresee doing this a significant number of further times? If so, the effort to preallocate for u isn't that great, the general idea is

nMaxU=SomeLargeNumberGreaterThanExpectedLength;
u=zero(nMaxU,nColOfu);
idu=1;
...
if ~isempty(U)
  U=U(any(U,2),:);
  idu2=idu1+size(U,1)-1;
  if(idu2>nMaxU)
    % additional reallocation of more room here
    ...
  else
    u(idu1:idu2,:)=U;
    idu1=idu2+1;
    U=[];
  end

Should take very little to flesh out the remainder...