How to use the index of a matrix in another one?

EDIT

I have a matrix y(k,sum(counter))=

     0     1     0     0     1     0
     1     0     0     1     0     0
     0     1     0     0     1     0
     1     0     0     1     0     0
     0     1     0     0     1     0

counter(j,1) =

         [ 2
           1
           3 ]

I want something that says for each j in the counter(j,1) find its value, then sum the columns of y that are equal to the value in the counter.

 for counter (1)= 2 , then sum the first (two) columns in  y 
 for counter(2)= 1  , sum the following (one) column ( will stay the same basically)
 for counter(3) = 3,  sum the following (three) columns in y

y_new = [

     1          0     1    
     1          0     1     
     1          0     1     
     1          0     1     
     1          0     1     ]

[Azzi Abdelmalek]

This is not clear, what is counter? , what does mean (which is equal 2)?

[Islam]

I clarified the question more below

Answer 1

[Wayne King]

You are missing something in your description, summing the first two columns of y does not result in y_new. How did you get a third column of all ones in y_new?

   idx = find(counter==1,1,'first');
   B = A;
   B(:,1) = sum(A(:,1:2),2);
   B(:,2) = [];

The above creates a new matrix B with the first column equal to sum of the first two columns of A. But again what you described does not result in y_new

[Islam]

I clarified the question below

Answer 2

[Azzi Abdelmalek]

y=[ 0     1     0     1    1     0
  1     0     0     1     0     0
  0     1     0     0     1     0
  1     0     0     1     0     0
  0     1     0     0     1     0]
counter =[2;1;3];
ii=1;
idx1=1;
for k=counter'
  idx2=idx1+k-1;
  out(:,ii)=sum(y(:,idx1:idx2),2);
  idx1=idx2+1;
  ii=ii+1;
end
out

Answer 3

[Andrei Bobrov]

iend = cumsum(counter);
ifs = iend - counter + 1;
idx = zeros(iend(end),1);
idx(ifs) = 1;
idx = cumsum(idx);
[ii,jj] = ndgrid(1:size(y,1;),idx);
out = accumarray([ii(:),jj(:)],y(:));

or

out = cell2mat(cellfun(@(x)sum(x,2),mat2cell(y,size(y,1),counter),'un',0));

or

i0 = cumsum(counter);
i1 = i0 - counter + 1;
for ii = numel(counter):-1:1
    out(:,ii) = sum(y(:,i1(ii):i0(ii)),2);
end