How to save matrix created in loop

I am running the below code which gives me (5,4) matrix output (generated by B matrix) for each "i"

I want to save the output (generated by B matrix) from each "i" one after one. Which means after the 2nd "i" I will have (10, 4) matrix and for 3rd "i" I will have (15, 4) matrix and so on. Final output should be of (length(i)*5, 4) matrix.

Please help. See the code below

   A=xlsread('Jan.xlsx','sheet1','B3:E2000');
   n=size(A,1);
   for k=5;
   for i=1:k:n-(k-1);
     a=A(i:i+(k-1),:);
     B=a(randperm(k),:);
   end
   end

Answer 1

[Azzi Abdelmalek]

   B{i}=a(randperm(k),:);

[Swaami Jan]

Thanks for your reply. But I want the final output in one matrix. That means 2nd output from B will be saved below the first output.

and my "i" is not continuous. It takes values 1, 6, 11, 16. .... etc.

Answer 2

[Jan]

   A = xlsread('Jan.xlsx','sheet1','B3:E2000');
   n = size(A, 1);
   k = 5;   % ??? Without a FOR
   index = 1:k:n-(k-1);
   len = length(index);
   B = cell(1, len); 
   for c = 1:len    % EDITED: "i" -> "1"
     i    = index(c);
     a    = A(i:i+(k-1), :);
     B{c} = a(randperm(k), :);
   end

[Swaami Jan]

Thanks Simon for the code. Your code is giving me below error

" Attempted to access index(0); index must be a positive integer or logical." i = index(c);

   for c = i:len
          i    = index(c);
     a    = A(i:i+(k-1), :);
     B{c} = a(randperm(k), :);
  end

I tried modifying the code and added one more condition. See below

    for c=i:len
       if (i>0)
           i = index(c);
           a= A(i+i(k-1),:);
           B{c}=a(randperm(k),:);
       end
   end

But its is giving " Warning: Colon operands must be real scalars." for the below line

     for c=i:len

[Jan]

This was a typo. See EDITED.

[Karanvir singh Sohal]

I am also facing such problem to save a multiple matrixs from loop in a single matrix

X=[300; 400; 600];
[m,n]=size(X);

for i=1:m
  Y=[1.5*X(i);2*X(i)];
  [mY,nY]=size(Y);
  for j=1:mY
    Mat(j,1)=X(i);
  end
  Mat(:,2)=Y
end

At the end I want a single matrix "c" that can combine all the resultant matrics from the loop

Expected Result

c= [ 300 450; 300 600; 400 600; 400 800; 600 900; 600 1200]