MATLAB: Can you help me

Salam: I want to combine two or more row with same first column in the matrix .

from

A = [1 2 3 4;
     1 5 6 1;
     2 2 3 4;
     2 5 6 1;
     2 6 7 8;
     3 1 2 3;
     4 1 2 3 
     4 5 8 7];

      A = [1 2 3 4 5 6 1 0 0 0;
           2 2 3 4 5 6 1 6 7 8;
           3 6 7 8 0 0 0 0 0 0;
           4 1 2 3 5 8 7 0 0 0];

Numeric arrays cannot have different number of columns for each row.

Start with "If an entry in B is greater than or equal to zero": You can either run a loop over all elements of B or use "logical indexing":

% Linear indexing:
for k = 1:numel(B)
   if B(k) >= 0
     ...
   end
end

Or two indices for rows and columns:

for r = 1:size(B, 2)
  for c = 1:size(B, 1)
    if B(r, c) >= 0
       ...
    end
  end
end

Or "logical indexing":

index = (B >= 0);
disp(B(index))

Now try to use this to modify B. Use the mentioned terms to search in the documentation and in the internet.