Sum of rows if elements in the first columns are same

Hello Guys, i have to do a sum operation on a matrix. for example x = [1 2 3 4 5 6; 1 3 5 7 9 11; 2 3 4 5 6 7; 2 4 6 8 10 12]

i need to check if the row elements in the first column are equal, if equal then add the rows together. the answer below is what i expect

x_sum = [1 5 8 11 14 17; 2 7 10 13 16 19].

It would be great if you guys can help. Thanks Raj

[José-Luis]

What have you tried so far?

[Rajesh]

I tried with Cumsum and Sum but wasnt able to get any where close. I can do it using loops but before using loops wanted to ask you guys the experts whether this can be done without loops....

[José-Luis]

Is there a particular reason you don't want loops, they are not intrinsically bad and can actually be faster than other approaches. Please show what you have done so we can guide you.

Answer 1

[Sean de Wolski]

One of many ways:

x = [1 2 3 4 5 6; 1 3 5 7 9 11; 2 3 4 5 6 7; 2 4 6 8 10 12];
[uv,~,idx] = unique(x(:,1));
nu = numel(uv);
x_sum = zeros(nu,size(x,2));
for ii = 1:nu
  x_sum(ii,:) = sum(x(idx==ii,:));
end

[Rajesh]

Sean, Thanks for the answer works great, to have the set of data as per my answer i changed a little bit the answer above, but it is a great solution. Thanks Raj

My changed code is as below

      x = [1 2 3 4 5 6; 1 3 5 7 9 11; 2 3 4 5 6 7; 2 4 6 8 10 12];
      [uv,~,idx] = unique(x(:,1));
      nu = numel(uv);
      x_sum = zeros(nu,size(x,2));
      for ii = 1:nu
              x_sum(ii,1) = uv(ii,1); 
        x_sum(ii,2:size(x,2)) = sum(x(idx==ii,2:size(x,2)));
      end