Multiply elements in second column according to labels in the first

Question

I\'m working in Matlab. I have a two-dimensional matrix with two columns. Lets consider elements in the first column as labels. Labels may be repeated.

How to mult

Answer 1

I would use accumarray. The preprocessing with unique assigns integer indices 1:n to the values in the first row, which allow accumarray to work without creating unnecessary bins for 2 and 4. It also enables the support for negative numbers and floats.

[ulable,~,uindex]=unique(matrix(:,1))
r=accumarray(uindex,matrix(:,2),[],@prod)
r=[ulable,r]

/You can also use splitapply:

[ulable,~,uindex]=unique(matrix(:,1))
r=splitapply(@prod,matrix(:,2),uindex)
r=[ulable,r]