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

Try something like this, I'm sure it can be improved...

unValues = unique(matrix(:,1));
bb = ones(size(unValues));

for ii = 1:length(unValues)
    bb(ii) = bb(ii)*prod(matrix(matrix(:, 1) == unValues(ii), 2));
end

a = [unValues bb];

Answer 2

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]

Answer 3

You can do it without loops using accumarray and the prod function:

clear
clc


matrix = [1,3,3,1,5; 2,3,7,8,3]';

A = unique(matrix,'rows');

group = A(:,1);

data = A(:,2);

indices = [group ones(size(group))];

prods = accumarray(indices, data,[],@prod); %// As mentionned by @Daniel. My previous answer had a function handle but there is no need for that here since prod is already defined in Matlab.

a = nonzeros(prods)

Out = [unique(group) a]

Out =

     1    16
     3    21
     5     3

Check Lauren blog's post here, accumarray is quite interesting and powerful!