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

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!