根据在第一标签乘以第二列元素 [英] Multiply elements in second column according to labels in the first

问题描述

我在Matlab工作。
我有两列的二维矩阵。让我们考虑在第一列作为标记元素。标签可以重复。

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.

如何乘以每一个标签的第二列的所有元素？

How to multiply all elements in the second column for every label?

例如：

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

我需要：

a = [1,3,5; 16,21,3]'

您可以帮我做这件事的没有 换，而周期？

Can you help me with doing it without for-while cycles?

推荐答案

您可以不用使用的 accumarray 和刺功能：

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

检查劳伦博客的帖子这里，accumarray相当有趣而强大！

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

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