根据一列对矩阵行进行分组 [英] Grouping matrix rows in terms of one column

问题描述

首先，我很难很好地描述问题，但我会尽力的.

First of all it is really hard for me to describe the problem really good but I'll try.

说我们有矩阵A

A = [23 1;
     45 1
     78 1
     86 1
     98 2
     1  2
     23 2
     14 3
     15 4
     85 4]

我想要的输出是

    B{1} = [23,45,78,86]
    B{2} = [98,1,23]
    B{3} = [14]
    B{4} = [15,85]

请记住，原始A是一个巨大的矩阵，我不想使用for循环来做到这一点.我想使用使用并行处理的函数.

Bear in mind that the original A is a huge matrix, and I do not wanna do this with for loops. I would like to use functions that uses parallel processing.

推荐答案

您可以在此处使用accumarray:

B = accumarray(A(:,2),A(:,1),[],@(x){x},{});

如果您知道A已排序，并且第二列中没有缺失的条目，则也可以使用mat2cell:

If you know that A is sorted, and that there is no missing entry from the second column, you can also use mat2cell:

counts = histc(A(:,2),unique(A(:,2)));
B = mat2cell(A(:,1),counts);

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