第二列子组的平均值，按第一列分组

问题描述

假设我有矩阵A.第一列是组".然后，我想计算每组第二列的平均值.所以我想创建B.

Suppose I have matrix A. 1st column is "group". Then I want to calculate the average of 2nd column for each group. So I want to create B.

A=
 1  2
 1  3
 2  4
 2  2

B=
 1  2.5
 2  3

到目前为止，我所做的最好的事情是构造一个long for和if循环，并使用average函数到达B.

The best thing I did until now is to construct a long for and if loop and use average function to get to B.

但是我想会有更简单的方法.有吗?

But I guess there will be more simple method. Is there?

推荐答案

我没有使用 accumarray 之前，因此由于@Dan的评论，我决定尝试一下.

I hadn't used accumarray before, so due to the comment by @Dan I decided to give it a try.

起初，我尝试过一个简单的版本，并使用histc对出现的次数进行计数以获得所需的平均值...(请注意，accumarray将对输出进行排序，使其与unique的顺序相同，因此均值将被正确计算)

At first I tried a naive version and used histc to count occurrences to get the desired mean values... (Note that accumarray will sort the output the same order as unique, so mean will be calculated correctly)

%// Naive version
ua = unique(A(:,1)); %// use as histc bins (or sorted "group" values)
result = accumarray(A(:,1), A(:,2)) ./ histc(A(:,1), uA);

在这里，accumarray通过对A(:,2)中与A(:,1)中相同下标相对应的所有条目求和.

Here, accumarray operates by summing over all entries in A(:,2) corresponding to identical subscripts in A(:,1).

但是后来我意识到，通过将可选的fun参数传递给accumarray，将"summing"更改为"mean"，您可以将其作为单行代码进行:

But then I realised that by passing the optional fun argument to accumarray changing "summing" to "mean", you can do this as a one-liner:

%// one-liner
result = accumarray(A(:,1), A(:,2), [], @mean);

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