仅查找所选条目的平均值 [英] Finding mean of selected entries only

问题描述

考虑两个向量:

 v= [1 2 3 4 5 6 7]

 a=['a' 'b' 'c' 'a' 'a' 'a' 'd']

我想查找v中所有条目的均值，其中a中的对应条目为'a'；

I want to find the mean of all entries in v whose corresponding entries in a is 'a';

即test =平均值(1,3,4,5)

i.e. test= mean(1,3,4,5)

我已经尝试过以此作为开始来捕获条目的方法:

I have tried this for a start to catch the entries:

for i=1:7
 if abs(char(a(i))-char(c))==0;
   test(i)=v(i);
 end
end

测试

test =      1     0     0     4     5     6  

问题:

1. 它为未找到的条目分配0
2. 它不在考虑上学期

推荐答案

尝试使用

Try using the ismember function:

>> help ismember
 ismember True for set member.
    ismember(A,S) for the array A returns an array of the same size as A
    containing 1 where the elements of A are in the set S and 0 otherwise.
    A and S can be cell arrays of strings.

ismember将您的test向量形成为逻辑数组，并在向量中找到字符'a'的位置分配1，在没有向量的地方分配0:

ismember forms your test vector as a logical array, assigning 1 where the character 'a' is found in your vector, and 0 where it isn't:

>> ismember(a, 'a')

ans =

 1     0     0     1     1     1     0

然后您可以将其用作逻辑索引，以从向量v中提取相应的条目:

You can then use this as a logical index to extract the corresponding entries from your vector v:

>> v(ismember(a, 'a'))

ans =

     1     4     5     6

最后，您可以求出该向量的均值:

Finally, you can take the mean of this vector:

>> mean(v(ismember(a, 'a')))

ans =

  4

---

编辑 我已经意识到，在您的情况下，您实际上可以使用比较运算符以更简单的方式形成逻辑数组:

---

EDIT I have realised that in your case, you can actually form your logical array in a much simpler way, using a comparison operator:

>> a == 'a'

ans =

     1     0     0     1     1     1     0

因此，您的最后一行代码将如下所示:

So your final line of code will look like so:

>> mean(v(a == 'a'))

ans =

     4

ismember在要测试是否存在多个字符的情况下很有用，例如，如果要查找'a'或'b'所在的位置.

ismember is useful where you want to test for the presence of more than one character, for example if you wanted to find locations where 'a' or 'b' were.

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