如何根据因子水平对观察结果进行子集化

Question

I have a data frame and need to group the observations based on one variable(vector)'s value into equal number of groups. Below is to illustrate, I used to cut function to create factor based on vector A' value. It create three levels, and each value of the vector A fall into one of the level/group. Now how do I extract/refer to the vector A value based on the cut level of X. For instance, if I want to select observations of A fall into level 1:[1.2.33], how to do that?

> A<-1:5  
> X<-cut(A,breaks=quantile(A,probs=c(0:3)/3),labels=1:3,include.lowest=TRUE)  
> A  
[1] 1 2 3 4 5  
> X  
[1] [1,2.33]    [1,2.33]    (2.33,3.67] (3.67,5]    (3.67,5]   
Levels: [1,2.33] (2.33,3.67] (3.67,5]

Answer 1

Here's a better example than what you provided:

> v <-1:10
> X <- cut(v, breaks=quantile(v,probs=c(0:3)/3), labels=letters[1:3], include.lowest=TRUE)
> X
 [1] a a a a b b b c c c
Levels: a b c

Two select values from v which correspond to level "a" , just run:

> v[X=="a"]
[1] 1 2 3 4