[R] how to count "associated" factors?

[Liaw, Andy]

table() is your friend:


> X <- factor(scan(what=""))
1: A B B C A C B A C C
11: 
Read 10 items
> Y <- factor(scan(what=""))
1: B B C C C A A A B B
11: 
Read 10 items
> table(X)
X
A B C 
3 3 4 
> table(X, Y)
   Y
X   A B C
  A 1 1 1
  B 1 1 1
  C 1 2 1

Andy

> From: zhihua li
> 
> hi netters
> 
> Suppose I have a factor X, with 10 elements and 3 levels: A B 
> B C A C B A C 
> C .
> 
> It is easy to count the number of elements for each level: 
> tapply(X,X,length).
> 
> Now I have another factor Y, which formed a matrix with X:
> 
>  X| A B B C A C B A C C
>  Y| B B C C C A A A B B
> 
> I wanna count the number of elements for each of these 
> conditions: when X=A 
> and Y=A; when X=A and Y=B; when X=A and Y=C; when X=B and 
> Y=A; when X=B and 
> Y=B; when X=B and Y=C; when X=C and Y=A; when X=C and Y=B; 
> when X=C and 
> Y=C.
> 
> The code I have written for this task is too complicated, 
> involving a lot 
> of for loops and if conditions. I believe there's some nice 
> code that can 
> do it far more efficiently. Can anyone give me a hint?
> 
> Thanks a lot!
> 
>