R help - Jun 2005 - how to count "associated" factors?

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!

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!
> 
>

try this:

X  <- factor(sample(c("A", "B", "C"), 10,
TRUE), levels = c("A", "B",
"C"))
Y  <- factor(sample(c("A", "B", "C"), 10,
TRUE), levels = c("A", "B",
"C"))
#############
table(X, Y)


I hope it helps.

Best,
Dimitris

----
Dimitris Rizopoulos
Ph.D. Student
Biostatistical Centre
School of Public Health
Catholic University of Leuven

Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: +32/16/336899
Fax: +32/16/337015
Web: http://www.med.kuleuven.be/biostat/
     http://www.student.kuleuven.ac.be/~m0390867/dimitris.htm


----- Original Message ----- 
From: "zhihua li" <lzhtom at hotmail.com>
To: <r-help at stat.math.ethz.ch>
Sent: Tuesday, June 21, 2005 2:46 PM
Subject: [R] how to count "associated" factors?

> 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!
>
>

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