[R] contingency table on data frame

[Karin Lagesen]

I am sorry if this is a faq or tutorial somewhere, but I am unable to
solve this one.

What I am looking for is a count of how many different
categories(numbers in this case) that appears for a given factor.

Example:

> l <- c("Yes", "No", "Perhaps")
> x <- factor( sample(l, 10, replace=T), levels=l )
> m <- c(1:5)
> y <- factor( sample(m, 10, replace=T), levels=m )
> z = c(1:10)
> my_df = data.frame("Z" = z, "Y"= y, "X" = x)
> my_df
    Z Y       X
1   1 4     Yes
2   2 1      No
3   3 2 Perhaps
4   4 3     Yes
5   5 4      No
6   6 5      No
7   7 1     Yes
8   8 4 Perhaps
9   9 4     Yes
10 10 2 Perhaps
> 

I am now looking for a table that will give me this:

Yes      3       # Yes has these ys: 4,3,1,4, two are the same, ergo 3
No       3       # No has these ys: 1,4,5
Perhaps  2       # Perhaps has these ys: 2,4,2

My dataframe has lots of other colums too, but I only want this
information out.


Thankyou for your help!

Karin
-- 
Karin Lagesen, PhD student
karin.lagesen at medisin.uio.no
http://folk.uio.no/karinlag