[R] data.frame indexing

[Ben Tupper]

On Apr 28, 2008, at 10:40 AM, Georg Ehret wrote:

> Dear R Community,     A simple problem (for some of you): I wish to  
> index a
> data.frame by all elements NOT in my index
> E.g.:
>> a<-as.data.frame(matrix(rnorm(100),nrow=10,ncol=10))
>> b<-which(a$V1>0.8)
>> b
> [1]  1  4  6 10
>> a_indexb<-a[b,]
>> a_notIndexB<-a[!b,]
>> nrow(a_notIndexB)
> [1] 0
>
> Indexing a on b is not a problem (a_indexb), but how can do get  
> only the
> elements left if I take out the elements indexed with b?
>

Hi,

I think one of the most wonderfully magic things about R is that it  
can use a logical array to index other things (like a data frame).   
You could use the extra step with which, but in your case you can  
bypass that step.  Here is a simple example...

 > a <- data.frame(V1 = seq(-5,5), V2 = seq(-5,5))
 > a
    V1 V2
1  -5 -5
2  -4 -4
3  -3 -3
4  -2 -2
5  -1 -1
6   0  0
7   1  1
8   2  2
9   3  3
10  4  4
11  5  5
 > b <- a$V1 > 0
 > b
  [1] FALSE FALSE FALSE FALSE FALSE FALSE  TRUE  TRUE  TRUE  TRUE  TRUE
 > a[b,]
    V1 V2
7   1  1
8   2  2
9   3  3
10  4  4
11  5  5
 > a[!b,]
   V1 V2
1 -5 -5
2 -4 -4
3 -3 -3
4 -2 -2
5 -1 -1
6  0  0

Hope that helps!

Ben