[R] behavior of "by"

[Jeff Laake]

Any insight into the behavior of "by" in the following case would be 
appreciated.  There is a note in the help details for "by" about 
documenting behavior since v2.7 but I don't entirely understand what it 
is saying.  I'm using R2.7.2 Windows.  I'm interested if the following 
behavior was a change or whether it has always worked this way.  I 
looked at RSiteSearch and read through version changes but found nothing.

Take a dataframe as follows:
 > samples
   Region.Label  Area Sample.Label Effort Label
1             1 10000            1    100    11
2             1 10000            2    100    12
3             1 10000            3    100    13
4             1 10000            4    100    14
5             1 10000            5    100    15
6             1 10000            6    100    16
7             1 10000            7    100    17
8             1 10000            8    100    18
9             1 10000            9    100    19
10            1 10000           10    100   110

Use "by" to tally number of entries with particular values of 
Region.Label (in this case there is only 1 value of Region.Label)

by(samples$Effort,samples$Region.Label,length)
INDICES: 1
[1] 1

I expected to get 10 instead of 1.  I debugged into by.data.frame and I 
can see that it used drop=FALSE, so length returned the number of 
columns which is 1.  But if I do any of the following, I get the 10 I 
expect.

 > by(rep(1,10),samples$Region.Label,length)
samples$Region.Label: 1
[1] 10
by(samples$Label,samples$Region.Label,length)
samples$Region.Label: 1
[1] 10

Also if I use "tapply" with samples$Effort instead of "by" I get the 10 
I expect.

tapply(samples$Effort,samples$Region.Label,length)
 1
10

I do not understand why I'm getting these differences but I can see that 
I'm going to use tapply from now on.