selection based on dates

Dear All
 
please provide your insigths on the following: 
 
I have:
 
a <-c("1/1/13",15,20)
b <-c("1/5/13",15,25)
c <-c("1/9/13",15,28)
d <-c("2/1/13",18,30)
e <-c("2/5/13",18,35)
f <-c("2/9/13",18,38)
x <-matrix(c(a,b,c,d,e,f),ncol=3,byrow=TRUE)
 
What I would like to do is to eliminate certain rows of this matrix based on the date column values. As you can see, in the second column my values (15 and 18) repeat 3 times each, so this column serves as an ID number if you will. Thus each ID numbers show up with 3 different date values in the first column. Now I would like to eliminate the rows with the earliest date per ID number. My result should look like this:
 
z <-x[-c(1,4),]
 

Hi,

It is not clear to me that the result you show in z is what you describe in words.  On the other hand, I think the following will "eliminate the rows with the earliest date per ID number".

x <- structure(c("1/1/13", "1/5/13", "1/9/13", "2/1/13", "2/5/13", 
"2/9/13", "15", "15", "15", "18", "18", "18", "20", "25", "28", 
"30", "35", "38"), .Dim = c(6L, 3L))

ix <- duplicated(x[,2])

z <- x[ix,]

Is that what you are looking to achieve?  By the way, the solution is not based upon selecting by date as your subject message suggests.  Do you need to filter on dates instead?  If that is so, then you will need a different solution.

Regards,
Ben

as allways, your help is greatly appreciated,
 
thanks,
 
Andras

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Ben Tupper
Bigelow Laboratory for Ocean Sciences
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