R- Several min values using ddply

R- Several min values using ddply

I have a df like this:
> head(datamelt)
TIMESTAMP ring dendro diameter ID Rain_mm_Tot year DOY
1373635 2013-05-02 00:00:00 1 1 3405 r1_1 0 2013 122
1373672 2013-05-02 00:15:00 1 1 3417 r1_1 0 2013 122
1373735 2013-05-02 00:30:00 1 1 3417 r1_1 0 2013 122
1373777 2013-05-02 00:45:00 1 1 3426 r1_1 0 2013 122
1373826 2013-05-02 01:00:00 1 1 3438 r1_1 0 2013 122
1373873 2013-05-02 01:15:00 1 1 3444 r1_1 0 2013 122
I used ddply to get the min diameter value for each of the year (DOY) in
each dendro (dendrometers) in two different ways: i) The first one do its
job, giving one value for each day and dendro. nrow(dailymin)=5784.
However, I don't know what it does when there are several min values in
one day, but in those cases it is not the result I need:
library(plyr)
dailymin <- ddply(datamelt, .(year,DOY,ring,dendro),
function(x)x[which.min(x$diameter), ])
ii) The second way returns several rows for each day if there are several
min values, which is OK. nrow(dailymin)=12634:
dailymin <- ddply(datamelt, .(year,DOY,ring,dendro),
function(x)x[x$diameter==min(x$diameter), ])
Here are my questions: - How is the i) way working when there are several
min values? - And more importantly, in the ii) way, when there are several
min values, how can I only have the min that happen further in time? For
example, image that for dendro #5 in ring #2 there are 3 min values in
3598mm diameter, happening at 13:15, 13:30 and 13:45. I would like to have
only the last one (13:45) and remove the others.
> str(dailymin$TIMESTAMP)
POSIXct[1:12634], format: "2013-05-02 13:45:00" "2013-05-02 08:45:00"
"2013-05-02 14:00:00" "2013-05-02 13:45:00" "2013-05-02 14:45:00" ...
Thanks