[R] Select rows based on matching conditions and logical operators

[kborgmann]

Hi,
I have a dataset in which I would like to select rows based on matching
conditions and return the maximum value of a variable else return one row if
duplicate counts exist.  My dataset looks like this:
PGID	PTID	Year	 Visit  Count
6755	53121	2009	1	0
6755	53121	2009	2	0
6755	53121	2009	3	0
6755	53122	2008	1	0
6755	53122	2008	2	0
6755	53122	2008	3	1
6755	53122	2009	1	0
6755	53122	2009	2	1
6755	53122	2009	3	2

I would like to select rows if PTID and Year match and return the maximum
count else return one row if counts are the same, such that I get this
output 
PGID	PTID	Year	 Visit  Count
6755	53121	2009	1	0
6755	53122	2008	3	1
6755	53122	2009	3	2

I tried the following code and the output is almost correct but duplicate
values were included
df2<-with(df, sapply(split(df, list(PTID, Year)),
function(x) if (nrow(x)) x[which(x$Count==max(x$Count)),]))
df<-do.call(rbind,df)
rownames(df)<-1:nrow(df)

Any suggestions? 
Thanks much for your responses!




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