[R] Extracting rows with latest date from a data frame

[dthomas]

Hi,

I have a list of unique patient IDs which I want to run against a dataframe
of unique patient IDs with dates. There are multiple date instances against
unique IDs. The end result I wish is that I have a list of IDs (which are
unique) with the latest date. I have begun with using test data (but I
haven't got very far!)

#Create test data
#Data frame creation
df1<-data.frame(uID=c("1","2"))
df2<-data.frame(uID2=c("1","1","1","2","2","2"),Date=c(as.Date("20/12/2010",
format="%d/%m/%Y"),
as.Date("12/05/2011", format="%d/%m/%Y"),as.Date("12/07/2011",
format="%d/%m/%Y"),
as.Date("12/07/2010",format="%d/%m/%Y"),as.Date("12/05/2009",format="%d/%m/%Y"),
as.Date("20/10/2012",format="%d/%m/%Y")))

for (i in uID) {                                                                                                    
#For all the unique patient IDs
                df3<-data.frame(i)                                                                         
#Create a one record data frame of record
                m1<-merge(df3,df2,by.x="i", by.y="uID2")                         
#Merge with list of IDs and dates of event
                                                for (j in m1) {
                                               #maybe an if statement in
here but I’m not sure how to seperate the dates out and then create a new
dataframe with the IDs and latest date??
                                }
                }

I am completely new to R so R vocabulary is limited at the moment. I'm not
sure if i'm on the right tracted here. For example I not sure if I need
dataframe of uniqueIDs (df1). Is there a way in a For loop  that you can
pinpoint all date instances for a given ID and then select the latest and
add to new dataframe?

Your help is much appreciated.
D

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