How to get lastest value based on date in R? - r

Question

I tried to find out(guess) current status based on lastest status.
Assume that we have following data frame(it's abbreviation of real data)
examineData
ID Date Status_Value
A 2012-01-01 100
A 2012-01-10 200
A 2012-02-20 500
B 2012-01-01 1100
B 2012-01-10 1200
B 2012-02-20 1500
C 2012-01-01 2100
C 2012-01-10 2200
C 2012-02-20 2500
In above, A,B and C are objects which have status_value. Status_values were examined on the Date.
asked
ID Date
A 2012-01-09
A 2012-02-28
B 2012-02-19
C 2012-01-10
But, someone asked about status from A,B and C (it could be less) on specific date.
As you can see, some of asked$Date does not match to the examinData$Date.
In that case, we decided to get lastest data from examineData$Date.
ID Date Status_Value
A 2012-01-09 100
A 2012-02-28 500
B 2012-02-19 1200
C 2012-01-10 2200
Would you give me a sample code? (Speed is important - 1,600,000 rows of examineData, 110,000 rows of asked)
In addition, There are over 60,000 kinds of ID. And, there are no duplicate date in a same ID in examineData

Answer 1

This seems to work:
examineData$Date <- as.Date(examineData$Date, format = "%Y-%m-%d")
asked$Date <- as.Date(asked$Date, format = "%Y-%m-%d")
#res <- unlist(lapply(split(examineData, examineData$ID),
# function(x) { merged <- c(x$Date, asked$Date[asked$ID == unique(x$ID)]) ;
# x$Status_Value[which(order(merged) %in% length(merged)) - 1] }))
I guess, though, a data.table solution might be more efficient than this.
EDIT Modified solution, provided -now- that there might be duplicate IDs in asked:
#dates should, still, be turned into actual dates if they aren't
#function to (m)apply over asked
fun <- function(id, date)
{
subsetted_examineData <- examineData[examineData$ID == id,]
merged <- c(subsetted_examineData$Date, date)
res <- subsetted_examineData$Status_Value[which(order(merged) %in% length(merged)) -1]
return(res)
}
res <- mapply(fun, asked$ID, asked$Date)
res
# A A B C
# 100 500 1200 2200
cbind(asked, Status_Value = unname(res))
# ID Date Status_Value
#1 A 2012-01-09 100
#2 A 2012-02-28 500
#3 B 2012-02-19 1200
#4 C 2012-01-10 2200

Answer 2

sel <- vector()
for(i in 1:length(unique(examineData$ID))){
id <- unique(examineData$ID)[i]
set <- subset(examineData,ID==id)
dif <- asked[asked$ID==id,"Date"] - set$Date
dif[dif<0] <- NA
sel[i] <- row.names(set)[which.min(dif)]
}
examineData[sel,]
To get this
ID Date Status_Value
1 A 2012-01-01 100
5 B 2012-01-10 1200
8 C 2012-01-10 2200
You can build in some "corrections" for missing values, but as you have not specified any of, this is the clean way.