[R] Best way/practice to create a new data frame from two given ones with last column computed from the two data frames?

[Marius Hofert]

Dear expeRts,

What is the best approach to create a third data frame from two given ones, when
the new/third data frame has last column computed from the last columns of the two given
data frames?

## Okay, sounds complicated, so here is an example. Assume we have the two data frames:
df1 <- data.frame(Year=rep(2001:2010, each=2), Group=c("Group 1","Group 2"), Value=1:20)
df2 <- data.frame(Year=rep(2001:2010, each=2), Group=c("Group 1","Group 2"), Value=21:40)

## To make this a bit more fun, let's say the order of elements is different...
(df1 <- df1[sample(1:nrow(df1)),])
(df2 <- df2[sample(1:nrow(df2)),])

## Now I would like to create a third data frame that has "Year" in column one, 
## "Group" in column two, and each entry of column three should consist of the 
## corresponding entry in df1 divided by the one in df2. 

## To achieve this, one could do:
df3 <- df1[with(df1, order(Year,Group)),]
df3$Value <- df3$Value/df2[with(df2, order(Year,Group)),]$Value
colnames(df3)[3] <- "New Value" # typically, the column name changes 

## or one could do:
df3 <- df1[with(df1, order(Year,Group)), -ncol(df1)]
df3 <- cbind(df3, "New Value"=df1[with(df1, order(Year,Group)),]$Value/df2[with(df2, order(Year,Group)),]$Value)

## Is there a more elegant solution? (maybe with ddply?)

## By the way:
df1[,"Value"] # works
df1[,-"Value"] # does not work
## Is there a way to exclude columns by names? that would make the code more readable.
## I know one could use...
subset(df1, select=c("Year","Group"))
## ... but it seems a bit tedious if you have lots of columns to first remove the 
## column name that should be dropped and then put the remaining column names in "select"


Cheers,

Marius