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

[Daniel Malter]

The "problem" with your first solution is that it relies on that the each
'year x group' combination is present in both data frames. To avoid this, I
would recommend to use merge()

df3<-merge(df1,df2,by.x=c("Year","Group"),by.y=c("Year","Group"))
df3$ratio<-with(df3,Value.x/Value.y)
df3

HTH,
Daniel


mhofert wrote:
> 
> 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
> 
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