extract data from only columns matching character strings

Question

I have a dataset that looks something like this (but much larger)

Jul_08 <- c(1,0,2,0,3)
Aug_08 <- c(0,0,1,0,1)
Sep_08 <- c(0,1,0,0,1)
month<-c(\"Jul         


        

Answer 1

You can use lapply :

value <- unlist(lapply(1:nrow(dataset),
                function(r){ 
                   dataset[r,as.character(dataset[r,'month'])] 
                }))
> value
[1] 1 0 2 0 3

Or, alternatively :

value <- diag(as.matrix(dataset[,as.character(dataset$month)]))
> value
[1] 1 0 2 0 3    

Then you can cbind the new column as you did in your example.

Some notes:

• I prefer unlist(lapply(...)) over sapply since automagic simplification implemented in sapply function tends to surprise me sometimes. But I'm pretty sure this time you can use it without any problem.
• as.character is necessary only if month column is a factor (as in the example), otherwise is redundant (but I would leave it, just to be safe).