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).

Answer 2

You can use matrix indexing:

w <- match(month, names(dataset))

dataset$value <- dataset[ cbind(seq_len(nrow(dataset)), w) ]

Here the w vector tells R which column to take the value from and seq_len is used to say use the same row, so the value column is constructed by taking the 1st column in the 1st row, then the 2nd column and 2nd row, 1st column for the 3rd row, etc.