r - Adding a new column to each element in a list of tables or data frames

Question

I have a list of files. I also have a list of "names" which I substr() from the actual filenames of these files. I would like to add a new column to each of the files in the list. This column will contain the corresponding element in "names" repeated times the number of rows in the file.

For example:

df1 <- data.frame(x = 1:3, y=letters[1:3])
df2 <- data.frame(x = 4:6, y=letters[4:6])
filelist <- list(df1,df2)
ID <- c("1A","IB")

Pseudocode

  for( i in length(filelist)){

       filelist[i]$SampleID <- rep(ID[i],nrow(filelist[i])

  }

// basically create a new column in each of the dataframes in filelist, and fill the column with repeted corresponding values of ID

my output should be like:

filelist[1] should be:

   x y SAmpleID
 1 1 a       1A
 2 2 b       1A
 3 3 c       1A

fileList[2]

   x y SampleID
 1 4 d       IB
 2 5 e       IB
 3 6 f       IB

and so on.....

Any Idea how it could be done.

Answer 1

An alternate solution is to use cbind, and taking advantage of the fact that R will recylce values of a shorter vector.

For Example

x <- df2  # from above
cbind(x, NewColumn="Singleton")
 #    x y NewColumn
 #  1 4 d Singleton
 #  2 5 e Singleton
 #  3 6 f Singleton

There is no need for the use of rep. R does that for you.

Therfore, you could put cbind(filelist[[i]], ID[[i]]) in your for loop or as @Sven pointed out, you can use the cleaner mapply:

filelist <- mapply(cbind, filelist, "SampleID"=ID, SIMPLIFY=F)