How to extract different columns from each row of a data.frame? [duplicate]

Answer 1

How about this:

Df <- data.frame(
  x=c("a","b","c"), 
  y=c("d","e","f"), 
  z=c("g","h","i"))
##
i <- c(3,1,2)
##
index2D <- function(v = i, DF = Df){
  sapply(1:length(v), function(X){
    DF[X,v[X]]
  })
}
##
> index2D()
[1] "g" "b" "f"

Answer 2

Just use matrix indexing, like this:

dframe[cbind(seq_along(i), i)]
# [1] "g" "b" "f"

The cbind(seq_along(i), i) part creates a two column matrix of the relevant row and column that you want to extract.