r - Remove quotes from vector element in order to use it as a value

Question

Suppose that I have a vector x whose elements I want to use to extract columns from a matrix or data frame M.

If x[1] = "A", I cannot use M$x[1] to extract the column with header name A, because M$A is recognized while M$"A" is not. How can I remove the quotes so that M$x[1] is M$A rather than M$"A" in this instance?

Answer 1

Don't use $ in this case; use [ instead. Here's a minimal example (if I understand what you're trying to do).

mydf <- data.frame(A = 1:2, B = 3:4)
mydf
#   A B
# 1 1 3
# 2 2 4

x <- c("A", "B")
x
# [1] "A" "B"

mydf[, x[1]]                  ## As a vector
# [1] 1 2

mydf[, x[1], drop = FALSE]    ## As a single column `data.frame`
#   A
# 1 1
# 2 2

I think you would find your answer in the R Inferno. Start around Circle 8: "Believing it does as intended", one of the "string not the name" sub-sections.... You might also find some explanation in the line The main difference is that $ does not allow computed indices, whereas [[ does. from the help page at ?Extract.

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Note that this approach is taken because the question specified using the approach to extract columns from a matrix or data frame, in which case, the [row, column] mode of extraction is really the way to go anyway (and the $ approach would not work with a matrix).