Function that takes variable names and concatenates them interally in R

Question

I'd like to build a function that takes a variable and then concatenates it with other letters within a function to refer to different variables in a frame. For example (code does not work):

set.seed(123)
frame <- data.frame(x = rnorm(100), p01u = rnorm(100), p01o = rnorm(100))

sum.fun <- function(frame, var){
        xu <-  cat(paste(var, "u", sep = ""))
        xo <-  cat(paste(var, "o", sep = ""))
        print(sum(frame$xu))
        print(sum(frame$xo))
}

sum.fun(frame, "p01")

The issue here is in the cat(paste()), but I can't quite figure out how to coerce it into a class that works. Any and all thoughts greatly appreciated.

Answer 1

You could remove the cat calls and use [[ to get the columns, as $ does not work well with character arguments.

sum.fun <- function(frame, var) {
    xu <-  paste0(var, "u")
    xo <-  paste0(var, "o")
    setNames(c(sum(frame[[xu]]), sum(frame[[xo]])), c(xu, xo))
}

sum.fun(frame, "p01")
#      p01u      p01o 
# -10.75468  12.04651 

To check:

with(frame, c(sum(p01u), sum(p01o)))
# [1] -10.75468  12.04651

Note: If you want multiple outputs from a function, I think it's best to use a list for the output. This way, you can have a vector of characters and a vector of numerics both classed as desired in the result. For example:

sum.fun2 <- function(frame, var){
    xu <-  paste0(var, "u")
    xo <-  paste0(var, "o")
    list(names = c(xu, xo), values = c(sum(frame[[xu]]), sum(frame[[xo]])))
}

(sf2 <- sum.fun2(frame, "p01"))
# $names
# [1] "p01u" "p01o"
#
# $values
# [1] -10.75468  12.04651
sapply(sf2, class)
#       names      values 
# "character"   "numeric"