Reference vector from data frame using custom function - r

Question

I'm trying to call a vector "a" from a data frame "df" using a function. I know I could do this just fine with the following:
> df$a
[1] 1 2 3
But I'd like to use a function where both the data frame and vector names are input separately as arguments. This is the best that I've come up with:
show_vector <- function(data.set, column) {
data.set$column
}
But here's how it goes when I try it out:
> show_vector(df, a)
NULL
How could I change this function in order to successfully reference vector df$a where the names of both are input to a function as arguments?

Answer 1

It's actually possible to do this without passing the column name as a string (in other words, you can pass in the unquoted column name:
show_vector <- function(data.set, column) {
eval(substitute(column), envir = data.set)
}
Usage example:
df <- data.frame(a = 1:3, b = 4:6)
show_vector(df, b)
# 4 5 6

Answer 2

I've wondered about this kind of thing a lot in the past and haven't found an easy fix. The best I've come up with is this:
df <- data.frame(c(1, 2, 3), c(4, 5, 6))
colnames(df) <- c("A", "B")
test <- function(dataframe, columnName) {
return(dataframe[, match(columnName, colnames(dataframe))])
}
test(df, "A")

Answer 3

Your code would work if you only put the column name in quotes i.e. show_vector(df, "a")
Other multiple ways to do this:
Using base functionality
func <- function(df, cname){
return(df[, grep(cname, colnames(df))])
}
Or even
func <- function(df, cname){
return(df[, cname])
}

Answer 4

You can use substitute to capture the input vector name as it is then use `as.character to make it as a character.
show_vector <- function(data.set, column) {
data.set[,as.character(substitute(column))]
}
Now lets take a look:
(dat=data.frame(a=1:3,b=4:6,c=10:12))
a b c
1 1 4 10
2 2 5 11
3 3 6 12
show_vector(dat,a)
[1] 1 2 3
show_vector(dat,"a")
[1] 1 2 3
It works.
we can also write a simple one where we just input a character string:
show_vector1 <- function(data.set, column) {
data.set[,column]
}
show_vector1(dat,"a")
[1] 1 2 3
Although this will not work if the column name is not a character:
show_vector1(dat,a)
**Show Traceback
Rerun with Debug
Error in `[.data.frame`(data.set, , column) : undefined columns selected**