Using dplyr::select within a function to select a fixed column from multiple data frames

Question

I'm trying to define a function that uses dplyr::select to select a column that has the same name in multiple dataframes, so the column's name shouldn't be a relevant input to the user. For instance, I'd like something like this to work for any data frame that has the "Sepal.Length" column inside of it:

sel_Sepal.Length <- function(df) {
        # The code we are looking for...
}

So that I can apply it

sel_Sepal.Length(iris)

To obtain a result like this:

    Sepal.Length
1            5.1
2            4.9
3            4.7
4            4.6
5            5.5
...          ...

I'm aware of this answer for a similar problem. But the difference is that, I'd like the function to work without inputting the column's name, which should be fixed inside the function's code.

This could possibly be considered a trivial question, since one can make the user input the column's name and make it work:

selectvar <- function(df, var) {
        var <- enquo(var)
        df %>%
                select(!!var)
}

selectvar(iris, Sepal.Length)

    Sepal.Length
1            5.1
2            4.9
3            4.7
4            4.6
5            5.5
...          ...

But I think there's a concept I'm missing so I can't make it work the way I asked (without inputting the column to be selected). This is a question asked just for the sake of finding that missing concept. Hope it could help others. Thank you in advance!

Answer 1

I may have misunderstood your question; since you explicitly want the column name to be hard-coded inside the function (at least that's what I infer from "I'd like the function to work without inputting the column's name, which should be fixed inside the function's code") you could do

sel_Sepal.Length <- function(df) {
    df %>% select(Sepal.Length)
}

But this means that there is really not much point to the whole function.

Perhaps you can clarify on the whole point of the exercise?