add_column in tibble with variable column name

Question

This code doesn't work to add a column in tibble:

  library(tidyverse)
  df <- data.frame("Oranges" = 5)
  mycols <- c("Apples", "Bananas", "Oranges")
  add_column(df, mycols[[2]] = 7)

I get the error message:

  Error: unexpected '=' in "add_column(df, mycols[[2]] ="

But this code works:

  add_column(df, "Bananas" = 7)

Why?

I don't know the values of 'mycols' ahead of time. That's why I wrote my code for it to be a variable. Is this not possible in dplry?

Answer 1

Well, add_column seems to come from tibble rather than dplyr, but it does use the new tidy eval syntax. You can use

add_column(df, !!(mycols[2]) := 7)

Note the !! and :=. The := allows you to use variables for parameter names and the !! expands the expression into a string.