I don't understand how to use expressions and quotation/unquotation/quasiquotation in the context of the R tidyverse. In the following example, I thought that using the unquote (!!) operator would allow me to generate the required arguments for add_row after I evaluate expression ex. However, I get this error instead. I've read the Metaprogramming Big Picture and Quasiquotation chapters in Advanced R, but I'm still having trouble understanding how to use these features correctly.
library(tidyverse)
# create sample data
df <- data.frame(x = 1:10, y = 11:20, z = 21:30)
df
#> x y z
#> 1 1 11 21
#> 2 2 12 22
#> 3 3 13 23
#> 4 4 14 24
#> 5 5 15 25
#> 6 6 16 26
#> 7 7 17 27
#> 8 8 18 28
#> 9 9 19 29
#> 10 10 20 30
mini_df <- data.frame(x = 33:35, y = 43:45, z = 53:55)
mini_df
#> x y z
#> 1 33 43 53
#> 2 34 44 54
#> 3 35 45 55
# store the expression I want to call in add_row
ex <- expr(paste0(names(df),':=',paste0('mini_df$',names(mini_df)),collapse=','))
# attempt to call add_row using arguments unquoted after evaluating expression ex
add_row(df,(!! eval(ex)), .after = 3L)
#> New rows in `add_row()` must use columns that already exist:
#> * Can't find column `"x:=mini_df$x,y:=mini_df$y,z:=mini_df$z"` in `.data`.
Created on 2019-05-17 by the reprex package (v0.3.0)
It turns out I should have been using unquote-splicing by calling UQS() or !!! instead. I didn't need to use the eval() or expr() functions at all. Instead, the proper usage was as follows:
library(tidyverse)
df <- data.frame(x = 1:10, y = 11:20, z = 21:30)
mini_df <- data.frame(x = 33:35, y = 43:45, z = 53:55)
add_row(df,!!!mini_df, .after = 3L)
#> x y z
#> 1 1 11 21
#> 2 2 12 22
#> 3 3 13 23
#> 4 33 43 53
#> 5 34 44 54
#> 6 35 45 55
#> 7 4 14 24
#> 8 5 15 25
#> 9 6 16 26
#> 10 7 17 27
#> 11 8 18 28
#> 12 9 19 29
#> 13 10 20 30
Created on 2019-05-17 by the reprex package (v0.3.0)
For more on unquoting and unquote-splicing see:
Programming with dplyr - in particular, the line that states "A very useful feature of unquote-splicing is that the vector names become argument names"