Dplyr Tidyyeval for built variable names

Question

I am trying to build a function that will allow me to perform functions on columns that start with a specific prefix. I am struggling to figure out how to get the names to evaluate to the correct thing.

I've looked at the dplyr website where it talks about programming but couldn't quite figure out how to make it evaluate properly.

I am making use of the most current version of dplyr on CRAN (v0.70) where @hadley has introduced tidyeval

Reprex

library(tidyverse)

tbl1 <- tibble(
  urn = c(1 ,2 ,3 ),
  a_width = c(10,20,30),
  a_height = c(12,13,14),
  b_width = c(25,50,75),
  b_height = c(25,50,75)
)

my_mean <- function(x, group) {
  width <- paste0(quo_name(group), "_width")
  height <- paste0(quo_name(group), "_height")

  summarise(x, 
  !!paste0(group, "_mean_width") := mean(!!width),          
  !!paste0(group, "_mean_height") := mean(!!height)          
  )  
}

my_mean(tbl1, "a")

# # A tibble: 1 x 2
# a_mean_width a_mean_height
# <dbl>         <dbl>
#   1           NA            NA
# Warning messages:
#   1: In mean.default("a_width") :
#   argument is not numeric or logical: returning NA
# 2: In mean.default("a_height") :
#   argument is not numeric or logical: returning NA

You are correct, the comma was missed however that is more a case of my poor typing, rather than the cause of the issue. — Dan, Jun 27 '17 at 22:48
Sure, I was just trying to reproduce your error and the missed comma made this impossible :) — amarchin, Jun 27 '17 at 22:53

Hong Ooi · Accepted Answer · 2017-06-27T23:06:13.470

2

!!width returns the string "a_width". You have to do !!sym(width) to turn it into a name. Similarly for !!height.

edited Jun 27 '17 at 23:06

answered Jun 27 '17 at 22:50

Hong Ooi

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3

I think you need `!!sym(width)` not `sym(!!width)` – aosmith Jun 27 '17 at 22:54
1

I get an error saying 'could not find function sym' – Dan Jun 27 '17 at 22:59
`sym` is a function from the rlang package which is what dplyr uses for this stuff – Hong Ooi Jun 27 '17 at 23:01
`!!sym(width)` is the one. Feel free to edit the answer and I'll confirm it. – Dan Jun 27 '17 at 23:05
Edited, not at my computer at the moment – Hong Ooi Jun 27 '17 at 23:19

Dplyr Tidyyeval for built variable names

1 Answers1