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More on Macros in R

Recently ran into something interesting in the R macros/quasi-quotation/substitution/syntax front:

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Romain François: “.@_lionelhenry reveals planned double curly syntax At #satRdayParis as a possible replacement, addition to !! and enquo()”

It appears !! is no longer the last word in substitution (it certainly wasn’t the first).

The described effect is actually already pretty easy to achieve in R.

suppressPackageStartupMessages(library("dplyr"))

group_by <- wrapr::bquote_function(group_by)
summarize <- wrapr::bquote_function(summarize)

my_average <- function(data, grp_var, avg_var) {
  data %>%
    group_by(.( grp_var )) %>%
    summarize(avg = mean(.( avg_var ), na.rm = TRUE))
}

data <- data.frame(x = 1:10, g = rep(c(0,1), 5))

my_average(data, as.name("g"), as.name("x"))

## # A tibble: 2 x 2
##       g   avg
##   <dbl> <dbl>
## 1     0     5
## 2     1     6

Or if you don’t want to perform the quoting by hand.

my_average <- function(data, grp_var, avg_var,
                       grp_var_name = substitute(grp_var),
                       avg_var_name = substitute(avg_var)
                       ) {
  data %>%
    group_by(.( grp_var_name )) %>%
    summarize(avg = mean(.( avg_var_name ), na.rm = TRUE))
}

my_average(data, g, x)

## # A tibble: 2 x 2
##       g   avg
##   <dbl> <dbl>
## 1     0     5
## 2     1     6

And we can use the same Thomas Lumley / bquote() notation for string interpolation.

group_var <- as.name("g")
avg_var <- as.name("x")

wrapr::sinterp("group_var was .(group_var), and avg_var was .(avg_var)")

# [1] "group_var was g, and avg_var was x"

The .() notation has a great history in R and has been used by data.table for years.

Categories: Coding Opinion Tutorials

Tagged as:

jmount

Data Scientist and trainer at Win Vector LLC. One of the authors of Practical Data Science with R.

1 reply

  1. BTW: the second function is a great way to simultaneously present a workable Non Standard Evaluation and standard evaluation interface at the same time (so that NSE isn’t a barrier to programming).

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