问题
Based on the section regarding capturing multiple arguments in Programming with dplyr, I am trying to specify
multiple variables to group by in
dplyr::group_by
without relying on
...
but using an explicit list argumentgroup_vars
insteadwithout needing to quote the list elements in arg
group_vars
Example data
df <- tibble::tribble(
~a, ~b, ~c,
"A", "a", 10,
"A", "a", 20,
"A", "b", 1000,
"B", "a", 5,
"B", "b", 1
)
Approach based on ...
from Programming with dplyr
# Approach 1 -----
my_summarise <- function(df, ...) {
group_vars <- dplyr::enquos(...)
df %>%
dplyr::group_by(!!!group_vars) %>%
dplyr::summarise(x = mean(c))
}
my_summarise(df, a, b)
#> # A tibble: 4 x 3
#> # Groups: a [2]
#> a b x
#> <chr> <chr> <dbl>
#> 1 A a 15
#> 2 A b 1000
#> 3 B a 5
#> 4 B b 1
Approach based on list argument with quoted elements:
# Approach 2 -----
my_summarise_2 <- function(df, group_vars = c("a", "b")) {
group_vars <- dplyr::syms(group_vars)
df %>%
dplyr::group_by(!!!group_vars) %>%
dplyr::summarise(x = mean(c))
}
my_summarise_2(df)
#> # A tibble: 4 x 3
#> # Groups: a [2]
#> a b x
#> <chr> <chr> <dbl>
#> 1 A a 15
#> 2 A b 1000
#> 3 B a 5
#> 4 B b 1
my_summarise_2(df, group_vars = "a")
#> # A tibble: 2 x 2
#> a x
#> <chr> <dbl>
#> 1 A 343.
#> 2 B 3
I can't find an approach that lets me supply unquoted column names:
# Approach 3 -----
my_summarise_3 <- function(df, group_vars = list(a, b)) {
group_vars <- dplyr::enquos(group_vars)
df %>%
dplyr::group_by(!!!group_vars) %>%
dplyr::summarise(x = mean(c))
}
my_summarise_3(df)
#> Error: Column `list(a, b)` must be length 5 (the number of rows) or one, not 2
I guess the crucial thing is to end up with an identical list structure as the
one after calling group_vars <- dplyr::enquos(...)
:
<list_of<quosure>>
[[1]]
<quosure>
expr: ^a
env: global
[[2]]
<quosure>
expr: ^b
env: global
I tried to tackle it with group_vars %>% purrr::map(dplyr::enquo)
, but of course R complains about a
and b
as they need to be evaluated.
回答1:
The main issue is that list(a, b)
does not capture unevaluated expressions a
and b
, but instead evaluates those expressions and creates a two-element list with results. You basically have two options:
Solution one: Use rlang::exprs()
to capture the actual expressions. Since the expressions are already unevaluated, you no longer need an enquos
inside your function, which simply becomes
my_summarise_3 <- function(df, group_vars = rlang::exprs(a, b)) {
df %>%
dplyr::group_by(!!!group_vars) %>%
dplyr::summarise(x = mean(c))
}
my_summarise_3(df)
# # A tibble: 4 x 3
# # Groups: a [2]
# a b x
# <chr> <chr> <dbl>
# 1 A a 15
# 2 A b 1000
# 3 B a 5
# 4 B b 1
The down side of this interface is that the user is now responsible for quoting (i.e, capturing the expressions of) the arguments:
# Note that it can be done using quote() from base R
my_summarise_3(df, group_vars=quote(a))
# # A tibble: 2 x 2
# a x
# <chr> <dbl>
# 1 A 343.
# 2 B 3
Solution two: Capture the unevaluated expression list(a,b)
in its entirety and parse it by hand.
## Helper function to recursively construct an abstract syntax tree
getAST <- function( ee ) { as.list(ee) %>% map_if(is.call, getAST) }
my_summarise_3 <- function(df, group_vars = list(a,b)) {
## Capture the expression and parse it
ast <- rlang::enexpr(group_vars) %>% getAST()
## Identify symbols present in the data
gvars <- unlist(ast) %>% map_chr(deparse) %>%
intersect(names(df)) %>% rlang::syms()
df %>%
dplyr::group_by(!!!gvars) %>%
dplyr::summarise(x = mean(c))
}
my_summarise_3(df, list(a,b))
# # A tibble: 4 x 3
# # Groups: a [2]
# a b x
# <chr> <chr> <dbl>
# 1 A a 15
# 2 A b 1000
# 3 B a 5
# 4 B b 1
my_summarise_3(df, b)
# # A tibble: 2 x 2
# b x
# <chr> <dbl>
# 1 a 11.7
# 2 b 500.
回答2:
I think you just want to reinvent vars()
:
library(magrittr)
library(dplyr,warn.conflicts = FALSE)
#> Warning: package 'dplyr' was built under R version 3.6.1
df <- tibble::tribble(
~a, ~b, ~c,
"A", "a", 10,
"A", "a", 20,
"A", "b", 1000,
"B", "a", 5,
"B", "b", 1
)
my_summarise <- function(data, group_vars) {
data %>%
group_by_at(group_vars) %>%
summarise(x = mean(c))
}
my_summarise(df, c("a","b"))
#> # A tibble: 4 x 3
#> # Groups: a [2]
#> a b x
#> <chr> <chr> <dbl>
#> 1 A a 15
#> 2 A b 1000
#> 3 B a 5
#> 4 B b 1
my_summarise(df, vars(a, b))
#> # A tibble: 4 x 3
#> # Groups: a [2]
#> a b x
#> <chr> <chr> <dbl>
#> 1 A a 15
#> 2 A b 1000
#> 3 B a 5
#> 4 B b 1
Created on 2019-07-26 by the reprex package (v0.3.0)
Here's a variation on @Artem's solution if you really want this (but why ?) :
my_summarise <- function(df, group_vars) {
quoted_group_vars <- rlang::list2(
!!!as.list(enexpr(group_vars)[-1]))
df %>%
dplyr::group_by(!!!quoted_group_vars) %>%
dplyr::summarise(x = mean(c))
}
my_summarise(df, list(a, b))
#> # A tibble: 4 x 3
#> # Groups: a [2]
#> a b x
#> <chr> <chr> <dbl>
#> 1 A a 15
#> 2 A b 1000
#> 3 B a 5
#> 4 B b 1
来源:https://stackoverflow.com/questions/57007865/specifying-multiple-variables-to-group-by-via-explicit-argument-with-unquoted-el