第 46 章 非标准性评估

Tidy Evaluation (Tidy Eval),不是一个宏包,而是一个非标准评估的框架,也叫延迟评估。主要目的是更方便地与tidyverse里的函数配合使用,事实上,很多时候我们不一定需要用到它。我这里尽可能规避较专业的词汇,用通俗的语言介绍一些简单用法,表述可能不准确。如果想了解背后复杂的机制请阅读advance R

46.1 编写函数

写代码的过程中,我们会遇到对不同的数据框,执行相同的操作。比如

df1 %>% group_by(x1) %>% summarise(mean = mean(y1))
df2 %>% group_by(x2) %>% summarise(mean = mean(y2))
df3 %>% group_by(x3) %>% summarise(mean = mean(y3))
df4 %>% group_by(x4) %>% summarise(mean = mean(y4))

为了减少代码的重复,我们考虑将共同的部分保留,变化的部分用参数名提取出来

data %>% group_by(group_var) %>% summarise(mean = mean(summary_var))

很自然地,我们想到写一个子函数的形式,比如

grouped_mean <- function(data, group_var, summary_var) {
  data %>%
    group_by(group_var) %>%
    summarise(mean = mean(summary_var))
}

当我们试图运行这段代码的时候,却发现报错了

grouped_mean(mtcars, cyl, mpg)
## Error in `group_by()`:
## ! Must group by variables found in `.data`.
## ✖ Column `group_var` is not found.

Hadley Wickham告诉我们,正确的写法应该是,

grouped_mean <- function(data, group_var, summary_var) {
  group_var <- enquo(group_var)
  summary_var <- enquo(summary_var)

  data %>%
    group_by(!!group_var) %>%
    summarise(mean = mean(!!summary_var))
}

然后再运行

grouped_mean(mtcars, cyl, mpg)
## # A tibble: 3 × 2
##     cyl  mean
##   <dbl> <dbl>
## 1     4  26.7
## 2     6  19.7
## 3     8  15.1

或者更简便的

grouped_mean <- function(data, group_var, summary_var) {
  data %>%
    group_by({{group_var}}) %>%
    summarise(mean = mean({{summary_var}}))
}

grouped_mean(mtcars, cyl, mpg)
## # A tibble: 3 × 2
##     cyl  mean
##   <dbl> <dbl>
## 1     4  26.7
## 2     6  19.7
## 3     8  15.1

dplyr1.0之后,可以这样写

sum_group_vars <- function(df, 
                           group_vars, 
                           sum_vars){
  df %>% 
    group_by(across({{ group_vars }})) %>% 
    summarise(n = n(), 
              across({{ sum_vars }}, 
                     list(mean = mean, sd = sd))
              )
}

sum_group_vars(mpg, c(model, year), c(hwy, cty))

下面我们讲讲为什么要这样写。

46.2 看看发生了什么

弄清楚之前,这里需要明白两个概念:

  • 环境变量(env-variables) ,一般你在Rstuido右上角的Environment中发现它。比如n <- 10这里的n

  • 数据变量(data-variables),一般指数据框的某个变量。比如data <- data.frame(x = 1, n = 2)中的data$n

那么,对于我们这里编写的函数中

grouped_mean(mtcars, cyl, mpg)

cylmpg是打算传递的参数,是环境变量,但我们期望他们在函数中当作mtcars中的数据变量,即当做mtcars的一个列的名字来使用, 那么要完成这个角色转换,就需要引用(quote)和解引用(unquote)两个工序:

  • 第一步,用 enquo()把用户传递过来的参数引用起来(引用可以理解为冷冻起来)

  • 第二步,用 !! 解开这个引用(解引用可以理解为解冷),然后使用参数的内容

这个quote-unquote的过程让环境变量名变成了数据变量,也可以理解为在函数评估过程中,数据变量(data-variable)遮盖了环境变量(env-variable),即数据遮盖(data masking),看到cyl,正常情况下,本来应该是到环境变量里去找这个cyl对应的值,然而,数据遮盖机制,插队了,让代码去数据变量中去找cyl以及对应的值。

我们通过rlang::qq_show()看看这个quote-unquote机制是怎么工作的

先看看qq_show()

var <- quote(height)
qq_show(!!var)
## height

再看看grouped_mean()的代码

  group_var <-  quote(cyl)
summary_var <-  quote(mpg)
    
rlang::qq_show( 
    data %>%
    group_by(!!group_var) %>%
    summarise(mean = mean(!!summary_var))
)
## data %>% group_by(cyl) %>% summarise(mean = mean(mpg))

关于数据遮盖更多细节请看Quote and unquote

46.3 处理多个参数

前面讲了如何传递分组参数和统计参数到子函数。如果传递更多的参数,可以用...代替group_var ,然后传递到group_by(),比如

grouped_mean <- function(data, summary_var, ...) {
  summary_var <- enquo(summary_var)
    group_var <- enquos(...)
 
  data %>%
    group_by(!!!group_var) %>%
    summarise(mean = mean(!!summary_var))
}

指定统计参数disp,分组参数(cyl am),然后运行代码,

grouped_mean(mtcars, disp, cyl, am)
## # A tibble: 6 × 3
## # Groups:   cyl [3]
##     cyl    am  mean
##   <dbl> <dbl> <dbl>
## 1     4     0 136. 
## 2     4     1  93.6
## 3     6     0 205. 
## 4     6     1 155  
## 5     8     0 358. 
## 6     8     1 326

或者指定统计参数disp,更多的分组参数(cyl, am, vs)

grouped_mean(mtcars, disp, cyl, am, vs)
## # A tibble: 7 × 4
## # Groups:   cyl, am [6]
##     cyl    am    vs  mean
##   <dbl> <dbl> <dbl> <dbl>
## 1     4     0     1 136. 
## 2     4     1     0 120. 
## 3     4     1     1  89.8
## 4     6     0     1 205. 
## 5     6     1     0 155  
## 6     8     0     0 358. 
## # … with 1 more row

注意到...代表的是多个参数,因此在引用的时候用的是enquos(),在解引用的时候 用的是group_by(!!!group_var). 事实上, ...是一个特殊的符号,我们可以省略引用后再解引用的过程,直接传给给group_by(), 比如

grouped_mean <- function(data, summary_var, ...) {
  summary_var <- enquo(summary_var)

  data %>%
    group_by(...) %>%
    summarise(mean = mean(!!summary_var))
}

grouped_mean(mtcars, disp, cyl, am, vs)
## # A tibble: 7 × 4
## # Groups:   cyl, am [6]
##     cyl    am    vs  mean
##   <dbl> <dbl> <dbl> <dbl>
## 1     4     0     1 136. 
## 2     4     1     0 120. 
## 3     4     1     1  89.8
## 4     6     0     1 205. 
## 5     6     1     0 155  
## 6     8     0     0 358. 
## # … with 1 more row

46.4 调整输入的表达式

46.4.1 修改引用参数的默认名

我们希望输出的统计结果中,统计参数名加一个前缀 “avg_”, 可以分三步完成

  • 获取引用参数的默认名
  • 修改参数的默认名,比如加前缀或者后缀
  • !! 解引用并放在 := 左边
grouped_mean2 <- function(.data, .summary_var, ...) {
  summary_var <- enquo(.summary_var)
  group_vars <- enquos(...)

  # Get and modify the default name
  summary_nm <- as_label(summary_var)
  summary_nm <- paste0("avg_", summary_nm)

  .data %>%
    group_by(!!!group_vars) %>%
    summarise(!!summary_nm := mean(!!summary_var))  # Unquote the name
}

grouped_mean2(mtcars, disp, cyl, am)
## # A tibble: 6 × 3
## # Groups:   cyl [3]
##     cyl    am avg_disp
##   <dbl> <dbl>    <dbl>
## 1     4     0    136. 
## 2     4     1     93.6
## 3     6     0    205. 
## 4     6     1    155  
## 5     8     0    358. 
## 6     8     1    326

或者更简洁的办法

my_summarise <- function(data, group_var, summarise_var) {
  data %>%
    group_by(across({{ group_var }})) %>%
    summarise(across({{ summarise_var }}, mean, .names = "mean_{col}"))
}

my_summarise(starwars, species, height)

如果想调整多个分组变量的默认名,比如加个前缀”groups_“,方法和上面的步骤类似

  • 引用传递过来的参数名,.enquos(..., .named = TRUE), 增加了控制语句.named = TRUE
  • 修改在每个参数的默认名,比如加前缀或者后缀
  • !! 解引用并放在 := 左边
grouped_mean3 <- function(.data, .summary_var, ...) {
  summary_var <- enquo(.summary_var)

  # Quote the dots with default names
  group_vars <- enquos(..., .named = TRUE)

  summary_nm <- as_label(summary_var)
  summary_nm <- paste0("avg_", summary_nm)

  # Modify the names of the list of quoted dots
  names(group_vars) <- paste0("groups_", names(group_vars))

  .data %>%
    group_by(!!!group_vars) %>%  # Unquote-splice as usual
    summarise(!!summary_nm := mean(!!summary_var))
}

grouped_mean3(mtcars, disp, cyl, am)
## # A tibble: 6 × 3
## # Groups:   groups_cyl [3]
##   groups_cyl groups_am avg_disp
##        <dbl>     <dbl>    <dbl>
## 1          4         0    136. 
## 2          4         1     93.6
## 3          6         0    205. 
## 4          6         1    155  
## 5          8         0    358. 
## 6          8         1    326

46.4.2 修改引用的表达式

有时候,我们不想“按多个变量分组,对一个变量统计”。而是“按一个变量分组,对多个变量统计”。这种情况,我们就需要调整引用的表达式

  • .group_var放分组的变量species
  • ... 放需要统计的多个变量height, mass,期望完成 mean(height), mean(mass)
  • 需要用purrr:map()配合调整表达式, 如
vars <- list(quote(mass), quote(height))

purrr::map(vars, function(var) expr(mean(!!var, na.rm = TRUE)))
## [[1]]
## mean(mass, na.rm = TRUE)
## 
## [[2]]
## mean(height, na.rm = TRUE)

完整代码可以这样写

grouped_mean4 <- function(.data, .group_var, ...) {
  group_var <- enquo(.group_var)
  summary_vars <- enquos(..., .named = TRUE)

  # Wrap the summary variables with mean()
  summary_vars <- purrr::map(summary_vars, function(var) {
    expr(mean(!!var, na.rm = TRUE))
  })

  # Prefix the names with `avg_`
  names(summary_vars) <- paste0("avg_", names(summary_vars))

  .data %>%
    group_by(!!group_var) %>%
    summarise(!!!summary_vars)
}
grouped_mean4(starwars, species, height, mass)
## # A tibble: 38 × 3
##   species  avg_height avg_mass
##   <chr>         <dbl>    <dbl>
## 1 Aleena          79      15  
## 2 Besalisk       198     102  
## 3 Cerean         198      82  
## 4 Chagrian       196     NaN  
## 5 Clawdite       168      55  
## 6 Droid          131.     69.8
## # … with 32 more rows

46.5 案例

46.5.1 统计并过滤

df <- tibble(index = sample(letters[1:4], size = 100, replace = TRUE) ) 
df
## # A tibble: 100 × 1
##   index
##   <chr>
## 1 a    
## 2 b    
## 3 b    
## 4 d    
## 5 d    
## 6 b    
## # … with 94 more rows
filter_which <- function(df, var, val) {
    
    which_var <- enquo(var)
    which_val <- as_name(enquo(val))
    
    df %>% 
        count(!!which_var) %>% 
        filter(!!which_var ==  which_val) 
    
}


df %>% 
    filter_which(index, a)
## # A tibble: 1 × 2
##   index     n
##   <chr> <int>
## 1 a        29

46.5.2 自定义统计输出

my_summarise <- function(data, expr) {
  data %>% summarise(
    "mean_{{expr}}" := mean({{ expr }}),
    "sum_{{expr}}" := sum({{ expr }}),
    "n_{{expr}}" := n()
  )
}

mtcars %>% my_summarise(mpg)
## # A tibble: 1 × 3
##   mean_mpg sum_mpg n_mpg
##      <dbl>   <dbl> <int>
## 1     20.1    643.    32

46.5.3 形成依次下滑的列

d <- tibble(x = seq_len(10))


jetlag <- function(data, variable, n = 10){
  variable <- enquo(variable)
  
  indices <- seq_len(n)
  quosures <- purrr::map( indices, ~quo(lag(!!variable, !!.x)) ) %>%
      purrr::set_names(nm = purrr::map_chr(indices, ~paste0("lag_", .x)))
  
  dplyr::mutate(data, !!!quosures)
  
}


d %>% jetlag(x, 3)
## # A tibble: 10 × 4
##       x lag_1 lag_2 lag_3
##   <int> <int> <int> <int>
## 1     1    NA    NA    NA
## 2     2     1    NA    NA
## 3     3     2     1    NA
## 4     4     3     2     1
## 5     5     4     3     2
## 6     6     5     4     3
## # … with 4 more rows

46.6 可能会用到的函数

enquo() vs quo() vs expr() vs as_name() vs as_label() vs sym()

a <- 1
b <- 1
var <- quote(a + b)
# returns a single quoted expression for the delayed computation
var
## a + b
qq_show(!!var)
## a + b
# quotes a new expression locally
expr(mean(!!var, na.rm = TRUE))
## mean(a + b, na.rm = TRUE)
var <- quo(height)

# transforms a quoted variable name into a string. 
as_name(var)
## [1] "height"
# also returns a single string but supports any kind of R object as input, including quoted function calls and vectors. Its purpose is to summarise that object into a single label. That label is often suitable as a default name.
as_label(var)
## [1] "height"
# creates a symbol from a string
sym("height")
## height