I'm trying to use double curly brackets in a function that makes a timeseries plot, and I've hit a wall, despite reading this

CodePudding user response：

The double curly braces work with unquoted variables names. With variables names as character strings, use aes_string, see also here.

suppressPackageStartupMessages({
  library(dplyr)
  library(ggplot2)
  library(lubridate)
})

#Sample data
test <- tibble(Week = seq(as.Date("2014/09/04"), by = "week", length.out = 8),
               Month = ymd(rep('2014-09-01', 4), rep('2014-10-01', 4)),
               Week_score = c(2, 3, 4, 6, 5, 7, 8, 9),
               Month_score = c(15, NA, NA, NA, 29, NA, NA, NA))

#My function
make_time_plot <- function(data, time_period = c("Week", "Month")) {
  
  time_period <- match.arg(time_period)
  time_period_score <- paste0(time_period, "_score")
  
  data %>%
    ggplot(aes_string(x = time_period, y = time_period_score))  
    geom_line()
}

#make_time_plot(data = test, time_period = "Month")
make_time_plot(data = test, time_period = "Week")

^{Created on 2022-04-02 by the reprex package (v2.0.1)}

CodePudding user response：

There are several things going on here. {{}} work on unquoted variables, and cannot be mixed and matched with strings.

For example: when you write mtcars %>% select(hp), you don't need to write "hp". This is due to "data masking", a process which makes R understand that hp is a variable in mtcars, and not a variable in your environment. It´t the process that makes this code work:

# Defining an env-variable
cyl <- 1000

# Referring to a data-variable
dplyr::summarise(mtcars, mean(cyl))
#>   mean(cyl)
#> 1    6.1875

^{Created on 2022-04-02 by the reprex package (v2.0.1)}

How to fix your function

There are easier ways to fix your function, without using {{}}, but if you want to see how to do it with {{}} I've included an example here.

make_time_plot <- function(data, time_period) {
  
  time_period_score <- paste0(rlang::as_name(enquo(time_period)), "_score")
  
  data %>%
    ggplot(
      aes(
        x = {{time_period}},
        y = .data[[time_period_score]]
      )
    )  
    geom_line()
}

make_time_plot(data = test, time_period = Week)

^{Created on 2022-04-02 by the reprex package (v2.0.1)}

I've changed the following:

• Since you want to paste a string to time_period, and time_period is a unquoted variable, you need to make it into a string using rlang::as_name(enquo(time_period)).
• Since time_period_score is a string, you need to supply it to aes() as a string. Since time_period is not a string, you cannot use aes_string(), so instead I use the .data pronoun with .data[[time_period_score]].

You can find more information about {{}} here.