Using nesting variable inside a function

Question

Is it possible to use a nesting variable inside a function used in purrr::map? For example, in the following example I want each plot to have a title showing the number of cylinders

    library(tidyverse)
    plot_mtcars <- function(df, cyl){
                ggplot(aes(x = disp, y = mpg), data = df) + 
                        geom_point() +
                        ggtitle(paste("Cylinders =", cyl))
            }
    plots <- mtcars %>% 
                nest(-cyl) %>% 
                mutate(plot = map(data, ~plot_mtcars(., cyl)))

The code above does not work, as all plots return: Cylinders = 6 (instead of 6,4,8)

akuiper · Accepted Answer · 2018-06-17 03:49:58Z

3

The issue here is that cyl is a vector, so it's setting a character vector in ggtitle, in which case only the first element is used; You need to loop through cyl and pass the corresponding element to the plot function:

plots <- mtcars %>% 
    nest(-cyl) %>% 
    # here use map2 to pass data and corresponding cyl to the plot function
    mutate(plot = map2(data, cyl, ~ plot_mtcars(.x, .y)))

Check the plot title:

plots$plot[[1]]$labels$title
# [1] "Cylinders = 6"

plots$plot[[2]]$labels$title
# [1] "Cylinders = 4"

plots$plot[[3]]$labels$title
# [1] "Cylinders = 8"

answered Jun 17, 2018 at 3:49

akuiper

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2 Comments

Sasha Over a year ago

What if nesting is done using 2 variables and I want to include both in the title. For example, plots <- mtcars %>% nest(-cyl, -am) %>% mutate(plot = map2...

akuiper Over a year ago

Do you need to access both cyl and am in the plot function ? In which case you might use pmap something like pmap(list(data, cyl, am), ~ plot_mtcars(..1, ..2, ..3)) and your plot_mtcars function need to access three parameters as well.

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