How would you write this in base R 4.1+? Is there a better way? More approaches here: https://gist.github.com/hadley/c430501804349d382ce90754936ab8ec

base-r-summarise.R

# Single grouping in base R: Multiple functions

library(dplyr, warn.conflicts = FALSE)
expected_output <- mtcars %>%
    group_by(cyl) %>%
    summarise(mean = mean(disp), n = n())  %>% 
    as.data.frame()
expected_output

by <- c("cyl")
result <- Reduce(function(df1, df2) merge(df1, df2, by = by), list(
    aggregate(mtcars$disp, mtcars[by], mean) |> setNames(c(by, "mean")),
    aggregate(mtcars$disp, mtcars[by], length) |> setNames(c(by, "n"))
))
result

testthat::expect_identical(result, expected_output)


# More complex grouping in base R: Multiple variariables and functions

library(dplyr, warn.conflicts = FALSE)
expected_output <- mtcars %>%
    group_by(cyl, am) %>%
    summarise(mean = mean(disp), n = n())  %>% 
    as.data.frame()
expected_output

by <- c("cyl", "am")
result <- Reduce(function(df1, df2) merge(df1, df2, by = by), list(
    aggregate(mtcars$disp, mtcars[by], mean) |> setNames(c(by, "mean")),
    aggregate(mtcars$mpg, mtcars[by], mean) |> setNames(c(by, "mpg")),
    aggregate(mtcars$disp, mtcars[by], length) |> setNames(c(by, "n"))
))
result

testthat::expect_identical(result, expected_output)