Pass function arguments by column position to mutate_at

Here's a map2 solution along the lines of Henrik's comment. You can then wrap this inside a custom function. I provided an rough first attempt but I have done minimal tests, so it probably breaks under all sorts of situations if evaluation is strange. It also doesn't use tidyselect for .at, but neither does modify_at...

library(tidyverse)

df <- tibble::tribble(
  ~name, ~life_expectancy,          ~poverty, ~household_income,
  "New Haven", 78.0580437642378, 0.264221051111753,  42588.7592521085,
  "New York", 12.349685329, 0.324067934, 32156.230974623
)

rounded <- df %>%
  select(life_expectancy, poverty, household_income) %>%
  map2_dfc(
    .y = c(1, 2, 0),
    .f = ~ round(.x, digits = .y)
  )
df %>%
  select(-life_expectancy, -poverty, -household_income) %>%
  bind_cols(rounded)
#> # A tibble: 2 x 4
#>   name      life_expectancy poverty household_income
#>   <chr>               <dbl>   <dbl>            <dbl>
#> 1 New Haven            78.1    0.26            42589
#> 2 New York             12.3    0.32            32156


modify2_at <- function(.x, .y, .at, .f) {
  modified <- .x[.at] %>%
    map2(.y, .f)
  .x[.at] <- modified
  return(.x)
}

df %>%
  modify2_at(
    .y = c(1, 2, 0),
    .at = c("life_expectancy", "poverty", "household_income"),
    .f = ~ round(.x, digits = .y)
  )
#> # A tibble: 2 x 4
#>   name      life_expectancy poverty household_income
#>   <chr>               <dbl>   <dbl>            <dbl>
#> 1 New Haven            78.1    0.26            42589
#> 2 New York             12.3    0.32            32156

^{Created on 2018-11-13 by the reprex package (v0.2.1)}

2 solutions

---

mutate with !!!

invoke was a good idea but you need it less now that most tidyverse functions support the !!! operator, here's what you can do :

digits <- c(life_expectancy = 1, poverty = 2, household_income = 0)  
df %>% mutate(!!!imap(digits, ~round(..3[[.y]], .x),.))
# # A tibble: 1 x 4
#          name life_expectancy poverty household_income
#         <chr>           <dbl>   <dbl>            <dbl>
#   1 New Haven            78.1    0.26            42589

..3 is the initial data frame, passed to the function as a third argument, through the dot at the end of the call.

Written more explicitly :

df %>% mutate(!!!imap(
  digits, 
  function(digit, name, data) round(data[[name]], digit),
  data = .))

If you need to start from your old interface (though the one I propose will be more flexible), first do:

digits <- setNames(c(1, 2, 0), c("life_expectancy", "poverty", "household_income"))

---

mutate_at and <<-

Here we bend a bit the good practice of avoiding <<- whenever possible, but readability matters and this one is really easy to read.

digits <- c(1, 2, 0)
i <- 0
df %>%
  mutate_at(vars(life_expectancy, poverty, household_income), ~round(., digits[i<<- i+1]))
# A tibble: 1 x 4
#     name      life_expectancy poverty household_income
#     <chr>               <dbl>   <dbl>            <dbl>
#   1 New Haven            78.1    0.26            42589

(or just df %>% mutate_at(names(digits), ~round(., digits[i<<- i+1])) if you use a named vector as in my first solution)