map_dbl fails when called with mean

Question

I'm trying to replicate a simple example from R for Data Science, using the map functions.

This works: map_dbl(mtcars, median). But surprisingly, this doesn't: map_dbl(mtcars, mean). I get this error message:

> map_dbl(mtcars, mean)
Error in `map_dbl()`:
i In index: 1.
i With name: mpg.
Caused by error:
! Result must be length 1, not 0.

This map_dbl(mtcars, ~mean(.))solves the problem.

But why does map_dbl(mtcars, median)work while map_dbl(mtcars, mean)doesn't?

Answer 1

At a guess, you've assigned another value to mean somewhere along the way, e.g.:

library(tidyverse)

map_dbl(mtcars, mean)
#>        mpg        cyl       disp         hp       drat         wt       qsec 
#>  20.090625   6.187500 230.721875 146.687500   3.596563   3.217250  17.848750 
#>         vs         am       gear       carb 
#>   0.437500   0.406250   3.687500   2.812500
map_dbl(mtcars, median)
#>     mpg     cyl    disp      hp    drat      wt    qsec      vs      am    gear 
#>  19.200   6.000 196.300 123.000   3.695   3.325  17.710   0.000   0.000   4.000 
#>    carb 
#>   2.000

mean <- list()
map_dbl(mtcars, mean)
#> Error in `map_dbl()`:
#> ℹ In index: 1.
#> ℹ With name: mpg.
#> Caused by error:
#> ! Result must be length 1, not 32.

map_dbl(mtcars, ~mean(.))
#>        mpg        cyl       disp         hp       drat         wt       qsec 
#>  20.090625   6.187500 230.721875 146.687500   3.596563   3.217250  17.848750 
#>         vs         am       gear       carb 
#>   0.437500   0.406250   3.687500   2.812500

^{Created on 2023-06-16 with reprex v2.0.2}

To fix it try restarting R, making sure you aren't saving history. Or just clear out your global environment variables.