Inconsistent behaviour of map and pluck when indexing by name and location

[JBGruber]

I was reading the pluck documentation when I stumbled on this interesting note in the examples:

# The map() functions use pluck() by default to retrieve multiple
# values from a list:

But when I wanted to try it, I got stuck. What my problem was is that mixing indexing by name and location does not work with map(), but I use it all the time with pluck. Here is an example where I would expect the outcome of both map calls to be the same:

obj1 <- list("a", list(1, elt = "foo"))
obj2 <- list("b", list(2, elt = "bar"))
x <- list(obj1, obj2)

purrr::map(x, c(2, "elt"))
#> [[1]]
#> NULL
#> 
#> [[2]]
#> NULL
purrr::map(x, \(i) purrr::pluck(i, 2, "elt"))
#> [[1]]
#> [1] "foo"
#> 
#> [[2]]
#> [1] "bar"

^{Created on 2024-10-01 with reprex v2.1.1}

Session info

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[joakimlinde]

The c(2, "elt") argument looks odd to me. For vectors in R all elements have to be of the same type. R will coerce this into c("2", "elt"). Did you mean to say list(2, "elt") ?

purrr::map(x, list(2, "elt"))
#> [[1]]
#> [1] "foo"
#>
#> [[2]]
#> [1] "bar"

[JBGruber]

It totally makes sense that list works and the approach with c does not! I didn't even consider that, thanks! But maybe it would make sense to add this as an example here:

https://github.com/tidyverse/purrr/blob/700ce1973aa2dac714b7fbbbc619ead0fe946b0d/R/pluck.R#L82

If I make this mistake, others might as well? Maybe:

#' # if you want to mix location and name indexing, use list
#' purrr::map_chr(x, list(2, "elt"))

[joakimlinde]

There is already an example similar to this.

# Use a list to build an extractor that mixes numeric indices and names,
# and .default to provide a default value if the element does not exist
l2 |> map(list("num", 3))
#> [[1]]
#> [1] 3
#> 
#> [[2]]
#> [1] 103
#> 
#> [[3]]
#> NULL
#> 
l2 |> map_int(list("num", 3), .default = NA)
#> [1]   3 103  NA

[JBGruber]

Great!