Problems with `invoke_map_dbl`'s argument

EconKid · November 26, 2018, 7:23am

I read a post on the R-blogger.

The writer use purrr's function in this way .

library(tidyverse)
spread <- list(sd = sd, iqr = IQR, mad = mad)
x <- rnorm(100)
invoke_map_dbl(spread, x = x)
#>       sd      iqr      mad 
#> 1.040133 1.393010 1.051002

^{Created on 2018-11-26 by the reprex package (v0.2.1)}

Session info

devtools::session_info()
#> - Session info ----------------------------------------------------------
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#>  date     2018-11-26                                         
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I read the help document, there is no argument named by x in it.

invoke_map_dbl(.f, .x = list(NULL), ..., .env = NULL)

I wonder is there any special meanin of x = x here?

jcblum · November 26, 2018, 7:50am

That particular usage is a bit of a clever shorthand (it’s the same thing that’s going on in examples 6 and 7 on the doc page, which are equivalent). I think it might be clearer written out more verbosely:

invoke_map_dbl(.f = spread, .x = list(NULL), x = x)

Combine that with this condensed excerpt from the docs:


`.f`	…for `invoke_map` a list of functions.
`.x`	…for `invoke_map` a list of argument-lists the same length as `.f` (or length 1). The default argument, `list(NULL)` , will be recycled to the same length as `.f` , and will call each function with no arguments (apart from any supplied in `...` ).
`...`	Additional arguments passed to each function.

All of the functions in the list spread take an argument x. Here, x is supplied as one of the “additional arguments” passed via .... It happens to refer to the object x, a vector of 100 values. Since nothing is passed to invoke_map_dbl()’s argument .x, nothing other than the ... argument is passed to the functions in spread, so each function is called with x = x.

As in example 6, the same thing could also be accomplished with:

invoke_map_dbl(spread, list(list(x = x)))

But you can maybe see why that’s an awkward syntax in this case!

EconKid · November 26, 2018, 10:29am

@jcblum Thanks, that is a very clear answer !!!

And, by the way, I want to know how to type a table in this way you do.

mara · November 26, 2018, 11:29am

system · December 3, 2018, 11:29am

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