R language: why i can get the below outcome like this by Sapply function?

Question

 x<- split(mtcars,mtcars$cyl)
 sapply(x,'[',"mpg")

"From the above code, could someone explain to me why I can get the following outcome and why putting '[' in the sapply can get the following outcome?"

$`4.mpg`
 [1] 22.8 24.4 22.8 32.4 30.4 33.9 21.5 27.3 26.0 30.4 21.4

$`6.mpg`
[1] 21.0 21.0 21.4 18.1 19.2 17.8 19.7

$`8.mpg`
 [1] 18.7 14.3 16.4 17.3 15.2 10.4 10.4 14.7 15.5 15.2 13.3 19.2 15.8 15.0

Answer 1

If you look at the args for sapply(), you will see that the first three unnamed arguments given to it will be treated the input data (X), the function to be applied (FUN) and additional arguments to pass to that function (...)

> args('sapply')
function (X, FUN, ..., simplify = TRUE, USE.NAMES = TRUE) 

> help('sapply')
...
     X: a vector (atomic or list) or an ‘expression’ object.  Other
          objects (including classed objects) will be coerced by
          ‘base::as.list’.

     FUN: the function to be applied to each element of ‘X’: see
          ‘Details’.  In the case of functions like ‘+’, ‘%*%’, the
          function name must be backquoted or quoted.

     ...: optional arguments to ‘FUN’.

So when you call sapply(x,'[',"mpg") on the list resulting from the split on mpg, you are effectively calling the indexing operator [ on each element in the list, and passing the string mpg to it, e.g.:

x$`4`['mpg']
                mpg
Datsun 710     22.8
Merc 240D      24.4
Merc 230       22.8
Fiat 128       32.4
Honda Civic    30.4
Toyota Corolla 33.9
Toyota Corona  21.5
Fiat X1-9      27.3
Porsche 914-2  26.0
Lotus Europa   30.4
Volvo 142E     21.4

Finally, in the process of assembling the results back into a list, the names are lost, so you end up with:

$`4.mpg`
 [1] 22.8 24.4 22.8 32.4 30.4 33.9 21.5 27.3 26.0 30.4 21.4