In R, I have, for example:
> foo <- list(a=1,b=2,c=3)
If I type foo, I get:
$a
[1] 1
$b
[1] 2
$c
[1] 3
How can I look through foo to get a list of "keys" only? In this case, (a, b, c).
An R list can have named elements and so function as a dictionary structure. You can just do:
> names(foo)
[1] "a" "b" "c"
If you are looking for a dictionary structure you might also consider using the hash packages which provides a Python and Perl like dictionary/hash with the expected functions such as keys, so you can say:
keys(hash)
In terms of performance a list serves as a better dictionary than a hash for several hundred elements or fewer (<200) because of the cost of the hashing. The hash package is much better for very large dictionaries.
> names(foo)
[1] "a" "b" "c"
来源：https://stackoverflow.com/questions/3737194/dictionary-data-structure-in-r