Is there syntactic sugar to define a data frame in R - r

Question

I want to regroup US states by regions and thus I need to define a "US state" -> "US Region" mapping function, which is done by setting up an appropriate data frame.
The basis is this exercise (apparently this is a map of the "Commonwealth of the Fallout"):
One starts off with an original list in raw form:
Alabama = "Gulf"
Arizona = "Four States"
Arkansas = "Texas"
California = "South West"
Colorado = "Four States"
Connecticut = "New England"
Delaware = "Columbia"
which eventually leads to this R code:
us_state <- c("Alabama","Arizona","Arkansas","California","Colorado","Connecticut",
"Delaware","District of Columbia","Florida","Georgia","Idaho","Illinois","Indiana",
"Iowa","Kansas","Kentucky","Louisiana","Maine","Maryland","Massachusetts","Michigan",
"Minnesota","Mississippi","Missouri","Montana","Nebraska","Nevada","New Hampshire",
"New Jersey","New Mexico","New York","North Carolina","North Dakota","Ohio","Oklahoma",
"Oregon","Pennsylvania","Rhode Island","South Carolina","South Dakota","Tennessee",
"Texas","Utah","Vermont","Virginia","Washington","West Virginia ","Wisconsin","Wyoming")
us_region <- c("Gulf","Four States","Texas","South West","Four States","New England",
"Columbia","Columbia","Gulf","Southeast","North West","Midwest","Midwest","Plains",
"Plains","East Central","Gulf","New England","Columbia","New England","Midwest",
"Midwest","Gulf","Plains","North","Plains","South West","New England","Eastern",
"Four States","Eastern","Southeast","North","East Central","Plains","North West",
"Eastern","New England","Southeast","North","East Central","Texas","Four States",
"New England","Columbia","North West","Eastern","Midwest","North")
us_state_to_region_map <- data.frame(us_state, us_region, stringsAsFactors=FALSE)
which is supremely ugly and unmaintainable as the State -> Region mapping is effectively
obfuscated.
I actually wrote a Perl program to generate the above from the original list.
In Perl, one would write things like:
#!/usr/bin/perl
$mapping = {
"Alabama"=> "Gulf",
"Arizona"=> "Four States",
"Arkansas"=> "Texas",
"California"=> "South West",
"Colorado"=> "Four States",
"Connecticut"=> "New England",
...etc...etc...
"West Virginia "=> "Eastern",
"Wisconsin"=> "Midwest",
"Wyoming"=> "North" };
which is maintainable because one can verify the mapping on a line-by-line basis.
There must be something similar to this Perl goodness in R?

Answer 1

It seems a bit open for interpretation as to what you're looking for.
Is the mapping meant to be a function type thing such that a call would return the region or vise-versa (Eg. similar to a function call mapping("alabama") => "Gulf")?
I am reading the question to be more looking for a dictionary style storage, which in R could be obtained with an equivalent named list
ncountry <- 49
mapping <- as.list(c("Gulf","Four States",
...
,"Midwest","North"))
names(mapping) <- c("Alabama","Arizona",
...
,"Wisconsin","Wyoming")
mapping[["Pennsylvania"]]
[1] "Eastern"
This could be performed in a single call
mapping <- list("Alabama" = "Gulf",
"Arizona" = "Four States",
...,
"Wisconsin" = "Midwest",
"Wyoming" = "North")
Which makes it very simple to check that the mapping is working as expected. This doesn't convert nicely to a 2 column data.frame however, which we would then obtain using
mapping_df <- data.frame(region = unlist(mapping), state = names(mapping))
note "not nicely" simply means as.data.frame doesn't translate the input into a 2 column output.
Alternatively just using a named character vector would likely be fine too
mapping_c <- c("Alabama" = "Gulf",
"Arizona" = "Four States",
...,
"Wisconsin" = "Midwest",
"Wyoming" = "North")
which would be converted to a data.frame in almost the same fashion
mapping_df_c <- data.frame(region = mapping_c, state = names(mapping_c))
Note however a slight difference in the two choices of storage. While referencing an entry that exists using either single brackets [ or double brackets [[ works just fine
#Works:
mapping_c["Pennsylvania"] == mapping["Pennsylvania"]
#output
Pennsylvania
TRUE
mapping_c[["Pennsylvania"]] == mapping[["Pennsylvania"]]
[1] TRUE
But when referencing unknown entries these differ slightly in behaviour
#works sorta:
mapping_c["hello"] == mapping["hello"]
#output
$<NA>
NULL
#Does not work:
mapping_c[["hello"]] == mapping[["hello"]]
Error in mapping_c[["hello"]] : subscript out of bounds
If you are converting your input into a data.frame this is not an issue, but it is worth being aware of this, so you obtain the behaviour expected.
Of course you could use a function call to create a proper dictionary with a simple switch statement. I don't think that would be any prettier though.

Answer 2

If us_region is a named list...
us_region <- list(Alabama = "Gulf",
Arizona = "Four States",
Arkansas = "Texas",
California = "South West",
Colorado = "Four States",
Connecticut = "New England",
Delaware = "Columbia")
Then,
us_state_to_region_map <- data.frame(us_state = names(us_region),
us_region = sapply(us_region, c),
stringsAsFactors = FALSE)
and, as a bonus, you also get the states as row names...
us_state_to_region_map
us_state us_region
Alabama Alabama Gulf
Arizona Arizona Four States
Arkansas Arkansas Texas
California California South West
Colorado Colorado Four States
Connecticut Connecticut New England
Delaware Delaware Columbia

Answer 3

As #tim-biegeleisen says it could be more appropriate to maintain this dataset in a database, a CSV file or a spreadsheet and open it in R (with readxl::read_excel(), readr::read_csv(),...).
However if you want to write it directly in your code you can use tibble:tribble() which allows to write a dataframe row by row :
library(tibble)
tribble(~ state, ~ region,
"Alabama", "Gulf",
"Arizona", "Four States",
(...)
"Wisconsin", "Midwest",
"Wyoming", "North")

Answer 4

One option could be to create a data frame in wide format (your initial list makes it very straightforward and this maintains a very obvious mapping. It is actually quite similar to your Perl code), then transform it to the long format:
library(tidyr)
data.frame(
Alabama = "Gulf",
Arizona = "Four States",
Arkansas = "Texas",
California = "South West",
Colorado = "Four States",
Connecticut = "New England",
Delaware = "Columbia",
stringsAsFactors = FALSE
) %>%
gather("us_state", "us_region") # transform to long format