检查字符值是否是有效的 R 对象名称

Question

Several months ago I asked something similar , but I was using JavaScript to check if provided string is a "valid" R object name. Now I'd like to achieve the same by using nothing but R. I suppose that there's a very nice way to do this, with some neat (not so) esoteric R function, so regular expressions seem to me as the last line of defence. Any ideas?

Oh, yeah, using back-ticks and stuff is considered cheating. =)

Answer 1

Edited 2013-1-9 to fix regular expression. Previous regular expression, lifted from page 456 of John Chambers' "Software for Data Analysis", was (subtly) incomplete. (ht Hadley Wickham)

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There are a couple of issues here. A simple regular expression can be used to identify all syntactically valid names --- but some of those names (like if and while ) are 'reserved', and cannot be assigned to.

• Identifying syntactically valid names:

  ?make.names explains that a syntactically valid name:

  [...] consists of letters, numbers and the dot or underline characters and starts with a letter or the dot not followed by a number. Names such as '".2way"' are not valid [...]

  Here is the corresponding regular expression:

   "^((([[:alpha:]]|[.][._[:alpha:]])[._[:alnum:]]*)|[.])$"

• Identifying unreserved syntactically valid names

  To identify unreserved names, you can take advantage of the base function make.names() , which constructs syntactically valid names from arbitrary character strings.

   isValidAndUnreserved <- function(string) { make.names(string) == string } isValidAndUnreserved(".jjj") # [1] TRUE isValidAndUnreserved(" jjj") # [1] FALSE

• Putting it all together

  isValidName <- function(string) { grepl("^((([[:alpha:]]|[.][._[:alpha:]])[._[:alnum:]]*)|[.])$", string) } isValidAndUnreservedName <- function(string) { make.names(string) == string } testValidity <- function(string) { valid <- isValidName(string) unreserved <- isValidAndUnreservedName(string) reserved <- (valid & ! unreserved) list("Valid"=valid, "Unreserved"=unreserved, "Reserved"=reserved) } testNames <- c("mean", ".j_j", ".", "...", "if", "while", "TRUE", "NULL", "_jj", " j", ".2way") t(sapply(testNames, testValidity))

   Valid Unreserved Reserved mean TRUE TRUE FALSE .j_j TRUE TRUE FALSE . TRUE TRUE FALSE ... TRUE TRUE FALSE if TRUE FALSE TRUE while TRUE FALSE TRUE TRUE TRUE FALSE TRUE NULL TRUE FALSE TRUE _jj FALSE FALSE FALSE j FALSE FALSE FALSE # Note: these tests are for " j", not "j" .2way FALSE FALSE FALSE

For more discussion of these issues, see the r-devel thread linked to by @Hadley in the comments below.

Answer 2

As Josh suggests, make.names is probably the best solution to this. Not only will it handle weird punctuation, it'll also flag reserved words:

make.names(".x")   # ".x"
make.names("_x")   # "X_x"
make.names("if")   # " if."
make.names("function")  # "function."