在R中引用的各种用法的描述。
'single quotes can be used more-or-less interchangeably'
"with double quotes to create character vectors"
## Single quotes inside single-quoted strings need backslash-escaping.
## Ditto double quotes inside double-quoted strings.
##
identical('"It\'s alive!", he screamed.',
"\"It's alive!\", he screamed.") # same
## Backslashes need doubling, or they have a special meaning.
x print(x) # shows it as above ("input-like")
writeLines(x) # shows it as you like it ;-)
## Single backslashes followed by a letter are used to denote
## special characters like tab(ulator)s and newlines:
x writeLines(x) # see also ?strwrap
## Backticks are used for non-standard variable names.
## (See make.names and ?Reserved for what counts as
## non-standard.)
`x y` `x y`
d d$`1st column`
## Backslashes followed by up to three numbers are interpreted as
## octal notation for ASCII characters.
"\110\145\154\154\157\40\127\157\162\154\144\41"
## \x followed by up to two numbers is interpreted as
## hexadecimal notation for ASCII characters.
(hw1
## Mixing octal and hexadecimal in the same string is OK
(hw2
## \u is also hexadecimal, but supported up to 4 numbers,
## using Unicode specification. In the previous example,
## you can simply replace \x with \u.
(hw3
## The last three are all identical to
hw stopifnot(identical(hw, hw1), identical(hw1, hw2), identical(hw2, hw3))
## Using Unicode makes more sense for non-latin characters.
(nn
## Mixing \x and \u throws a _parse_ error (which is not catchable!)
## Not run:
"\x48\u65\x6c\u6c\x6f\u20\x57\u6f\x72\u6c\x64\u21"
## End(Not run)
## --> Error: mixing Unicode and octal/hex escapes .....
## \U works like \u, but supports up to eight numbers.
## So we can replace \u with \U in the previous example.
n2 stopifnot(identical(nn, n2))
## Under systems supporting multi-byte locales (and not Windows),
## \U also supports the rarer characters outside the usual 16^4 range.
## See the R language manual,
## https://cran.r-project.org/doc/manuals/r-release/R-lang.html#Literal-constants
## and bug 16098 https://bugs.r-project.org/bugzilla3/show_bug.cgi?id=16098
"\U1d4d7" # On Windows this gives the incorrect value of "\Ud4d7"
## nul characters (for terminating strings in C) are not allowed (parse errors)
## Not run:
"foo\0bar" # Error: nul character not allowed (line 1)
"foo\u0000bar" # same error
## End(Not run)
## A Windows path written as a raw string constant:
r"(c:\Program files\R)"
## More raw strings:
r"{(\1\2)}"
r"(use both "double" and 'single' quotes)"
r"---(\1--)-)---"