This is perhaps rather a minor question...
but just a moment ago I was looking through some code I had written and noticed that I tend to just use ="something" and ='something_else' completely interchangeably, often in the same function.
So my question is: Is there R code in which using one or other (single or double quotes) has different behaviour? Or are they totally synonymous?
According to http://stat.ethz.ch/R-manual/R-patched/library/base/html/Quotes.html, "[s]ingle and double quotes delimit character constants. They can be used interchangeably but double quotes are preferred (and character constants are printed using double quotes), so single quotes are normally only used to delimit character constants containing double quotes."
Just for curiosity, there is a further explaination in R-help mailing list for Why double quote is preferred in R:
To avoid confusion for those who are accustomed to programming in the
C family of languages (C, C++, Java), where there is a difference in
the meaning of single quotes and double quotes.
A C programmer reads 'a' as a single character and "a" as a character
string consisting of the letter 'a' followed by a null character to
terminate the string.
In R there is no character data type, there are
only character strings. For consistency with other languages it helps
if character strings are delimited by double quotes. The single quote
version in R is for convenience.
(Since) On most keyboards you don't need to
use the shift key to type a single quote but you do need the shift for
a double quote.
> print(""hi"")
Error: unexpected symbol in "print(""hi"
> print("'hi'")
[1] "'hi'"
> print("hi")
[1] "hi"
Related
I'm trying to understand what backticks do in R.
From what I can tell, this is not explained in the ?Quotes documentation page for R.
For example, at the R console:
"[["
# [1] "[["
`[[`
# .Primitive("[[")
It seem to be returning the equivalent to:
get("[[")
A pair of backticks is a way to refer to names or combinations of symbols that are otherwise reserved or illegal. Reserved are words like if are part of the language, while illegal includes non-syntactic combinations like c a t. These two categories, reserved and illegal, are referred to in R documentation as non-syntactic names.
Thus,
`c a t` <- 1 # is valid R
and
> `+` # is equivalent to typing in a syntactic function name
function (e1, e2) .Primitive("+")
As a commenter mentioned, ?Quotes does contain some information on the backtick, under Names and Identifiers:
Identifiers consist of a sequence of letters, digits, the period (.) and the underscore. They must not start with a digit nor underscore, nor with a period followed by a digit. Reserved words are not valid identifiers.
The definition of a letter depends on the current locale, but only ASCII digits are considered to be digits.
Such identifiers are also known as syntactic names and may be used directly in R code. Almost always, other names can be used provided they are quoted. The preferred quote is the backtick (`), and deparse will normally use it, but under many circumstances single or double quotes can be used (as a character constant will often be converted to a name). One place where backticks may be essential is to delimit variable names in formulae: see formula
This prose is a little hard to parse. What it means is that for R to parse a token as a name, it must be 1) a sequence of letters digits, the period and underscores, that 2) is not a reserved word in the language. Otherwise, to be parsed as a name, backticks must be used.
Also check out ?Reserved:
Reserved words outside quotes are always parsed to be references to the objects linked to in the 'Description', and hence they are not allowed as syntactic names (see make.names). They are allowed as non-syntactic names, e.g.inside backtick quotes.
In addition, Advanced R has some examples of how backticks are used in expressions, environments, and functions.
They are equivalent to verbatim. For example... try this:
df <- data.frame(20a=c(1,2),b=c(3,4))
gives error
df <- data.frame(`20a`=c(1,2),b=c(3,4))
doesn't give error
Here is an incomplete answer using improper vocabulary: backticks can indicate to R that you are using a function in a non-standard way. For instance, here is a use of [[, the list subsetting function:
temp <- list("a"=1:10, "b"=rnorm(5))
extract element one, the usual way
temp[[1]]
extract element one using the [[ function
`[[`(temp,1)
The lintr package has a check for single-quoted strings, and single quotes are discouraged elsewhere (e.g. ?Quotes). Single and double quoted strings work identically in R, so is there a reason why single quotes are considered bad practice?
Citations to canonical documents especially welcome. To be clear: I'm asking about the reasons given by the R core team for discouraging single quotes; not about people's own opinions on the topic.
I guess because R by default prints using double quotes.
This way the 'other quote character' in a string gets
a) handled normally if its a single quote
b) escaped when its a double quote
when printing. Using double-quotes discourages the use of double-quotes in a string.
Take the following example:
Double Quoted
"It's my car"
Prints
"It's my car"
Single Quoted
'It"s my car'
Prints
"It\"s my car"
I'm trying to understand what backticks do in R.
From what I can tell, this is not explained in the ?Quotes documentation page for R.
For example, at the R console:
"[["
# [1] "[["
`[[`
# .Primitive("[[")
It seem to be returning the equivalent to:
get("[[")
A pair of backticks is a way to refer to names or combinations of symbols that are otherwise reserved or illegal. Reserved are words like if are part of the language, while illegal includes non-syntactic combinations like c a t. These two categories, reserved and illegal, are referred to in R documentation as non-syntactic names.
Thus,
`c a t` <- 1 # is valid R
and
> `+` # is equivalent to typing in a syntactic function name
function (e1, e2) .Primitive("+")
As a commenter mentioned, ?Quotes does contain some information on the backtick, under Names and Identifiers:
Identifiers consist of a sequence of letters, digits, the period (.) and the underscore. They must not start with a digit nor underscore, nor with a period followed by a digit. Reserved words are not valid identifiers.
The definition of a letter depends on the current locale, but only ASCII digits are considered to be digits.
Such identifiers are also known as syntactic names and may be used directly in R code. Almost always, other names can be used provided they are quoted. The preferred quote is the backtick (`), and deparse will normally use it, but under many circumstances single or double quotes can be used (as a character constant will often be converted to a name). One place where backticks may be essential is to delimit variable names in formulae: see formula
This prose is a little hard to parse. What it means is that for R to parse a token as a name, it must be 1) a sequence of letters digits, the period and underscores, that 2) is not a reserved word in the language. Otherwise, to be parsed as a name, backticks must be used.
Also check out ?Reserved:
Reserved words outside quotes are always parsed to be references to the objects linked to in the 'Description', and hence they are not allowed as syntactic names (see make.names). They are allowed as non-syntactic names, e.g.inside backtick quotes.
In addition, Advanced R has some examples of how backticks are used in expressions, environments, and functions.
They are equivalent to verbatim. For example... try this:
df <- data.frame(20a=c(1,2),b=c(3,4))
gives error
df <- data.frame(`20a`=c(1,2),b=c(3,4))
doesn't give error
Here is an incomplete answer using improper vocabulary: backticks can indicate to R that you are using a function in a non-standard way. For instance, here is a use of [[, the list subsetting function:
temp <- list("a"=1:10, "b"=rnorm(5))
extract element one, the usual way
temp[[1]]
extract element one using the [[ function
`[[`(temp,1)
To declare a character or a string on R, one can use both following ways:
x <- 'Some string'
x <- "Some string"
Both work, but is there any difference ?
From ?"'":
Details
Three types of quotes are part of the syntax of R: single and double
quotation marks and the backtick (or back quote, `). In addition,
backslash is used to escape the following character inside character
constants.
Character constants
Single and double quotes delimit character constants. They can be used
interchangeably but double quotes are preferred (and character
constants are printed using double quotes), so single quotes are
normally only used to delimit character constants containing double
quotes.
Backslash is used to start an escape sequence inside character
constants. Escaping a character not in the following table is an
error.
Single quotes need to be escaped by backslash in single-quoted
strings, and double quotes in double-quoted strings.
No. These are identical.
......
Windows copies path with backslash \, which R does not accept. So, I wanted to write a function which would convert \ to /. For example:
chartr0 <- function(foo) chartr('\','\\/',foo)
Then use chartr0 as...
source(chartr0('E:\RStuff\test.r'))
But chartr0 is not working. I guess, I am unable to escape /. I guess escaping / may be important in many other occasions.
Also, is it possible to avoid the use chartr0 every time, but convert all path automatically by creating an environment in R which calls chartr0 or use some kind of temporary use like using options
From R 4.0.0 you can use r"(...)" to write a path as raw string constant, which avoids the need for escaping:
r"(E:\RStuff\test.r)"
# [1] "E:\\RStuff\\test.r"
There is a new syntax for specifying raw character constants similar to the one used in C++: r"(...)" with ... any character sequence not containing the sequence )". This makes it easier to write strings that contain backslashes or both single and double quotes. For more details see ?Quotes.
Your fundamental problem is that R will signal an error condition as soon as it sees a single back-slash before any character other than a few lower-case letters, backslashes themselves, quotes or some conventions for entering octal, hex or Unicode sequences. That is because the interpreter sees the back-slash as a message to "escape" the usual translation of characters and do something else. If you want a single back-slash in your character element you need to type 2 backslashes. That will create one backslash:
nchar("\\")
#[1] 1
The "Character vectors" section of _Intro_to_R_ says:
"Character strings are entered using either matching double (") or single (') quotes, but are printed using double quotes (or sometimes without quotes). They use C-style escape sequences, using \ as the escape character, so \ is entered and printed as \, and inside double quotes " is entered as \". Other useful escape sequences are \n, newline, \t, tab and \b, backspace—see ?Quotes for a full list."
?Quotes
chartr0 <- function(foo) chartr('\\','/',foo)
chartr0('E:\\RStuff\\test.r')
You cannot write E:\Rxxxx, because R believes R is escaped.
The problem is that every single forward slash and backslash in your code is escaped incorrectly, resulting in either an invalid string or the wrong string being used. You need to read up on which characters need to be escaped and how. Take a look at the list of escape sequences in the link below. Anything not listed there (such as the forward slash) is treated literally and does not require any escaping.
http://cran.r-project.org/doc/manuals/R-lang.html#Literal-constants