This question already has answers here:
How to get help in R?
(6 answers)
Closed 2 years ago.
I wanted to use "help in R" in order to see some information about some commands such as "for", "if", "while", "repeat" etc. But there is no information in "R help" regarding such commands. I would like to know why?
I use "R help" for the above commands like below:
?for
?while
?if
?repeat
R requires that keywords are used in syntactically valid form. The way R works, it expects that if for instance is followed by an expression in parentheses, and a body. ?if is not valid R syntax.
Conversely, ? is an operator that expects an identifier after it.
To make it valid, you should quote the if identifier in backticks. That way, R parses the expression as ? followed by an identifier, rather than ? followed by an incomplete if expression:
?`if`
Backtick-quoting is R’s way of saying: “hey, that thing between backticks is a valid identifier, even if it totally doesn’t look like one”. You could (but generally shouldn’t!) totally use it to use wonky variable names:
`name with spaces` = 2
message(`name with spaces` + 5)
# 7
This feature is more useful when applied to column names of externally imported data (which sometimes contains spaces or other invalid identifier characters), or when defining operators.
Related
This question already has answers here:
Regular expression to stop at first match
(9 answers)
Closed 1 year ago.
In long passages using bookdown, I have inserted numerous images. Having combined the passages into a single character string (in a data frame) I want to remove the markdown text associated with inserting images, but not any text in between those inserted images. Here is a toy example.
text.string <- "writing  writing to keep  more writing"
str_remove_all(string = text.string, pattern = "!\\[.+\\)")
[1] "writing more writing"
The regex expression doesn't stop at the first closed parenthesis, it continues until the last one and deletes the "writing to keep" in between.
I tried to apply String manipulation in R: remove specific pattern in multiple places without removing text in between instances of the pattern, which uses gsubfn and gsub but was unable to get the solutions to work.
Please point me in the right direction to solve this problem of a regex removal of designated strings, but not the characters in between the strings. I would prefer a stringr solution, but whatever works. Thank you
You have to use the following regex
"!\\[[^\\)]+\\)"
alternatively you can also use this:
"!\\[.*?\\)"
both solution offer a lazy match rather than a greedy one, which is the key to your question
I think you could use the following solution too:
gsub("!\\[[^][]*\\]\\([^()]*\\)", "", text.string)
[1] "writing writing to keep more writing"
This question already has answers here:
What is the meaning of the dollar sign "$" in R function()?
(2 answers)
Closed 2 years ago.
There are multiple post in the internet regarding the differences and similarities about [ and $. I see some post where $ is recommended only for interactive use but not for programming. However, I am not sure I understand if this is a preference or there is an explanation behind this idea.
Now lets say I am writing a package or function, if I am extracting an element by name (e.g., mtcars[["mpg"]]) why I should avoid using mtcars$mpg?
There are two differences that really matter between [[ and $:
[[ - works with strings (i.e. it supports variable substitution), $ doesn't. If you have my_var = "mpg", you can use mtcars[[my_var]], but there isn't a good way to use my_var with $.
$ auto-completes, if a partial column name is unambiguous. mtcars$m will return the mpg column, mtcars[["m"]] will return NULL. mtcars$d will return NULL because multiple columns start with a "d".
#1 makes [[ more flexible for programming - it's extremely common in programmatic use to be working with column names stored as strings.
#2 makes $ more dangerous - you should not use abbreviated column names in programming, however in interactive use it can be nice and quick. (Though this is largely moot with RStudio's auto-completion features, if you use that IDE.)
$ does partial matching: if you have a column named xxx in a dataframe dat, then dat$xx will return the xxx column (unless you also have a xx column). This can be dangerous.
I always use [["..."]] for another reason: I use RStudio, and there is a nice highlighting for strings, whereas there's no highlighting with $.
R 4.0.0 brings in a new syntax for raw strings:
r"(raw string here can contain anything except the closing sequence)"
But this same construct in R 3.x.x produced a syntax error:
Error: unexpected string constant in "r"(asdasd)""
Does it mean that the interpreter was changed in R 4.0.0. ?
And if so - does R 4.0.0. provide a mechanism to define custom functions like foo"()" ?
No, that's not possible at the moment (nor would I anticipate it becoming possible anytime soon).
Here's the NEWS item:
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.
https://cran.r-project.org/doc/manuals/r-devel/NEWS.html
Then from ?Quotes:
Raw character constants are also available using a syntax similar to
the one used in C++: r"(...)" with ... any character
sequence, except that it must not contain the closing sequence
)". The delimiter pairs [] and {} can also be
used, and R can be used in place of r. For additional
flexibility, a number of dashes can be placed between the opening quote
and the opening delimiter, as long as the same number of dashes appear
between the closing delimiter and the closing quote.
https://github.com/wch/r-source/blob/trunk/src/library/base/man/Quotes.Rd
Here's the (git mirror of the SVN patch of the) commit where this functionality was added:
https://github.com/wch/r-source/commit/8b0e58041120ddd56cd3bb0442ebc00a3ab67ebc
I work with knitr() and I wish to transform inline Latex commands like "\label" and "\ref", depending on the output target (Latex or HTML).
In order to do that, I need to (programmatically) generate valid R strings that correctly represent the backslash: for example "\label" should become "\\label". The goal would be to replace all backslashes in a text fragment with double-backslashes.
but it seems that I cannot even read these strings, let alone process them: if I define:
okstr <- function(str) "do something"
then when I call
okstr("\label")
I directly get an error "unrecognized escape sequence"
(of course, as \l is faultly)
So my question is : does anybody know a way to read strings (in R), without using the escaping mechanism ?
Yes, I know I could do it manually, but that's the point: I need to do it programmatically.
There are many questions that are close to this one, and I have spent some time browsing, but I have found none that yields a workable solution for this.
Best regards.
Inside R code, you need to adhere to R’s syntactic conventions. And since \ in strings is used as an escape character, it needs to form a valid escape sequence (and \l isn’t a valid escape sequence in R).
There is simply no way around this.
But if you are reading the string from elsewhere, e.g. using readLines, scan or any of the other file reading functions, you are already getting the correct string, and no handling is necessary.
Alternatively, if you absolutely want to write LaTeX-like commands in literal strings inside R, just use a different character for \; for instance, +. Just make sure that your function correctly handles it everywhere, and that you keep a way of getting a literal + back. Here’s a suggestion:
okstr("+label{1 ++ 2}")
The implementation of okstr then needs to replace single + by \, and double ++ by + (making the above result in \label{1 + 2}). But consider in which order this needs to happen, and how you’d like to treat more complex cases; for instance, what should the following yield: okstr("1 +++label")?
This question already has answers here:
How to escape backslashes in R string
(3 answers)
Closed 5 years ago.
I have an array:
t <- c("IMCR01","IMFA02","IMFA03")
I want to make it look like this:
"\'IMCR01\'","\'IMFA02\'","\'IMFA03\'"
I tried different ways like:
paste0("\'",t,"\'")
paste0("\\'",t,"\\'")
paste0("\\\\'",t,"\\\\'")
But none of them is correct. Any other functions are OK as well.
Actually your second attempt is correct:
paste0("\\'",t,"\\'")
If you want to tell paste to use a literal backslash, you need to escape it once (but not twice, as you would need within a regex pattern). This would output the following to the console in R:
[1] "\\'IMCR01\\'" "\\'IMFA02\\'" "\\'IMFA03\\'"
The trick here is that the backslash is even being escaped by R in the console output. If you were instead to write t to a text file, you would only see a single backslash as you wanted:
write(t, file = "/path/to/your/file.txt")
But why does R need to escape backslash when writing to its own console? One possibility is that if it were to write a literal \n then this would actually be interpreted by the console as a newline. Hence the need for eacaping is still there.