[R] How to convert "c:\a\b" to "c:/a/b"?

[Spencer Graves]

Hi, Henrik:

	  Several functions, e.g., "grep", "sub", "gsub", and "regexpr", have 
an argument "perl", FALSE by default.  Moreover, "?regexp" has a section 
on "Perl Regular Expressions".  If you can do it in perl, might that 
transfer to "gsub(..., perl=TRUE)"?

	  Thanks,
	  spencer graves
p.s.  I skimmed the discussion of "Pearl Regular Expressions", and 
experimented with "gsub(..., perl=TRUE)" without success.  However, 
there may be a way to do it, and I just don't know perl and regexp well 
enough to have figured it out in the time available.

Henrik Bengtsson wrote:

> Spencer Graves wrote:
> 
>>       Thanks, Dirk, Gabor, Eric:
>>
>>       You all provided appropriate solutions for the stated problem. 
>> Sadly, I oversimplified the problem I was trying to solve:  I copy a 
>> character string giving a DOS path from MS Windows Explorer into an R 
>> script file, and I get something like the following:
>>
>>       D:\spencerg\statmtds\R\Rnews
>>
>>       I want to be able to use this in R with its non-R meaning, e.g., 
>> in readLine, count.fields, read.table, etc., after appending a file 
>> name. Your three solutions all work for my oversimplified toy example 
>> but are inadequate for the problem I really want to solve.
> 
> 
> Hmmm. It should work as long as you do not source() the file (see 
> below).  There are two things to watch out for here.
> 
> First, you have to be careful with backslashes, that is, a backslash is 
> a single character ('\') in memory, but to be typed at the R prompt, you 
> have to escape it (with a backslash), which is why we type "\\", cf. 
> nchar("\\") == 0.  Consider the file foo.txt containing the 28 
> characters (==28 bytes in plain ASCII format)
> 
> D:\spencerg\statmtds\R\Rnews
> 
> You can create such a file in R by
> 
>  > cat(file="foo.txt", "D:\\spencerg\\statmtds\\R\\Rnews")
>  > str(file.info("foo.txt"))
> `data.frame':   1 obs. of  6 variables:
>  $ size : num 28
>  $ isdir: logi FALSE
>  $ mode :Class 'octmode'  int 438
>  $ mtime:'POSIXct', format: chr "2005-06-27 19:14:20"
>  $ ctime:'POSIXct', format: chr "2005-06-27 19:14:20"
>  $ atime:'POSIXct', format: chr "2005-06-27 19:14:20"
> 
> Re-read it into R:
>  > bfr <- readLines("foo.txt")
> Warning message:
> incomplete final line found by readLines on 'foo.txt'
>  > bfr
> [1] "D:\\spencerg\\statmtds\\R\\Rnews"
>  > cat("bfr='", bfr, "'\n", sep="")
> bfr='D:\spencerg\statmtds\R\Rnews'
> 
> Now, convert backslashes to "forwardslashes":
> bfr2 <- gsub("\\\\", "/", bfr)
>  > bfr2
> [1] "D:/spencerg/statmtds/R/Rnews"
>  > cat("bfr2='", bfr2, "'\n", sep="")
> bfr2='D:/spencerg/statmtds/R/Rnews'
> 
> Second, regular expression patterns have their own escaping rules. This 
> is why the following happens:
> 
> bfr3 <- gsub("\\", "/", bfr)
> Error in gsub(pattern, replacement, x, ignore.case, extended, fixed) :
>         invalid regular expression '\'
> 
> The pattern "\\", which is a single '\' in memory, is passed to gsub(). 
> Then gsub() tries to interpret this single backslash as a pattern, but 
> it is invalid. gsub() uses backslashed to escape some characters in 
> patterns.  So, when you think what gsub() needs, this about the 
> characters (bytes) that are really stored in memory, not what you see.
> 
> A side comment: Wouldn't it be nice if the R parser had an alternative 
> way to quote string such that, say, Perl strings could be used? Example:
> 
> bfr3 <- gsub("\\\\", "/", bfr)
> bfr3 <- gsub('\\', "/", bfr)
> 
> would be equal (if now single quotes wouldn't have been reserved already).
> 
> Back to your problem: You must not paste the 28 characters into an R 
> script that you source()! If you want to include you pathname (copied 
> from the command prompt), you have to escape each '\' with a '\' to 
> '\\'. Thus, if you use Emacs or another text editor, you pretty much 
> should see '\\' if you want R(!) to interpret this as the single 
> character '\'. Note the difference between using source() and, say, 
> readLines().
> 
> Hope this helps
> 
> Henrik
> 
> 
>>       Thanks,
>>       spencer graves
>>
>> Gabor Grothendieck wrote:
>>
>>
>>> On 6/27/05, Dirk Eddelbuettel <edd at debian.org> wrote:
>>>
>>>
>>>> On 26 June 2005 at 20:30, Spencer Graves wrote:
>>>> |         How can one convert back slashes to forward slashes, e.g, 
>>>> changing
>>>> | "c:\a\b" to "c:/a/b"?  I tried the following:
>>>> |
>>>> |  > gsub("\\\\", "/", "c:\a\b")
>>>> | [1] "c:\a\b"
>>>>
>>>> This does work, provided you remember that single backslashed "don't 
>>>> exist"
>>>> as e.g. \a is a character in itself. So use doubles are you should 
>>>> be fine:
>>>>
>>>>
>>>>
>>>>> gsub("\\\\", "/", "c:\\a\\b")
>>>>
>>>>
>>>> [1] "c:/a/b"
>>>>
>>>
>>>
>>> Also, if one finds four backslashes confusing one can avoid the use 
>>> of four via any of these:
>>>
>>> gsub("[\\]", "/", "c:\\a\\b")
>>> gsub("\\", "/", "c:\\a\\b", fixed = TRUE)
>>> chartr("\\", "/", "c:\\a\\b")
>>>
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>>
>>
>>
> 

-- 
Spencer Graves, PhD
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