Re: [R] tempfile problem

From: Romain Francois <romain.francois_at_dbmail.com>
Date: Thu, 17 Jun 2010 19:17:57 +0200

Le 17/06/10 18:59, Duncan Murdoch a écrit :
>
> On 17/06/2010 12:43 PM, Ben Madin wrote:
>> G'day all,
>>
>> The documentation for tempfile states :
>>
>> "The names are very likely to be unique among calls to tempfile in an
>> R session and across simultaneous R sessions. The filenames are
>> guaranteed not to be currently in use."
>>
>> My problem I think relates to the second part of the sentence, which
>> is the guarantee... and it is being met ... but I need to save the
>> files as .png files, in the same directory, so I am adding the suffix
>> and I suppose therefore the next offering can be unique (as it doesn't
>> have the prefix)
>>
>> I am using a command like :
>>
>> > fname <- basename(tempfile("nahis",
>> "/Library/WebServer/Documents/nahis/tmp"))
>>
>> on a mac, or
>> > fname <- basename(tempfile("nahis", "/htdocs/nahis/tmp"))
>>
>> on a FreeBSD system, as I need to be able to find the file from the
>> web browser up to 24 hours later.
>>
>> and then
>> > this_filename <- paste(fname, ".png", sep = "")
>>
>> and saving the file as this_filename, hence the next call doesn't find
>> it's own suggestion, and starts again.
>
> It sounds as though you are doing something strange with the random
> number seed, because those names are chosen at random, and then checked
> for uniqueness. If
> the seed is being reset you could get the same name twice in a row, but
> otherwise it's very unlikely. (And it's the C library function rand(),
> not R's RNG that is used.)
>> Is there any alternative filenameing approach I can use to get around
>> this? Do I need to manually scan and reject the name if it matches the
>> names I already have? Should I just digest the current time ? (It's
>> working so far!)
>
> If you use the current time, watch out for timer accuracy and fast
> computers. You may be able to get more than one file created before the
> next timer tick.
>
> I'd suggest that you should generate more than enough filenames once at
> the start, confirm they're all unique, and then just take them one by
> one as needed. Alternatively, create the tempfile() as well as the
> tempfile().png, but this is likely to be really slow if the seed is the
> same each time, because checking for the existence of the first n tries
> is going to be slow.
>
> Duncan Murdoch

Would it not make sense to change the signature of tempfile to this:

function (pattern = "file", tmpdir = tempdir(), suffix = "" )

and include the suffix in the "does the file exist" test ?

Romain

-- 
Romain Francois
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Received on Thu 17 Jun 2010 - 17:21:14 GMT

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