[R] read.*: How to read from a URL?

[Prof Brian Ripley]

On Wed, 10 Dec 2008, hadley wickham wrote:

> Hi Michael,
>
> In general, I think you should be able to do:
>
> gimage <- read.jpeg(url(gimageloc))

Note that would not be really correct: it would need to be

gimage <- read.jpeg(con <- url(gimageloc))
close(con)

since it otherwise leaks a connection (which would eventually be closed).

However, from ?read.jpeg

Arguments:

filename: filename of JPEG image

so it does not accept a connection (and the source code wll confirm that). 
In fact virtually all functions that accept a 'file name or connection' 
will work with URLs, as file() accepts URLs as well as file names (see 
?file).

The issue is that writers of third-party readers should be encouraged to 
support connections (which have been around for ca 7 years in R).
It is ammazing how people take such innovations for granted.

> or alternatively use the EBImage from bioconductor which will read
> from a url automatically (it also opens a much wider range of file
> types)
>
> library(EBImage)
> img <- readImage(gimageloc, TrueColor)
>
> Hadley
>
>
> On Wed, Dec 10, 2008 at 1:17 PM, Michael Friendly <friendly at yorku.ca> wrote:
>> The question is how to use a URL in place of a file= argument for
>> read.*.functions that do
>> not support this internally.
>>
>> e.g., utils::read.table() and her family all support a file= argument that
>> can take a URL
>> equally well as a local file.  So, if I have a file on the web, I can
>> equally well do
>>
>>> langren <- read.csv("langrens.csv", header=TRUE)
>>> langren <-
>>> read.csv("http://euclid.psych.yorku.ca/SCS/Gallery/Private/langrens.csv",
>>> header=TRUE)
>>
>> where the latter is more convenient for posts to this list or distributed
>> examples.
>> rimage::read.jpeg() doesn't support URLs, and the only way I've found is to
>> download the
>> image file from a URL to a temp file, in several steps.
>> This is probably a more general problem than just read.jpeg,
>> so maybe there is a general idiom for this case, or better-- other read.*
>> functions could
>> be encouraged to support URLs.
>>
>>> library(rimage)
>>> # local file: OK
>>> gimage <-
>>> read.jpeg("C:/Documents/milestone/images/vanLangren/google-toledo-rome3.jpg")
>>>
>>> gimageloc <-
>>> "http://euclid.psych.yorku.ca/SCS/Gallery/images/Private/Langren/google-toledo-rome3.jpg"
>>> dest <- paste(tempfile(),'.jpg', sep='')
>>> download.file(gimageloc, dest, mode="wb")
>> trying URL
>> 'http://euclid.psych.yorku.ca/SCS/Gallery/images/Private/Langren/google-toledo-rome3.jpg'
>> Content type 'image/jpeg' length 35349 bytes (34 Kb)
>> opened URL
>> downloaded 34 Kb
>>
>>> dest
>> [1] "C:\\DOCUME~1\\default\\LOCALS~1\\Temp\\Rtmp9nNTdV\\file5f906952.jpg"
>>> #  Is there something simpler??
>>> gimage <- read.jpeg(dest)
>>
>>> #  I thought file() might work, but evidently not.
>>> gimage <- read.jpeg(file(gimageloc))
>> Error in read.jpeg(file(gimageloc)) : Can't open file.
>>>
>>
>> --
>> Michael Friendly     Email: friendly AT yorku DOT ca Professor, Psychology
>> Dept.
>> York University      Voice: 416 736-5115 x66249 Fax: 416 736-5814
>> 4700 Keele Street    http://www.math.yorku.ca/SCS/friendly.html
>> Toronto, ONT  M3J 1P3 CANADA
>>
>> ______________________________________________
>> R-help at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>
>
>
> -- 
> http://had.co.nz/
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595