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

Michael Friendly friendly at yorku.ca
Wed Dec 10 20:17:06 CET 2008

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 <- 

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 <- 
 > gimageloc <- 
 > dest <- paste(tempfile(),'.jpg', sep='')
 > download.file(gimageloc, dest, mode="wb")
trying URL 
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

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