[R] jpeg() creating empty files with qplot() in a loop

[Henrik Bengtsson]

First,

please provide us with enough unambiguous information so we don't have
to second guess that you are using qplot() in the 'ggplot' package.
Report sessionInfo() is recommended.

It has nothing to do with jpeg() or any other device driver.  You get
the same problem with:

library(ggplot)
x <- y <- 1:10
for (kk in 1:3) { qplot(x,y,main=kk) }

or

foo <- function(x,y,...) {
  qplot(x,y, ...)
  invisible(FALSE)
}
foo(x,y)

You have to explicitly print() the value that qplot() returns, e.g.

for (kk in 1:3) { print(qplot(x,y,main=kk)) }

foo <- function(x,y,...) {
  print(qplot(x,y, ...))
  invisible(FALSE)
}
foo(x,y)

Your last example worked only because you did it at the prompt.

/Henrik

On Thu, Feb 21, 2008 at 10:51 AM, B. Bogart <bbogart at sfu.ca> wrote:
> Hello all,
>
>  I'm stuck with a strange issue with writing jpegs of plots to a folder
>  in a loop.
>
>  This works:
>
>  for (step in 1:length(steps)) {
>   jpeg(filename=paste("frame_",sprintf("%05d",step),".jpg",sep=""))
>   plot(steps[[step]])
>   dev.off()
>  }
>
>  But if I use qplot to generate the plot (which is my aim):
>
>  for (step in 1:length(steps)) {
>   jpeg(filename=paste("frame_",sprintf("%05d",step),".jpg",sep=""))
>   qplot(x, y, data=steps[[step]], geom="tile", fill=rgb(V1,V2,V3)) +
>  scale_fill_identity() + opts(aspect.ratio = .75)
>   dev.off()
>  }
>
>  I end up with a directory of empty files named correctly.
>
>  The following does work:
>
>  step <- 10
>  jpeg(filename=paste("frame_",sprintf("%05d",step),".jpg",sep=""))
>  qplot(x, y, data=steps[[step]], geom="tile", fill=rgb(V1,V2,V3)) +
>  scale_fill_identity() + opts(aspect.ratio = .75)
>  dev.off()
>
>  So what could be the difference between the last example and it running
>  in a loop, that the former generates a proper jpeg file and the lattter
>  does not?
>
>  Does something more special than dev.off() need to be done with qplot
>  output when in a loop?
>
>  Thanks,
>  B. Bogart
>
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