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

From: B. Bogart <bbogart_at_sfu.ca>
Date: Thu, 21 Feb 2008 10:51:33 -0800

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]])

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?

B. Bogart

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