R help - Jul 2011 - dev.off and Cairo and ggplot2

I have a weird example which fails and I don't understand why.  If I
put dev.off() inside my function, it produces a bad image file.  But
if I run what I think should be equivalent code with dev.off() running
outside my function, it works.  Why does the location of dev.off
matter?


library(Cairo)
library(ggplot2)

foo<-function(){
   Cairo(file="a.png")
   cat(dev.cur(), "\n")
   qplot(1:10,1:10)
   dev.off();
}

foo()

# file a.png is bad

foo1<-function(){
   Cairo(file="a.png")
   cat(dev.cur(), "\n")
   qplot(1:10,1:10)
}

foo1(); dev.off()

# file a.png is good

On Jul 13, 2011, at 5:55 PM, Stephen Eick wrote:
> I have a weird example which fails and I don't understand why.  If I
> put dev.off() inside my function, it produces a bad image file.  But
> if I run what I think should be equivalent code with dev.off() running
> outside my function, it works.  Why does the location of dev.off
> matter?
It's in the misc section of the R-FAQ:

(Title is something along the lines of "Why don't lattice plots show  
up?" but its more general than lattice and also applies to ggplot/ 
ggplot2)

-- 
David.
>
>
> library(Cairo)
> library(ggplot2)
>
> foo<-function(){
>   Cairo(file="a.png")
>   cat(dev.cur(), "\n")
>   qplot(1:10,1:10)
>   dev.off();
> }
>
> foo()
>
> # file a.png is bad
>
> foo1<-function(){
>   Cairo(file="a.png")
>   cat(dev.cur(), "\n")
>   qplot(1:10,1:10)
> }
>
> foo1(); dev.off()
>
> # file a.png is good
>
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David Winsemius, MD
West Hartford, CT