[R] pdf() and histogram() in function call

[qian z]

   Here is a function I wrote. It runs no problem, but generate empty pdf
   files.

   I can't find what is the problem.


   create.pdf<- function(x, dir)
    {
     dir.create(dir, showWarnings = FALSE)
     plist<- c("a", "b" , "c", "d")

     for(j in plist)
     {

      filedir<- paste(dir, "/", j, ".pdf", sep="")

      form1<- as.formula(paste("~ ", j, " | var1", sep=" "))
      form2<- as.formula(paste("~ ", j, " | var2", sep=" "))
      form3<- as.formula(paste("~ ", j, " | var3", sep=" "))
      pdf(filedir)

      histogram(form1,data=x,type="count", xlab=j,main="Histogram conditioned
   on the levels of var1")
      histogram(form2,data=x, type="count",xlab=j,main="Histogram conditioned
   on the levels of var2")
      histogram(form3, data=x, type="count", xlab=j, main="Histogram
   conditioned on the levels of var3" )

      dev.off()
     }


    }


   I have tried to find the problem, but no luck!  Seriously need HELP! Thanks.

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References

   1. http://us.rd.yahoo.com/evt=51733/*http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ