Device problems in a loop.

I have tried the following code to plot and save many box plots in files.  The code works by itself, but does not run in a loop.  The error indicates that there is no device.  The error is printed below the code.

Code:

for(i in 1:len)
{
   A = as.numeric(delta[Delta[i,1]])
   B = as.numeric(delta[Delta[i,2]])
   C = as.numeric(delta[Delta[i,3]])
   D = as.numeric(delta[Delta[i,4]])
   data <- scan(filenames52[i], quiet=TRUE)
   data <- matrix(data, ncol=8)
   grp <- c('K1','K2','K3','K4','N','NBC','NE','NEBC')
   grp <- as.factor(rep(grp, each=200))
   bwplot(c(data)~grp, ylab='MSE', pch=19,
       par.settings = list(plot.symbol = list(pch = "+")),
       main=eval(substitute(expression(paste('        ', delta[phi[1]],
       " = ", tval, '  |  ',
       delta[phi[2]], ' = ', tval2, '  |  ',
       delta[theta[1]], ' = ', tval3, '  |  ',
       delta[theta[2]], ' = ', tval4,)),
       list(tval=A, tval2=B, tval3=C, tval4=D))))

   dev.print(file=filenameplot[i], device=png, width=600, height=480)
   dev.off()
}

Error is:

Error in dev.print(file = filenameplot[i], device = png, width = 600,  :
 no device to print from

I cannot figure out why the loop won't work but setting i <- 1 or 2 ... 
and then running the code inside the loop will work.  Does anyone have 
any suggestions.




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