R help - Nov 2005 - assign() problem

I've written a piece of code (see below) to do a wavelet image 
decomposition, during  the evaluation of this code I would like to write 
the results of some calculations back to the R root directory. I used 
assign() to do so because the names should vary when going thrue a 
while() loop. For some unknown reason I get an error that says:

Error in assign(varname[i], imwrImage) :
         invalid first argument

what could be wrong, when I do it on the commandline everything works 
out just fine. But within the function it doesn't. When I disable the 
assign statement, everything runs fine so the rest of the code should be 
clean.

My code:

# Wavelet multiresolution decomposition

wmra <- function(image_file){
#require(rimage)
require(wavethresh)

#reading image file and converting it to grayscale
#dimage <- read.jpeg(image_file)
#dimage <- imagematrix(dimage)

# discrete wavelet decomposition
imwdImage <- imwd(image_file)

# get a value for the number of decomposition levels
#nlevels <- imwdImage$nlevels

i <- 0
varname <- paste("level",0:(imwdImage$nlevels-1),sep="")
print(varname)
while ( i < imwdImage$nlevels)
	 {
	
	 # set the threshold to 0 on all levels except the one in evaluation
	 # thresholdLevels is the list of levels to set to zero
	 thresholdLevels <- 1:(imwdImage$nlevels-1)
	 thresholdLevels[i] <- 0
	 thresholdedCoeff <- threshold.imwd(imwdImage,levels=thresholdLevels, 
policy=c("manual"), type="hard", value=10000)
	
          # calculate the inverse wavelet transform
	 imwrImage <- imwr(thresholdedCoeff)

	 # assign the various decomposition level data a name
	 # starting at i+1 because i = 0.
          assign(varname[i+1],imwrImage) #  --> gives an error
	    	
	 # export the multiresolution decomposition for level i as graph
	 jpeg(paste("level",i,".jpg",sep=""),quality=100)
	 image(imwrImage,xlab=c("MRA image of level:
",i),col=gray(255:0/256))
	 dev.off()
    	 	 	
	 i <- i + 1
	  }

}

Best regards, Koen

Don't use assign(), named lists are much better (check the stuff on
indexing lists). Here's an example:
a <- list()
a[["one"]] <- c(1,2,3)
a[["two"]] <- c(4,5,6)
a[["two"]]
do.call("rbind",a)
do.call("cbind",a)
lapply(a,sum)

With regards to your question, did you try printing varname[i] in your
loop to see which value causes the error message?

> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Koen Hufkens
> Sent: Wednesday, November 23, 2005 7:37 AM
> To: R help mailinglist
> Subject: [R] assign() problem
>
>
> I've written a piece of code (see below) to do a wavelet image
> decomposition, during  the evaluation of this code I would
> like to write
> the results of some calculations back to the R root directory. I used
> assign() to do so because the names should vary when going thrue a
> while() loop. For some unknown reason I get an error that says:
>
> Error in assign(varname[i], imwrImage) :
>          invalid first argument
>
> what could be wrong, when I do it on the commandline everything works
> out just fine. But within the function it doesn't. When I disable the
> assign statement, everything runs fine so the rest of the
> code should be
> clean.
>
> My code:
>
> # Wavelet multiresolution decomposition
>
> wmra <- function(image_file){
> #require(rimage)
> require(wavethresh)
>
> #reading image file and converting it to grayscale
> #dimage <- read.jpeg(image_file)
> #dimage <- imagematrix(dimage)
>
> # discrete wavelet decomposition
> imwdImage <- imwd(image_file)
>
> # get a value for the number of decomposition levels
> #nlevels <- imwdImage$nlevels
>
> i <- 0
> varname <-
paste("level",0:(imwdImage$nlevels-1),sep="")
> print(varname)
> while ( i < imwdImage$nlevels)
> 	 {
> 	
> 	 # set the threshold to 0 on all levels except the one
> in evaluation
> 	 # thresholdLevels is the list of levels to set to zero
> 	 thresholdLevels <- 1:(imwdImage$nlevels-1)
> 	 thresholdLevels[i] <- 0
> 	 thresholdedCoeff <-
> threshold.imwd(imwdImage,levels=thresholdLevels,
> policy=c("manual"), type="hard", value=10000)
> 	
>           # calculate the inverse wavelet transform
> 	 imwrImage <- imwr(thresholdedCoeff)
>
> 	 # assign the various decomposition level data a name
> 	 # starting at i+1 because i = 0.
>           assign(varname[i+1],imwrImage) #  --> gives an error
> 	    	
> 	 # export the multiresolution decomposition for level i as graph
> 	
jpeg(paste("level",i,".jpg",sep=""),quality=100)
> 	 image(imwrImage,xlab=c("MRA image of level:
> ",i),col=gray(255:0/256))
> 	 dev.off()
>     	 	 	
> 	 i <- i + 1
> 	  }
>
> }
>
> Best regards, Koen
>
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