[R] dealing with NA in readBin() and writeBin()

[Duncan Murdoch]

On 04/01/2015 5:13 PM, Mike Miller wrote:
> The help doc for readBin writeBin tells me this:
> 
> Handling R's missing and special (Inf, -Inf and NaN) values is discussed 
> in the ‘R Data Import/Export’ manual.
> 
> So I go here:
> 
> http://cran.r-project.org/doc/manuals/r-release/R-data.html#Special-values
> 
> Unfortunately, I don't really understand that.  Suppose I am using 
> single-byte integers and I want 255 (binary 11111111) to be translated to 
> NA.  Is it possible to do that?  Of course I could always do something 
> like this:
> 
> X[ X==255 ] <- NA
> 
> The problem with that is that I want to process the data on the fly, 
> dividing the integer to produce a double in the range from 0 to 2:
> 
> X <- readBin( file, what="integer", n=N, size=1, signed=FALSE)/127

Why?  Why not do it in three steps, i.e.

X <- readBin( file, what="integer", n=N, size=1, signed=FALSE)
X[ X==255 ] <- NA
X <- X/127

If you are worried about the extra typing, then write a function to
handle all three steps.

> 
> It looks like this still works:
> 
> X[ X==255/127 ] <- NA

I suspect that would work on all current platforms, but I wouldn't trust
it.  Don't use == on floating point values unless you know they are
fractions with 2^n in the denominator.

> It would be neater if there were some kind of translation option for the 
> input stream, like the way GNU tr (Linux/UNIX) works.  I'm looking around 
> and not finding such a thing.  I can use gsub() to translate on the fly 
> and then coerce back to integer format:

It's really trivial to write a wrapper for readBin to do what you want:

myReadBin <- function(...) {
  X <- readBin(...)
  X[ X==255 ] <- NA
  X
}

Duncan Murdoch

> 
> X <- as.integer(gsub("255", NA, readBin( file, what="integer", n=N, size=1, signed=FALSE)))/127
> 
> What is your opinion of that tactic?  Is there a better way?  I don't know
> if that has any advantage on the postprocessing tactic above.  Maybe what 
> I need is something like gsub() that can operate on numeric values...
> 
> X <- numsub(255, NA, readBin( file, what="integer", n=N, size=1, signed=FALSE))/127
> 
> ...but if that isn't better in terms of speed or memory usage than 
> postprocessing like this...
> 
> X[ X==255/127 ] <- NA
> 
> ...then I really don't need it (for this, but it would be good to know 
> about).
> 
> 
> The na.strings = "NA" functionality of scan() is neat, but I guess that 
> doesn't work with the binary read system.  I don't think I can scan the 
> readBin input because it isn't a file or stdin.
> 
> Mike
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