Can I get the byte representation of an R float?

Question

I'm trying to read in a complicated data file that has floating point values. Some C code has been supplied that handles this format (Met Office PP file) and it does a lot of bit twiddling and swapping. And it doesn't work. It gets a lot right, like the size of the data, but the numerical values in the returned matrix are nonsensical, have NaNs and values like 1e38 and -1e38 liberally sprinkled.

However, I have a binary exe ("convsh") that can convert these to netCDF, and the netCDFs look fine - nice swirly maps of wind speed.

What I'm thinking is that the bytes of the PP file are being read in in the wrong order. If I could compare the bytes of the floats returned correctly in the netCDF data with the bytes in the floats returned wrongly from the C code, then I might figure out the correct swappage.

So is there a plain R function to dump the four (or eight?) bytes of a floating point number? Something like:

> as.bytes(pi)
[1] 23 54 163 73 99 00 12 45 # made up values

searches for "bytes" and "float" and "binary" haven't helped.

Its trivial in C, I could probably have written it in the time it took me to write this...

Answer 1

rdyncall might give you what you're looking for:

library(rdyncall)
as.floatraw(pi)
# [1] db 0f 49 40
# attr(,"class")
# [1] "floatraw"

Or maybe writeBin(pi, raw(8))?

Answer 2

Yes, that must exist in the serialization code because R merrily sends stuff across the wire, taking care of endianness too. Did you look at eg Rserve using it, or how digest passes the char representation to chosen hash functions?

After a quick glance at digest.R:

R> serialize(pi, connection=NULL, ascii=TRUE)
 [1] 41 0a 32 0a 31 33 34 39 31 34 0a 31 33 31 38 34 30 0a
[19] 31 34 0a 31 0a 33 2e 31 34 31 35 39 32 36 35 33 35 38
[37] 39 37 39 33 0a

and

R> serialize(pi, connection=NULL, ascii=FALSE)
 [1] 58 0a 00 00 00 02 00 02 0f 02 00 02 03 00 00 00 00 0e
[19] 00 00 00 01 40 09 21 fb 54 44 2d 18
R> 

That might get you going.

Come to think about it, this includes header meta-data.