For people who want to play with these, here are some functions that let
you get or set the "payload" value in a NaN. NaN and NA, Inf and -Inf
are stored quite similarly; these functions don't distinguish which of
those you're working with. Regular finite values give NA for the
payload value, and elements of x are unchanged if you try to set their
payload to NA.
By the way, this also shows that R *can* distinguish different NaN
values, but you need some byte-level manipulations.
Duncan Murdoch
showBytes <- function(x) {
bytes <- rawConnection(raw(0), "w")
on.exit(close(bytes))
writeBin(x, bytes)
rawConnectionValue(bytes)
}
NaNpayload <- function(x) {
if (typeof(x) != "double") stop("Can only handle doubles")
bytes <- as.integer(showBytes(x))
base <- 1 + (seq_along(x)-1)*8
S <- bytes[base + 7] %/% 128
E <- (bytes[base + 7] %% 128)*16 + bytes[base + 6] %/% 16
F <- bytes[base + 6] %% 16
for (i in 5:0) {
F <- F*256 + bytes[base + i]
}
nan <- E == 2047 # Add " & F != 0 " if you don't want to include
infinities
ifelse(nan, (1-2*S)*F/2^52, NA)
}
"NaNpayload<-" <- function(x, value) {
x <- as.double(x)
payload <- value
new <- payload[!is.na(payload)]
if (any( new <= -1 | new >= 1 )) stop("The payload values must be
between -1 and 1")
payload <- rep(payload, len=max(length(x), length(payload)))
x <- rep(x, len=length(payload))
bytes <- as.integer(showBytes(x))
base <- 1 + (seq_along(x)-1)*8
base[is.na(payload)] <- NA
F <- trunc(abs(payload)*2^52)
for (i in 0:5) {
bytes[base + i] <- F %% 256
F <- F %/% 256
}
bytes[base + 6] <- F + 0xF0
bytes[base + 7] <- (payload < 0)*128 + 0x7F
con <- rawConnection(as.raw(bytes), "r")
on.exit(close(con))
readBin(con, "double", length(x))
}
Example:
> x <- c(NA, NaN, 0, 1, Inf)
> NaNpayload(x)
[1] 0.5 -0.5 NA NA 0.0
> NaNpayload(x) <- -0.4
> x
[1] NaN NaN NaN NaN NaN
> y <- x
> NaNpayload(y) <- 0.6
> y
[1] NaN NaN NaN NaN NaN
> NaNpayload(x)
[1] -0.4 -0.4 -0.4 -0.4 -0.4
> NaNpayload(y)
[1] 0.6 0.6 0.6 0.6 0.6
> identical(x, y)
[1] TRUE
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