roundX {round}

Rounding Numbers to Decimal Digits – Variants

Description

Provide several versions of algorithms for round(x, digits), i.e., rounding to decimal digits. In particular, provides previous and current implementations of R's round().

Usage

roundX  (x, digits, version = roundVersions, trace = 0)

roundAll(x, digits, versions = roundVersions)

round_r3(x, d, info=FALSE, check=TRUE)

roundVersions # "sprintf" "r0.C" "r1.C" "r1a.C" "r2.C"  "r3.C" "r3d.C"  "r3"

Arguments

x

numeric vector

digits, d

integer number (for d) or numeric vector.

version

a character string specifying the version of rounding. Must match roundVersions (via match.arg).

trace

integer; if positive, the corresponding computations should be “traced” (possibly proportionally to the value of trace); currently only implemented for version = "r3.C".

versions

a character vector, a subset of roundVersions.

info

logical specifying if round_r3(*) should result in a list with components

"r":

the rounded x,

"D":

the difference (xu - x) - (x - xd), where xd and xu are the round down and up versions of x,

"e":

the modulo-2 remainder of floor(x * 10^d), which determines rounding to even (only) in case D = 0.

check

logical indicating if x and digits should be checked for validity. Is set to FALSE when used in roundX() (or roundAll), as the checks happen before round_r3() is called.

Details

Rounding to decimal digits is non-trivial in binary arithmetic. ISO standard “round to even”, see round()'s (help page), is more rare than typically assumed as most decimal fractions are not exactly representable in binary double precision numbers.

Decimal rounding is well defined when digits = 0, and calls the (C99 standard) C library function nearbyint() (which provide in this package as well, for completeness): round(x) is (R level) equivalent to round(x, digits = 0) and is also equivalent to (R and C level) nearbyint(x) which is defined to return the closest integer number (as double) and in the case of “doubt”, where both integer number neighbours are of the same distance, i.e., distance 0.5 the famous “round to even” strategy is used, such that, e.g., round(0:7 + 0.5) == c(0, 2, 2, 4, 4, 6, 6, 8).

The following strategy / algorithms are used for the different roundVersions; note that we only consider the crucial case digits > 0 in the following description:

"sprintf":

diverts the operation to sprintf("%.*f", digits, x) which in turn diverts to the corresponding C library function sprintf(); consequently may be platform dependent (though we have not yet seen differences from what we get by the most widely used GNU ‘glibc’ library, https://www.gnu.org/software/libc/). This version does not work with negative digits, returning NA with a warning there.

"r0.C":

a (too much) simplified version of R's "r1.C", just skipping the whole integer part computations; this was the first patch proposal in R-bugs' report PR#17668. However, this completely breaks down in extreme cases.

"r1.C":

the version of round() as in R 3.6.2 and earlier. It first removes the integer part(s) of x, then rounds and re-adds the integer part.

"r1a.C":

a slightly improved version of "r1.C", notably for |digits| > 308.

"r2.C":

the version of round() as added to ‘R-devel’ (the development version of R) with ‘svn’ revision ...... It does not remove and re-add the integer part(s) of x but ensures that no unnecessary overflow to +/-Inf or underflow to 0 happens when numbers are multiplied and divided by 10^d.

"r2a.C":

a slightly improved version of "r2.C", notably for large negative digits.

"r3":

(R level) implementation of “correct” rounding, rounding to the nearest double precision number (with “round to even” in case of equal distance) as seen in the function definition of round_r3(). Note that info=TRUE is only applied when when the digits d fulfill |d| <= 308.

"r3.C":

a C translation of "r3", using long double for intermediate computations which is particularly convenient for digits 308 < d < ~324 as overflow is not a possible then.

"r3d.C":

a version of "r3.C", only using double precision, and hence typically fast and less platform dependent, and also more often identical to "r3".

Value

roundX() returns a numeric vector (of length of recycled x and digits, i.e., typically (when digits is of length one) of length(x).

round_r3() is the workhorse of roundX(.., version = "r3"); it vectorizes in x but needs length(d) == 1.

roundVersions is a character vector of the versions available for roundX().

roundAll() applies roundX() for all versions, returning a matrix if one of x or digits is not of length one.

Author(s)

Martin Maechler (R Core for version "r1.C")

References

Wikipedia, Rounding, notably "Round half to even": https://en.wikipedia.org/wiki/Rounding#Round_half_to_even

See Also

round, also signif which is relatively sophisticated (also by code from M.M.).

Examples

roundVersions

round (55.55, 1)
roundX(55.55, 1, "r3")

## round() with all  roundVersions; quite simple (w/ recycling!)
roundAll # shows the function's definition

roundAll(55.55, 1)
roundAll(55.555, 2)
roundAll(55.5555, 3)
roundAll(55.55555, 4)
roundAll(55.555555, 5)
roundAll(55.5555555, 6)

## other "controversial" cases
rEx <- cbind( x =  c(10.7775, 12.345, 9.18665),
         digits =  c(   3  ,    2   ,    4   ))

resEx <- matrix(, length(roundVersions), nrow(rEx),
                dimnames = list(roundVersions, as.character(rEx[,"x"])))
for(i in 1:nrow(rEx))
  resEx[,i] <-  roundAll(rEx[[i,"x"]], digits = rEx[[i,"digits"]])

resEx  # r0.C & r2* agree and differ from the r1*;
       # "r3*" is close to "r2*" but not for 12.345
## The parts of "r3" :
r3rE <- sapply(1:nrow(rEx), function(i)
               round_r3(rEx[[i,"x"]], rEx[[i,"digits"]], info=TRUE))
colnames(r3rE) <- sapply(rEx[,"x"], format)
r3rE # rounding to even when D=0, but not when D < 0

## "Deterministic" Simulation - few digits only:
long <- interactive() # save time/memory e.g. when checking
I <- if(long) 0:9999 else 0:999
Ix <- I + 0.5
ndI <- 1L + as.integer(log10(pmax(1,I))) # number of (decimal) digits of I
nd2 <- outer(ndI, if(long) -3:4 else -2:3, `+`)
x   <- c(t( Ix / (10^nd2) ))
nd2 <- c(t( nd2 ))
 x  <-  x [nd2 > 0]
nd2 <- nd2[nd2 > 0]
rx <- roundAll(x, digits = nd2)

formatF <- function(.) format(., scientific=FALSE, drop0trailing=TRUE)
rownames(rx) <- formatF(x)
options(width = 123)
noquote(cbind(d = nd2, formatF(rx))[1:140,])
## -> The first cases already show a diverse picture; sprintf() a bit as outlier

## Error, assuming "r3" to be best, as it *does* really go to nearest:
Err <- rx - rx[, "r3"]
## careful : allowing small "noise" differences:
tErr <- abs(Err) > 1e-3* 10^-nd2 # "truly" differing from "r3"
colSums(tErr)  ## --> old R "r1*" is best here, then sprintf (among non-r3):
## For F30 Linux 64-bit (gcc),  and this selection of cases, r0+r2 are worst; r1 is best
## sprintf   r0.C   r1.C  r1a.C    r2.C  r2a.C   r3.C r3d.C  r3
##   15559  19778  14078  14078   19778  19778      8     0   0  {  long }
##    1167   1457   1290   1290    1457   1457      0     0   0  { !long }
if(long) { ## Q: where does  "r3.C"  differ from "r3" == "r3d.C" ?    A: in 10 cases; 8 "real"
  i3D <- which(Err[,"r3.C"] != 0)
  print(cbind(d = nd2[i3D], formatF(rx[i3D,])), quote=FALSE)
  print.table(zapsmall(Err[i3D,]), zero.print = ".")# differences (not very small ones!)
}

## Visualization of error happening (FIXME: do zapsmall()-like not count "noise")
cumErr <- apply(tErr[,colnames(rx) != "r3"], 2L, cumsum)
matPm <- function(y) {
   matplot(y=y, type = "l", lwd = 2, xlab = "i", ylab = deparse(substitute(y)))
   abline(h = 0, lty=2, col="gray")
   legend("topleft", legend = setdiff(roundVersions, "r3"),
          col = 1:6, lty = 1:5, lwd = 2, bty = "n")
}
matPm(head(cumErr,  100)) # sprintf seems worst
matPm(head(cumErr,  250)) # now r0+2 is worst, sprintf best
matPm(head(cumErr, 1000)) # now sprintf clearly worst again
matPm(head(cumErr, 2000)) # 0r/r2 best sprintf catching up
if(long) {
matPm(head(cumErr, 5000)) # now sprintf clearly worst again
matPm(head(cumErr,10000)) # now r0+2 is worst,  r1 best
}
matPm( cumErr )

same_cols <- function(m) all(m == m[,1])
stopifnot(same_cols(Err[, c("r0.C", "r2.C", "r2a.C")]))
stopifnot(same_cols(Err[, c("r1.C", "r1a.C")]))
if(FALSE) ## *not* in 'long' case, see above
stopifnot(same_cols(Err[, c("r3", "r3.C", "r3d.C")]))

sp <- search()
if(long && require("Matrix")) {
  showSp <- function(m) print(image(as(m, "sparseMatrix"), aspect = 4,
         ## fails, bug in lattice?  useRaster = !dev.interactive(TRUE) && (nrow(m) >= 2^12),
                                border.col = if(nrow(m) < 1e3) adjustcolor(1, 1/2) else NA))
  showSp(head(Err, 100))
  showSp(head(Err, 1000))
  showSp(Err)
  showSp(Err != 0) # B&W version ..
  if(!any(sp == "package:Matrix")) detach("package:Matrix")
}

## More digits random sample simulation tend go against "sprintf";
## see ../tests/  and also the vignette