jitter-bug? problematic behaviour of the jitter function

Hello,

I believe that though Duncan's explanation is right it is also not
explaining the value of the digits argument. round makes the first 2
numbers 0 but why?

then outputs d. The code is taken from jitter.


f <- function(x){
    z <- diff(r <- range(x[is.finite(x)]))
    cat("digits:", 3 - floor(log10(z)), "\n")
    diff(xx <- unique(sort.int(round(x, 3 - floor(log10(z))))))
}


Now see what cat outputs for 'digits'.


f(c(1,2,10^4))  # desired behaviour
#digits: 0
#[1]    1 9998
f(c(0,1,10^4))  # bad behaviour
#digits: -1
#[1] 10000
f(c(-1,0,10^4))  # bad behaviour
#digits: -1
#[1] 10000
f(c(1,2,10^5))  # bad behaviour
#digits: -1
#[1] 1e+05



And according to the documentation of ?round, negative digits are allowed:


Rounding to a negative number of digits means rounding to a power of
ten, so for example round(x, digits = -2) rounds to the nearest hundred.


But in this case two of the numbers are closer to 0 than they are of 10.
And unique keeps only 0 and the largest, then diff is big.


round(c(1,2,10^4),0)  # desired behaviour
#[1]     1     2 10000
round(c(0,1,10^4),-1)  # bad behaviour
#[1]     0     0 10000
round(c(-1,0,10^4),-1)  # bad behaviour
#[1]     0     0 10000
round(c(1,2,10^5),-1)  # bad behaviour
#[1] 0e+00 0e+00 1e+05



Isn't it still a bug?

Rui Barradas


?s 15:57 de 23/09/20, Duncan Murdoch escreveu:

On 23/09/2020 6:32 a.m., Martin Keller-Ressel wrote:

Dear all,

i have noticed some strange behaviour in the ?jitter? function in R.
On the help page for jitter it is stated that

"The result, say r, is r <- x + runif(n, -a, a) where n <- length(x)
and a is the amount argument (if specified).?

and

"If amount is NULL (default), we set a <- factor * d/5 where d is the
smallest difference between adjacent unique (apart from fuzz) x values.?

This works fine as long as there is no (very) large outlier

jitter(c(1,2,10^4))? # desired behaviour

[1]??? 1.083243??? 1.851571 9999.942716

But for very large outliers the added noise suddenly ?jumps? to a much
larger scale:

jitter(c(1,2,10^5)) # bad behaviour

[1] -19535.649?? 9578.702 115693.854
# Noise should be of order (2-1)/5? = 0.2 but is of much larger order.

This probably does not matter much when jitter is used for plotting,
but it can cause problems when jitter is used to break ties.

I think this is kind of documented:? "apart from fuzz" is what counts.
If you look at the code for jitter, you'll see this important line:

  ?d <- diff(xx <- unique(sort.int(round(x, 3 - floor(log10(z))))))

By the time you get here, z is the length of the rante of the data, so
it's 99999 in your example.? The rounding changes your values to
0,0,1e5, so the smallest difference is 1e5.

Duncan Murdoch