Bug report: Function ppois(0:20, lambda=0.9) does not generate a non-decreasing result.

function ppois is a function calculate the CDF of Poisson distribution, it should generate a non-decreasing result, but what I got is:

any(diff(ppois(0:19,lambda=0.9))<0)

[1] TRUE

Actually,

ppois(19,lambda=0.9)<ppois(18,lambda=0.9)

[1] TRUE

Which could not be TRUE.

This is just another manifestation of

0.1 * 3 > 0.3
#> [1] TRUE

This discussion returns to this list from time to time. TLDR; this is
not an R issue, but an unavoidable floating point issue. Solution:
work with log-probabilities instead.

any(diff(ppois(0:40, lambda=0.9, log.p=TRUE))<0)
#> [1] FALSE

I?aki