Dear R-devel list,
the following code shows NA's appearing in the result of pmatch() when
comparing a vector to itself when the length of the vector is more than 100.
The main specificity of this example is that elements are repeated in
the vector which is matched to itself.
The results when they do not include any NA (i.e. for argument of length
<=100) are exactly as I expect from the documentation.
I see that the source C code uses distinct algorithms whether n_input <=
100 || n_target <= 100 or not. Could there by a problem in the source
code for larger values ?
The NA's correspond to the first positions of the second replicate of
the integer sequence, e.g. positions 52 to 56 if n=51 in the example below.
But as shown below, the NA's do not appear when the sequence is reversed
before being compared to itself.
This was first detected with R 4.5.3 and is reproducible with a
just-downloaded, virgin R-devel installation.
Thanks in advance for any feedback,
F.
======================
countNAs <- function(n, rev.=FALSE) {
seqn <- seq(n)
if (rev.) seqn <- rev(seqn)
seqn <- rep(seqn,2) # of length 2 n: NA's
appear when 2 n > 100
chk <- pmatch(seqn, seqn) # I expect the result to be
seq(2*n)
sum(is.na(chk))
}
sapply(1:100, countNAs )
sapply(1:100, countNAs , rev.=TRUE)
=======================
Results:
> sapply(1:100, countNAs )
[1] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0
[29] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
5 5 5 5 5 5
[57] 5 5 5 6 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7
7 8 8 8 8 8
[85] 8 8 8 8 8 9 9 9 9 9 9 9 9 9 9 10
> sapply(1:100, countNAs , rev.=TRUE)
[1] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
[43] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
[85] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
> sessionInfo()
R Under development (unstable) (2026-06-14 r90150 ucrt)
Platform: x86_64-w64-mingw32/x64
Running under: Windows 10 x64 (build 19045)
Matrix products: default
LAPACK version 3.12.1
locale:
[1] LC_COLLATE=French_France.utf8 LC_CTYPE=French_France.utf8
[3] LC_MONETARY=French_France.utf8 LC_NUMERIC=C
[5] LC_TIME=French_France.utf8
time zone: Europe/Paris
tzcode source: internal
attached base packages:
[1] stats graphics grDevices utils datasets methods base
loaded via a namespace (and not attached):
[1] compiler_4.7.0 tools_4.7.0
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