Q re: logical indexing with is.na
logical indexing requires the logical index to be of the same length as the vector being indexed. If it is not, then the index is wrapped to be of sufficient length. The result on line 3 is y[c(TRUE, TRUE, FALSE, TRUE)] where the last TRUE was originally the first component of !is.na(y[1:3]) Grant Izmirlian, Ph.D. Mathematical Statistician izmirlig at mail.nih.gov Delivery Address: 9609 Medical Center Dr, RM 5E130 Rockville MD 20850 Postal Address: BG 9609 RM 5E130 MSC 9789 9609 Medical Center Dr Bethesda, MD 20892-9789 ofc: 240-276-7025 cell: 240-888-7367 fax: 240-276-7845
From: David Goldsmith <eulergaussriemann at gmail.com>
Sent: Saturday, March 9, 2019 8:36 PM
To: r-help at r-project.org
Subject: [R] Q re: logical indexing with is.na
Sent: Saturday, March 9, 2019 8:36 PM
To: r-help at r-project.org
Subject: [R] Q re: logical indexing with is.na
Hi! Newbie (self-)learning R using P. Dalgaard's "Intro Stats w/ R"; not
new to statistics (have had grad-level courses and work experience in
statistics) or vectorized programming syntax (have extensive experience
with MatLab, Python/NumPy, and IDL, and even a smidgen--a long time ago--of
experience w/ S-plus).
In exploring the use of is.na in the context of logical indexing, I've come
across the following puzzling-to-me result:
> y; !is.na(y[1:3]); y[!is.na(y[1:3])]
[1] 0.3534253 -1.6731597 NA -0.2079209
[1] TRUE TRUE FALSE
[1] 0.3534253 -1.6731597 -0.2079209
As you can see, y is a four element vector, the third element of which is
NA; the next line gives what I would expect--T T F--because the first two
elements are not NA but the third element is. The third line is what
confuses me: why is the result not the two element vector consisting of
simply the first two elements of the vector (or, if vectorized indexing in
R is implemented to return a vector the same length as the logical index
vector, which appears to be the case, at least the first two elements and
then either NA or NaN in the third slot, where the logical indexing vector
is FALSE): why does the implementation "go looking" for an element whose
index in the "original" vector, 4, is larger than BOTH the largest index
specified in the inner-most subsetting index AND the size of the resulting
indexing vector? (Note: at first I didn't even understand why the result
wasn't simply
0.3534253 -1.6731597 NA
but then I realized that the third logical index being FALSE, there was no
reason for *any* element to be there; but if there is, due to some
overriding rule regarding the length of the result relative to the length
of the indexer, shouldn't it revert back to *something* that indicates the
"FALSE"ness of that indexing element?)
Thanks!
DLG
> sessionInfo()
R version 3.5.2 (2018-12-20)
Platform: x86_64-apple-darwin15.6.0 (64-bit)
Running under: macOS High Sierra 10.13.6
Matrix products: default
BLAS:
/Library/Frameworks/R.framework/Versions/3.5/Resources/lib/libRblas.0.dylib
LAPACK:
/Library/Frameworks/R.framework/Versions/3.5/Resources/lib/libRlapack.dylib
locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
attached base packages:
[1] stats graphics grDevices utils datasets methods base
other attached packages:
[1] ISwR_2.0-7
loaded via a namespace (and not attached):
[1] compiler_3.5.2 tools_3.5.2
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