On 3/10/19 2:36 PM, David Goldsmith wrote:
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!
It happens because R is eco-concious and re-cycles. :-)
Try:
ok <- c(TRUE,TRUE,FALSE)
(1:4)[ok]
In general in R if there is an operation involving two vectors then
the shorter one gets recycled to provide sufficiently many entries to
match those of the longer vector.
This in the foregoing example the first entry of "ok" gets used again,
to make a length 4 vector to match up with 1:4. The result is the same
as (1:4)[c(TRUE,TRUE,FALSE,TRUE)].
If you did (1:7)[ok] you'd get the same result as that from
(1:7)[c(TRUE,TRUE,FALSE,TRUE,TRUE,FALSE,TRUE)] i.e. "ok" gets
recycled 2 and 1/3 times.
Try 10*(1:3) + 1:4, 10*(1:3) + 1:5, 10*(1:3) + 1:6 .
Note that in the first two instances you get warnings, but in the third
you don't, since 6 is an integer multiple of 3.
Why aren't there warnings when logical indexing is used? I guess
because it would be annoying. Maybe.
Note that integer indices get recycled too, but the recycling is
limited
so as not to produce redundancies. So
(1:4)[1:3] just (sensibly) gives
[1] 1 2 3
and *not*
[1] 1 2 3 1
Perhaps a bit subtle, but it gives what you'd actually *want* rather
than being pedantic about rules with a result that you wouldn't want.
cheers,
Rolf Turner
P.S. If you do
y[1:3][!is.na(y[1:3])]
i.e. if you're careful to match the length of the vector and the that
of
the indices, you get what you initially expected.
R. T.
P^2.S. To the younger and wiser heads on this list: the help on "["
does not mention that the index vectors can be logical. I couldn't
find
anything about logical indexing in the R help files. Is something
missing here, or am I just not looking in the right place?
R. T.
