On 19/05/17 23:38, S Ellison wrote:
TRUE & FALSE is FALSE but TRUE & TRUE is TRUE, so TRUE & NA could be
either TRUE or FALSE and consequently is NA.
OTOH FALSE & (anything) is FALSE so FALSE & NA is FALSE.
As I said *think* about it; don't just go with your immediate knee-jerk
(simplistic) reaction.
Hmm... not sure that was quite fair to the OP.
The OP complained that the logical operators in R are inconsistent.
This is an arrogant and presumptuous assertion that deserves a
reprimand. One should be very, very circumspect about presuming to know
better than R.
Yes, FALSE &
<anything>== FALSE. But 'NA' does not mean 'anything'; it means 'missing' (see
?'NA').
Well, duh. Yes, I know what NA means. If it's missing, you don't know
what it's value is. But it doesn't *matter* what its value is; FALSE &
<anything> is FALSE. So FALSE & NA is FALSE, irrespective of what NA
"really" is.
It is much less obvious that FALSE & <missing> should generate a
non-missing value. SQL, for example, generally takes the view that any
expression involving 'missing' is 'missing'.
Well, then SQL gets it wrong.
And R's behaviour can look odd if the vagaries of real data intervene:
b1 <- c(A=TRUE, C=FALSE)
b2 <- c(A=FALSE, B=FALSE, C=TRUE)
b1['B'] & b2['B']
#Which returns
# <NA>
# FALSE
which - particularly since it appears without warning ....
Everything appears without warning. Nobody expected the Spanish
Inquisition.
.... - is not an obviously sensible outcome.
Why not? It's obviously sensible to me, and to anyone else who is
thinking. Since b1['B'] is NA (b1 doesn't have an entry named 'B') and
b2['B'] is FALSE we get NA & FALSE which is FALSE. Where's the problem?
The "<NA>" in the output that you show is the *name* of the 'B' entry of
b1, and since there isn't one, it doesn't have a name. So the name is
missing whence it is rendered as "<NA>" (missing character).
I am not suggesting a change to R's logical operations, which have
clearly been thought through (that is evident from NA&FALSE ==
FALSE&NA == FALSE). But R's behaviour looks to me like a choice among
difficult alternatives, rather than the only possible choice. I'd
give the OP some credit for that.
You seem to be arguing for the sake of arguing. It's really quite
straightforward *if* you *think* about it.
cheers,
Rolf
--
Technical Editor ANZJS
Department of Statistics
University of Auckland
Phone: +64-9-373-7599 ext. 88276
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