Understanding when the && operator short circuits

Question

Perhaps I'm missing something obvious.

In R, TRUE && NA evalues to NA. This doesn't make sense to me, because && should evaluate left to right, and stop as soon as one of its conditions is true.

It's touched on in [this answer](http://stackoverflow.com/a/1535492/288545) but I still don't understand *why* — Daniel Kessler, May 30 '13 at 19:32
see http://stackoverflow.com/questions/16844139/na-true-and-false-properties and especially @joran answer about ambiguity of `NA`. — Beasterfield, May 30 '13 at 19:33
AND requires both conditions to be true, so will only short-circuit on `FALSE` — James, May 30 '13 at 19:39
http://spikedmath.com/445.html , http://mrburkemath.blogspot.ca/2011/05/coffee-logic.html — Ben Bolker, May 30 '13 at 20:17

csgillespie · Accepted Answer · 2013-05-30T19:49:29.407

20

This doesn't make sense to me, because && should evaluate left to right, and stop as soon as one of its conditions is true.

This is wrong. You are mixing up && with ||:

TRUE && FALSE gives FALSE
- && requires both conditions to be TRUE
- && will short-circuit on FALSE
TRUE || FALSE gives TRUE
- || requires a single condition to be TRUE
- || will short-circuit on TRUE

Also,

TRUE || NA

gives

TRUE

edited May 30 '13 at 19:49

answered May 30 '13 at 19:38

csgillespie

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Understanding when the && operator short circuits

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