Re^2: interpretation of "neither...nor"

See DeMorgan's laws in formal logic. The solution is correct because (as truth tables will reveal),

! (defined $ENV{EDITOR} || defined $ENV{VISUAL})
[download]

is equivalent to,

(! defined $ENV{EDITOR} && ! defined $ENV{VISUAL})
[download]

After Compline,
Zaxo

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Re^3: interpretation of "neither...nor" by cgmd (Beadle) on Jun 17, 2007 at 15:50 UTC
Thank you, Zaxo, for that reference. Following up, I have found: Augustus De Morgan originally observed that in classical propositional logic the following relationships hold: not (P and Q) = (not P) or (not Q) not (P or Q) = (not P) and (not Q) This explains why the following are equivalent instructions: `print "no editor found\n" if ! (defined $ENV{EDITOR} \|\| defined $ENV{VISUAL}); print "no editor found\n" if (!defined $ENV{EDITOR} && !defined $ENV{VISUAL});` [download] Thank you for setting me straight! This has been very helpful!	[reply] [d/l]

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Re^3: interpretation of "neither...nor"
by cgmd (Beadle) on Jun 17, 2007 at 15:50 UTC

Following up, I have found:

Augustus De Morgan originally observed that in classical propositional logic the following relationships hold:

not (P and Q) = (not P) or (not Q)

not (P or Q) = (not P) and (not Q)

This explains why the following are equivalent instructions:

print "no editor found\n"
   if ! (defined $ENV{EDITOR} || defined $ENV{VISUAL});

print "no editor found\n"
   if (!defined $ENV{EDITOR} && !defined $ENV{VISUAL});
[download]

Thank you for setting me straight! This has been very helpful!

[reply]
[d/l]