Re: confused about == vs === in this equality

*To*: mathgroup at smc.vnet.net*Subject*: [mg103756] Re: [mg103732] confused about == vs === in this equality*From*: DrMajorBob <btreat1 at austin.rr.com>*Date*: Mon, 5 Oct 2009 07:37:30 -0400 (EDT)*References*: <20091003104738.LCJ3I.416659.imail@eastrmwml34>*Reply-to*: drmajorbob at yahoo.com

Examples: a = ComplexInfinity; a == ComplexInfinity a === ComplexInfinity ComplexInfinity == ComplexInfinity True a is identical to ComplexInfinity, but it isn't Equal to it, because ComplexInfinity is an indeterminate form, not Equal to anything... not even itself. Clear[a, b] a == b a === b a == b False Mathematic doesn't know the values of a and b, so it doesn't know whether a is equal to b, hence a == b remains unevaluated. But Mathematica DOES know a and b are not identical. a == a a === a True True No matter what value "a" might have (OTHER THAN AN INDETERMINATE FORM), a == a is True. Mathematic is assuming "a" ISN'T indeterminate, even though it could be. Meanwhile, a === a because any expression is identical to itself. Bobby On Sun, 04 Oct 2009 04:35:38 -0500, Nasser Abbasi <nma at 12000.org> wrote: > ?=== > lhs===rhs yields True if the expression lhs is identical to rhs, and > yields > False otherwise. > > ?== > lhs==rhs returns True if lhs and rhs are identical. > > But looking at this example: > > a = ComplexInfinity; > If[a == ComplexInfinity, Print["YES"]] > > Expecting it would print "YES", but it does not. it just returns the > whole > thing unevaluated? But > > If[a === ComplexInfinity, Print["YES"]] > > does return YES. > > I guess I am a little confused about the "expression" bit in the > definition. > > So, when using the 3"=", it is looking at the _value_ of the expression, > but > when using the 2"=", it is looking at the expression _as it is_, i.e. > without evaluating it? Is this the difference? I've always used the > 2"=" > for equality, now I have to be more careful which to use. > > --Nasser > > > __________ Information from ESET NOD32 Antivirus, version of virus > signature database 4478 (20091003) __________ > > The message was checked by ESET NOD32 Antivirus. > > http://www.eset.com > > > > -- DrMajorBob at yahoo.com