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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
>
>
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>
--
DrMajorBob at yahoo.com
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