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

• To: mathgroup at smc.vnet.net
• Subject: [mg103775] Re: [mg103747] Re: confused about == vs === in this equality
• From: Leonid Shifrin <lshifr at gmail.com>
• Date: Mon, 5 Oct 2009 13:17:07 -0400 (EDT)
• References: <20091003104738.LCJ3I.416659.imail@eastrmwml34>

```Apparently, at the bottom of that page I made an incorrect statement: Nasser
just found an example where SameQ gives True while Equal evaluates to
itself. I should post it in the <errata> section and then eventually correct
it.

Regards,
Leonid

On Mon, Oct 5, 2009 at 3:35 PM, Chris Degnen <degnen at cwgsy.net> wrote:

> I refer you to Leonid's Mathematica introduction:
>
> http://www.mathprogramming-intro.org/book/node109.html
>
>
> "Nasser Abbasi" <nma at 12000.org> asked:
> >
> > ?===
> > 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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