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confused about == vs === in this equality

• To: mathgroup at smc.vnet.net
• Subject: [mg103732] confused about == vs === in this equality
• From: "Nasser Abbasi" <nma at 12000.org>
• Date: Sun, 4 Oct 2009 05:35:38 -0400 (EDT)
• References: <20091003104738.LCJ3I.416659.imail@eastrmwml34>
• Reply-to: "Nasser Abbasi" <nma at 12000.org>

?===
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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• Follow-Ups:
  • Re: confused about == vs === in this equality
    • From: Andrzej Kozlowski <akoz@mimuw.edu.pl>