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

• To: mathgroup at smc.vnet.net
• Subject: [mg103760] Re: confused about == vs === in this equality
• From: David Bailey <dave at removedbailey.co.uk>
• Date: Mon, 5 Oct 2009 07:38:15 -0400 (EDT)
• References: <20091003104738.LCJ3I.416659.imail@eastrmwml34> <ha9qqp$7vi$1@smc.vnet.net>

Nasser Abbasi wrote:

> 
> 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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No - both comparisons evaluate both sides before comparing. The real 
issue is that i==3 will return unevaluated if i does not have a value, 
whereas i===3 will return False unless i has the value 3 . === compares 
things as they are (after evaluation), it never returns unevaluated.

The comparison operator == is a little picky as to what it compares. I 
am not certain, but possibly it does not 'like' comparing one infinity 
with another for mathematical reasons.

David Bailey
http://www.dbaileyconsultancy.co.uk