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A newbie question about case and ===

Sonny Chee

8/29/2006 12:54:00 AM

Hey Guys,

Let me apologize upfront if this is not the appropriate place to ask
newbie questions. I can repost, if someone can point me to the correct
location.

Assuming no is offended, my question is:

I read that the case statement uses the === method for comparison of
each of its clauses. So, why doesn't the second comparison in the
following code snippet evaluate identically?

a_fix_num = 1
puts a_fix_num.class
case a_fix_num
when Integer
puts 'Yes, this is an Integer subclass'
else
puts 'No, this is not an Integer subclass'
end

if a_fix_num === Integer
puts 'Yes, this is an Integer subclass'
else
puts 'No, this is not an Integer subclass'
end

# Fixnum
# Yes, this is an Integer subclass.
# No, this is not an Integer subclass.

Sonny.

--
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4 Answers

e

8/29/2006 1:03:00 AM

0

Sonny Chee wrote:
> Hey Guys,
>
> Let me apologize upfront if this is not the appropriate place to ask
> newbie questions. I can repost, if someone can point me to the correct
> location.
>
> Assuming no is offended, my question is:
>
> I read that the case statement uses the === method for comparison of
> each of its clauses. So, why doesn't the second comparison in the
> following code snippet evaluate identically?
>
> a_fix_num = 1
> puts a_fix_num.class
> case a_fix_num
> when Integer
> puts 'Yes, this is an Integer subclass'
> else
> puts 'No, this is not an Integer subclass'
> end
>
> if a_fix_num === Integer
> puts 'Yes, this is an Integer subclass'
> else
> puts 'No, this is not an Integer subclass'
> end

case actually evaluates the other way.

Integer === a_fix_num # true

> # Fixnum
> # Yes, this is an Integer subclass.
> # No, this is not an Integer subclass.

Open up a terminal and type in ri Class#===
(and play around with ri otherwise too:)

> Sonny.


--
Posted via http://www.ruby-....

Just Another Victim of the Ambient Morality

8/29/2006 2:18:00 AM

0


"Eero Saynatkari" <eero.saynatkari@kolumbus.fi> wrote in message
news:f6a9600f8e87240669e788b674456558@ruby-forum.com...
> Sonny Chee wrote:
>>
>> I read that the case statement uses the === method for comparison of
>> each of its clauses. So, why doesn't the second comparison in the
>> following code snippet evaluate identically?
>>
>> a_fix_num = 1
>> puts a_fix_num.class
>> case a_fix_num
>> when Integer
>> puts 'Yes, this is an Integer subclass'
>> else
>> puts 'No, this is not an Integer subclass'
>> end
>>
>> if a_fix_num === Integer
>> puts 'Yes, this is an Integer subclass'
>> else
>> puts 'No, this is not an Integer subclass'
>> end
>
> case actually evaluates the other way.
>
> Integer === a_fix_num # true

That is hilarious!
It's obvious when you think about it but, otherwise, it's very
surprising...

Gary Wright

8/29/2006 3:23:00 AM

0


On Aug 28, 2006, at 9:02 PM, Eero Saynatkari wrote:
> case actually evaluates the other way.
>
> Integer === a_fix_num # true

I think the problem is that the visual representation (===)
of the operator i symmetric and this tends to imply that the
operator follows associative behavior ((a op b) the same as
(b op a)) but that isn't true for === in general.

If the operator token was asymmetric I don't think it would
be so confusing. Actually, I wonder why the match operator (=~)
couldn't be used instead. I've looked into this a bit, but I
I suspect that there are very few cases where === and =~
could not be aliases for each other.

Gary Wright

Sonny Chee

8/29/2006 4:29:00 AM

0

Eero Saynatkari wrote:
> case actually evaluates the other way.
>
> Integer === a_fix_num # true
>

Thanks Eero!

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