From: Rob Burrowes <rob@...>
Date: 2006-05-04T10:00:21+09:00
Subject: Re: Newbee Question on what if x === 2..5 does

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I mistyped that (left out an =), but the same result.

rob1:~ rbur004$ ruby -e 'a = [1,2,3,4,5,6,7,8,9,10]; a.each { |x|  
print x, "->"; if (2...5 === x) then print x, " " end; print "\n"}'
1->
2->
3->
4->
5->
6->
7->
8->
9->
10->

On 4/05/2006, at 12:42 PM, Rob Burrowes wrote:

> I get what you mean. It makes sense to have the === as a method of  
> the range, not of x, but this produces no result at all.
>
> rob1:~ rbur004$ ruby -e 'a = [1,2,3,4,5,6,7,8,9,10]; a.each { |x|  
> print x, "->"; if (2...5 == x) then print x, " " end; print "\n"}'
> 1->
> 2->
> 3->
> 4->
> 5->
> 6->
> 7->
> 8->
> 9->
> 10->
>
>
> On 4/05/2006, at 11:06 AM, Jacob Fugal wrote:
>
>> On 5/3/06, Rob Burrowes <rob@cs.auckland.ac.nz> wrote:
>>> I'm new to ruby and don't understand why the following code does  
>>> what
>>> is does. Can anyone enlighten me?
>>>
>>> ruby -e 'a = [1,2,3,4,5,6,7,8,9,10]; a.each { |x| if (x === 2..5 )
>>> then print x, " " end}; puts '
>>> 2 3 4 5 6 7 8 9 10
>>>
>>> I would have thought this would be the same as
>>>
>>> ruby -e 'a = [1,2,3,4,5,6,7,8,9,10]; a.each { |x| case x; when 2..5
>>> then print x, " "; end}; puts '
>>> 2 3 4 5
>>
>> Try switching x === 2..5 to 2..5 === x. The case equality operator
>> (===) is not cummutative. The conditions of a case statement are
>> evaluated using the when condition as the *receiver* of the case
>> equality operator, not the argument. Ie.
>>
>>  case a
>>  when b
>>    ...
>>  end
>>
>> is equivalent to:
>>
>>  if b === a
>>    ...
>>  end
>>
>>> I am also confused that
>>>
>>> ruby -e 'while gets; print  if /<div id=.*$/ .. /<\/body>/; end'  
>>> < fred
>>> <div id="eutpuhe">
>>> UETHUEPUK
>>> THEUTEUH
>>> teuhepxn
>>> thueonhoe
>>> ononueuouh
>>> </div>
>>> </body>
>>>
>>> and a three . range test give the same output.
>>>
>>> ruby -e 'while gets; print  if /<div id=.*$/ ... /<\/body>/; end'  
>>> < fred
>>> <div id="eutpuhe">
>>> UETHUEPUK
>>> THEUTEUH
>>> teuhepxn
>>> thueonhoe
>>> ononueuouh
>>> </div>
>>> </body>
>>
>> Sorry, can't help you there. I always have a hard time remembering
>> what the difference between .. and ... are in that context. I stay
>> away from it in favor of more descriptive flagging in a while loop.
>> E.g.:
>>
>>  flag = false
>>  while line = gets
>>    flag = true if line =~ /<div id=.*$/
>>    flag = false if line =~ /<\/body>/
>>    print line if flag
>>  end
>>
>> Jacob Fugal
>>
>
> Rob
>
> Rob Burrowes                     Ph:   +64 (9) 3737 599 Extn 87972
> Computing Services Manager        Fax: +64 (9) 3737 453
> Computer Science Department    Mob:+64 (27) 4731 856
> The University of Auckland
> Rm 391, Level 3
> 38 Princes Street                http://www.cs.auckland.ac.nz/~rob
> Private bag 92019                http://www.wikarekare.org
> Auckland 1001                    http://www.burrowes.org
>
>

Rob

Rob Burrowes                     Ph:   +64 (9) 3737 599 Extn 87972
Computing Services Manager        Fax: +64 (9) 3737 453
Computer Science Department    Mob:+64 (27) 4731 856
The University of Auckland
Rm 391, Level 3
38 Princes Street                http://www.cs.auckland.ac.nz/~rob
Private bag 92019                http://www.wikarekare.org
Auckland 1001                    http://www.burrowes.org



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