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Re: Rational[a,b] vs Rational[1,2]

  • To: mathgroup at smc.vnet.net
  • Subject: [mg116709] Re: Rational[a,b] vs Rational[1,2]
  • From: Daniel Lichtblau <danl at wolfram.com>
  • Date: Thu, 24 Feb 2011 06:25:13 -0500 (EST)

ADL wrote:
> On 22 Feb, 12:25, Andrzej Kozlowski <a... at mimuw.edu.pl> wrote:
>> Yes, but are there any likely circumstances under which someone might type "Rational[a,b]" (rather than a/b - which is quite different) unless that person knew quite well what he was doing?
>>
>> Andrzej Kozlowski
>>
>> On 22 Feb 2011, at 10:45, Richard Fateman wrote:
>>
>>
>>
>>> On 2/21/2011 6:04 PM, DrMajorBob wrote:
>>>> The head of 1/2 is Rational, but the head of a/b is Times.
>>> I know that.
>>> What is the head of Rational[a,b]?    where  a, b  are symbols, not
>>> numbers...
>>> FullForm[Rational[a,b]]
>>> says
>>> Rational[a,b]
>>>  also
>>> Head  says it is Rational.
>>>  BUT IT IS NOT A Rational.
>>> For example, Numerator[Rational[a,b]]  is not a, but   Rational[a,b]
>>> I am aware of the info about atoms and Rational, and Complex too.
>>> However, the situation above looks to me like it is hazardous.  Do you
>>> see what I am getting at?
>>> Regards
>>> RJF
>>>> a/b//FullForm
>>>> Times[a,Power[b,-1]]
>>>> Replace looks at the arguments of Times, but it doesn't look within
>>>> "atoms". Rationals are "atoms", so there you go.
>>>> Help explains this under "Possible Issues", and under "More
>>>> Information", it says:
>>>> "You have to use Numerator and Denominator to extract parts of
>>>> Rational numbers."
>>>> Apparently that's the ONLY way to extract parts of a Rational, and
>>>> that's what we're supposed to glean from "have to".
>>>> First[1/2] returns 1/2 and an error message, for instance.
>>>> Bobby
>>>> On Mon, 21 Feb 2011 18:30:54 -0600, Richard Fateman
>>>> <fate... at eecs.berkeley.edu> wrote:
>>>>> I find it potentially hazardous that Head[]
>>>>> does not distinguish the cases of Head[1/2]
>>>>> and Head[Rational[a,b]]
>>>>> Thus these both get transformed...
>>>>> 1/2  /. Rational[r_,s_]-> foo[r,s]
>>>>> Rational[a,b] /. Rational[r_,s_] -> foo[r,s]
>>>>> but compare ...
>>>>> 1/2  /. 2->4
>>>>> Rational[a,2] /. 2->4
>>>>> I'm not saying I'm surprised by this, of course.
>>>>> Perhaps Rational[a,b] should be converted to a/b unless both
>>>>> a and b are numbers.
>>>>> Oh, for some fun, try Rational[1/2,4].
>>>>> RJF
>>>>> PS, I used version 7.
> 
> Letting aside the never-ending debate, I anyway think that:
> 
> Rational[1/2, 4] // N
> 0.5/4.
> 
> is a little too much and should be fixed.
> 
> ADL
> 

What exactly is it you want "fixed"? StandardForm of objects with Head 
of Rational?

Daniel Lichtblau
Wolfram Research




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