Re: Sequence and Or
- To: mathgroup at smc.vnet.net
- Subject: [mg34214] Re: [mg34179] Sequence and Or
- From: Andrzej Kozlowski <andrzej at platon.c.u-tokyo.ac.jp>
- Date: Thu, 9 May 2002 05:17:08 -0400 (EDT)
- Sender: owner-wri-mathgroup at wolfram.com
Yes, in this case all the arguments will be evaluated. You can see the
difference in performance quite clearly:
In[1]:=
p1 := 1 == 1
In[2]:=
p2 := FullSimplify[Gamma[z]*z == z!]
In[3]:=
Timing[Or[p1,p2]]
Out[3]=
{0. Second,True}
In[4]:=
Timing[Or@@{p1,p2}]
Out[4]=
{0.84 Second,True}
Andrzej Kozlowski
Toyama International University
JAPAN
http://platon.c.u-tokyo.ac.jp/andrzej/
On Thursday, May 9, 2002, at 02:02 PM, DrBob wrote:
> Good explanation! We don't always EXPECT reasons that make sense.
>
> Then, does
>
> Or@@{expr1,expr2,...,exprn}
>
> sacrifice that efficiency?
>
> Bobby
>
> -----Original Message-----
> From: Andrzej Kozlowski [mailto:andrzej at platon.c.u-tokyo.ac.jp]
To: mathgroup at smc.vnet.net
> Sent: Wednesday, May 08, 2002 11:56 PM
> To: drbob at bigfoot.com
> Cc: mathgroup at smc.vnet.net
> Subject: [mg34214] Re: [mg34179] Sequence and Or
>
> It seems to me this is just a matter of efficiency. Suppose you want to
> evaluate Or[p1,p2,p3,...,pn] for some statements p1, p2 ...pn. It is
> enough that just one pi is True for the answer to be true. So
> Mathematica evaluates each statement in turn and stops whenever it finds
>
> one that turns out to be true. This can produce a big gain in
> efficiency, particularly if you sensibly arranged the statements so that
>
> those more likely to be true come earlier. Of course for this sort of
> thing to be of any use you need to have the HoldAll attribute otherwise
> Mathematica would first evaluate all the statements even if only p1 was
> true and easy to check and the rest hard.
>
>
>
>
> On Wednesday, May 8, 2002, at 11:54 PM, DrBob wrote:
>
>> Thanks for the workaround! It makes me wonder, though, why Or should
>> have the HoldAll attribute at all.
>>
>> Bobby
>>
>> -----Original Message-----
>> From: Andrzej Kozlowski [mailto:andrzej at platon.c.u-tokyo.ac.jp]
To: mathgroup at smc.vnet.net
>> Sent: Wednesday, May 08, 2002 5:27 AM
>> To: drbob at bigfoot.com
>> Cc: mathgroup at smc.vnet.net
>> Subject: [mg34214] Re: [mg34179] Sequence and Or
>>
>> Compare:
>>
>> In[19]:=
>> Or[Sequence@@{True,False,True}]
>>
>> Out[19]=
>> Sequence[True,False,True]
>>
>> In[20]:=
>> Or[Evaluate[Sequence@@{True,False,True}]]
>>
>> Out[20]=
>> True
>>
>> In[21]:=
>> Or[Sequence[True,False,True]]
>>
>> Out[21]=
>> True
>>
>> The point is the HoldAll attribute of Or. Because of it, when you
> enter
>>
>> Or[Sequence@@{True,False,True}] only a single argument is passed to
> Or
>> but Or with a single argument returns:
>>
>> In[22]:=
>> Or[x]
>>
>> Out[22]=
>> x
>>
>>
>>
>>
>> On Wednesday, May 8, 2002, at 02:57 PM, DrBob wrote:
>>
>>> When I evaluate the following...
>>>
>>> {True,False,True}
>>> Or@@%
>>> Sequence@@%
>>> Or[%]
>>>
>>> the outputs are:
>>>
>>> {True,False,True}
>>> True
>>> Sequence[True,False,True]
>>> Sequence[True,False,True]
>>>
>>> while I had expected:
>>>
>>> {True,False,True}
>>> True
>>> Sequence[True,False,True]
>>> True
>>>
>>> What am I missing?
>>>
>>> Bobby Treat
>>>
>>>
>>>
>>>
>>>
>>
>>
>>
>
>
>