Re: RE: Re: ValueQ
- To: mathgroup at smc.vnet.net
- Subject: [mg34411] Re: [mg34376] RE: [mg34340] Re: ValueQ
- From: Andrzej Kozlowski <andrzej at platon.c.u-tokyo.ac.jp>
- Date: Sat, 18 May 2002 03:51:34 -0400 (EDT)
- Sender: owner-wri-mathgroup at wolfram.com
The purpose of ValueQ is to check if a symbol is a "value", which means
not just a DownVlaue, but also OwnVlaue, SubValue and so on, e.g.:
In[1]:=
a/:f[a]=2;
In[2]:=
f[b]=3;
In[3]:=
c=5;
In[4]:=
ReleaseHold[Map[ValueQ,Hold[{f[a],f[b],f[c],c,d,f[d]}],{2}]]
Out[4]=
{True,True,True,True,False,False}
Andrzej Kozlowski
Toyama International University
JAPAN
http://platon.c.u-tokyo.ac.jp/andrzej/
On Friday, May 17, 2002, at 07:31 PM, DrBob wrote:
> Here's a more direct method:
>
> Position[DownValues[a], HoldPattern[a[#]]] != {} & /@ {1, 2}
>
> {True,False}
>
> This is what I'd have expected ValueQ to do, if I hadn't seen the
> examples and read the fine print.
>
> Bobby
>
> -----Original Message-----
> From: Jens-Peer Kuska [mailto:kuska at informatik.uni-leipzig.de]
To: mathgroup at smc.vnet.net
> Subject: [mg34411] [mg34376] [mg34340] Re: ValueQ
>
> Hi,
>
> Table[ ValueQ @@ (Hold[a[ii]] /. ii -> i), {i, 1, 2}]
>
> Regards
> Jens
>
> rainer wrote:
>>
>> Hi,
>>
>> I#m searching for a simple workaround of the following behaviour.
>>
>> For the symbol 'a' I've defined
>>
>> In[1]:=
>> a[1] = 2;
>>
>> When I evaluate ValueQ for a defined and for not a defined expression
> I
>> get what I expect:
>>
>> In[3]:=
>> ValueQ[a[1]]
>> Out[3]=
>> True
>>
>> In[4]:=
>> ValueQ[a[2]]
>> Out[4]=
>> False
>>
>> But when I evaluate ValueQ e. g. within a Table I always get True:
>>
>> In[5]:=
>> Table[ValueQ[a[i]], {i, 1, 2}]
>> Out[5]=
>> {True, True}
>>
>> The 2nd 'True' is because 'a[i]' is not equal to 'a[2]'. A first
>> solution to get the expected result is
>>
>> In[6]:=
>> Table[ToExpression@("ValueQ[a[" <> ToString[i] <> "]]"), {i, 1, 2}]
>> Out[6]=
>> {True, False}
>>
>> Does anybody know something better?
>>
>> Rainer Gruber
>> JOHANNES KEPLER UNIVERSITY LINZ
>> Institute of Experimental Physics
>> Atomic Physics and Surface Science
>
>
>
>
>
>