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Re: Re: Re: Set in Scan

• To: mathgroup at smc.vnet.net
• Subject: [mg22056] Re: [mg22008] Re: [mg21900] Re: [mg21856] Set in Scan
• From: Hartmut Wolf <hwolf at debis.com>
• Date: Fri, 11 Feb 2000 02:38:40 -0500 (EST)
• Organization: debis Systemhaus
• References: <200002100725.CAA05624@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

Dear Tomas,

what I wrote this morning ....

Hartmut Wolf schrieb:
> 
....
> 
> Or the somewhat more obfuscated
> 
> Scan[Evaluate,
>   Scan[((#[[3]] = If[#[[3]] < 0.5, "-XXX-", "-YYY-"])&
> @Unevaluated[#])&,
>     Unevaluated /@ Unevaluated[{v1, v2, v3}]]]
> 
> no Hold attributes in. This even works when you replace the outer Scan
> (to get the replaced elements evaluated) by Map although the inner Scan
> doesn't return anything.
> I guess this is neither a bug nor any intended use (of Map), but a
> consequence of the requirement that repeated Scans should be possible.
> This example at least is an argument that they should!
>

.....was a little bit too "obfuscated" (to myself, I confess)

And what I had talked about that second Scan (or Map) is pure nonsense.
It simply does nothing, as I had explained before. I somehow tricked
myself into that error, having not restored {v1,v2,v3} after an
experimentation with Hold, tjaaa ...
 
In[1]:=
{v1, v2, v3} = {{0.308624, 0.0113816, 0.452114, 0.100156},
        {0.368705, 0.44588, 0.878839, 0.736246},
        {0.71764, 0.18652, 0.667573, 0.338795}};

In[2]:=
Scan[((#[[3]] = If[#[[3]] < 0.5, "-XXX-", "-YYY-"]) &@Unevaluated[#]) &, 
          Unevaluated /@ Unevaluated[{v1, v2, v3}]]

In[3]:= {v1, v2, v3}
Out[3]=
{{0.308624, 0.0113816, "-XXX-", 0.100156}, {0.368705, 0.44588, "-YYY-", 
    0.736246}, {0.71764, 0.18652, "-YYY-", 0.338795}}


Another possibility would be

 
In[4]:=
{v1, v2, v3} = {{0.308624, 0.0113816, 0.452114, 0.100156},
        {0.368705, 0.44588, 0.878839, 0.736246},
        {0.71764, 0.18652, 0.667573, 0.338795}};

In[5]:=
Scan[((#[[3]] = If[#[[3]] < 0.5, "-XXX-", "-YYY-"]) &[#]) &, 
  Unevaluated /@ Unevaluated /@ Unevaluated[{v1, v2, v3}]]

In[6]:= {v1, v2, v3}
Out[6]=
{{0.308624, 0.0113816, "-XXX-", 0.100156}, {0.368705, 0.44588, "-YYY-", 
    0.736246}, {0.71764, 0.18652, "-YYY-", 0.338795}}

Each application removes one level of "Unevaluated".


Kind regards,  Hartmut

• References:
  • Re: Re: Set in Scan
    • From: "Tomas Garza" <tgarza@mail.internet.com.mx>