Re: Indexed element treated as number?

• To: mathgroup at smc.vnet.net
• Subject: [mg80182] Re: Indexed element treated as number?
• From: "Jung-Tsung Shen" <jushen at gmail.com>
• Date: Tue, 14 Aug 2007 07:07:33 -0400 (EDT)
• References: <f9jl5e\$3c6\$1@smc.vnet.net> <46BDDE70.50103@gmail.com>

```Jean-Marc and Bob,

Thanks very much for the very helpful solutions!

Sincerely,
JT

On 8/11/07, Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com> wrote:
> Jung-Tsung Shen wrote:
> > To get an array of length n, where n is large and post-given, one
> > issues the command, for example,
> >
> > n=100;
> > kList1=Array[a, n];
> >
> > but it seems that there are some limitations to this approach. For
> > example, in cases that I know all of the elements are real, and would
> > like to issue, say,
> >
> > a[37]/:Im[a[37]]=0;
> >
> > I get an error message saying
> > "TagSet::sym: Argument a[37] at position 1 is expected to be a
> > symbol." To declare the element to be explicitly zero sometimes speed
> > up the computation.
>
> In this case, the correct syntax for a tag set is the name of the array
> without any index followed by the tag set to the desired element as in
>
> In[1]:= a /: Im[a[2]] = 0;
>
> Alternatively, we could use *UpSet* as in
>
> In[2]:= Im[a[3]] ^= 0;
>
> Now we can compare the imaginary part of the elements.
>
> In[3]:= Im[a[2]] == Im[a[3]]
>
> Out[3]= True
>
> In[4]:= Im[a[2]] == Im[a[4]]
>
> Out[4]= 0 == Im[a[4]]
>
> > For a list such as
> >
> > kList2={k1, k2, k3, ..., k100};
> >
> > there's no such limitations, but since the number of elements is
> > large, and is post-given, I couldn't have this before hand.
> >
> > Is there any solution(s) to this "problem"?
>
> Well, you could use a function like the one below to create your array
> of symbol when you know what the length of the list is.
>
> In[1]:= createArray[n_Integer /; n > 0, sym_Symbol: k] :=
>     Table[ToExpression[StringJoin[ToString /@ {sym, i}]], {i, n}]
>
> In[2]:= createArray[5, a]
>
> Out[2]= {a1, a2, a3, a4, a5}
>
> In[3]:= kList2 = createArray[10]
>
> Out[3]= {k1, k2, k3, k4, k5, k6, k7, k8, k9, k10}
>
> HTH,
> --
> Jean-Marc
>

```

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