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Re: Changing names of variables in a loop

  • To: mathgroup at smc.vnet.net
  • Subject: [mg92218] Re: Changing names of variables in a loop
  • From: Albert Retey <awnl at gmx-topmail.de>
  • Date: Tue, 23 Sep 2008 07:33:07 -0400 (EDT)
  • References: <gb7obt$ngj$1@smc.vnet.net>

kazik.lakomy at gmail.com wrote:
> Hi,
> do you know maybe is there a possibility to change a name of a
> variable in a loop? I'm not exactly sure how to explain it, so maybe
> instead of my words I paste a basic example that express my query:
> 
> a1 = 1; a2 = 2; a3 = 3;
> func[g_] := g;
> Summ = 0;
> 
> For[i = 1, i <= 3, i += 1,
>  Summa += func[ai]]
> 
> So something like this... I have tried to deal with this like with
> strings and have put func[a<>"i"] but it didn't work (quite logic).
> The general problem is much more complicated, this is only a part of
> it, so please do not suggest me how can I sum up three numbers, this I
> believe I know :) I would very appreciate any hints,

this does literally what you want:

For[i = 1, i <= 3, i += 1,
 summ += func[ToExpression["a" <> ToString[i]]]]

there are alternatives which save you from error prone typing of
irrelevant code:

summ = 0;
Do[summ += func[ToExpression["a" <> ToString[i]]], {i, 3}]

or:

summ = Total[Table[func[ToExpression["a" <> ToString[i]]], {i, 3}]]

if possible I would suggest to use different naming conventions for your
variables if you need to generate them programmatically:

a[1] = 1; a[2] = 2; a[3] = 3;

which lets you reduce your code to:

summ = 0;
Do[summ += func[a[i]], {i, 3}]
summ

or even:

summ=Total[Table[func[a[i]], {i, 3}]]

or:

Total[func /@ a /@ Range[3]]

hth,

albert


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