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Re: problem with append

  • To: mathgroup at smc.vnet.net
  • Subject: [mg129540] Re: problem with append
  • From: <bar at robox.ii.up.krakow.pl>
  • Date: Mon, 21 Jan 2013 00:10:22 -0500 (EST)
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  • References: <kdg2kv$hf8$1@smc.vnet.net>

Paolo cachecho <paolocach at gmail.com> wrote:
> hello i am new to mathmatica.i have the following problem if anyone could help.
> 
> For[a = 2, a < 6, a++,
>        For[b = 2, b < 6, b++,
>  Appendto[rr = {}, a^b]]]
> rr
> 
> but rr remains empty. how can i solve this
> thank you
> 
Try this:
r = {};
For[a = 2, a < 6, a++,
 For[b = 2, b < 6, b++,
  AppendTo[rr, a^b]]]
rr

or 
Table[
 Table[a^b, {b, 2, 5}], {a, 2, 5}]


Table[a^b, {a, 2, 5}, {b, 2, 5}]


but it is not the same, I'm not sure what You mean



remember : Table is faster then For , AppendTo is extremally slow
see
http://blog.wolfram.com/2011/12/07/10-tips-for-writing-fast-mathematica-code/

Try examples:
n = 200;
rr = {};
Timing@For[a = 2, a < n, a++,
  For[b = 2, b < n, b++,
   AppendTo[rr, a + b]]]
{18.4572, Null}

and

Timing[rr = Table[a + b, {a, 2, n - 1}, {b, 2, n - 1}];]
{0.104007, Null}


(I changed a^b -> a+b )  


Olaf, 



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