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MathGroup Archive 2008

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Re: loops?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg92814] Re: loops?
  • From: David Bailey <dave at Remove_Thisdbailey.co.uk>
  • Date: Tue, 14 Oct 2008 04:56:02 -0400 (EDT)
  • References: <gcv784$dsn$1@smc.vnet.net>

Bob Hanlon wrote:
> n = 100; m = 100;
> 
> data1 = {};
> Timing[Do[
>    Do[data1 = AppendTo[data1, {aa, bb, xx, yy}], {bb, 10, m, 
>      10}], {aa, 1, n}];]
> 
> {0.03825,Null}
> 
> Timing[data2 = 
>    Table[{aa = kk[[1]]; bb = kk[[2]]; aa, bb, xx, yy}, {kk, 
>      Flatten[Outer[{#1, #2} &, Range[n], Range[10, m, 10]], 1]}];]
> 
> {0.003871,Null}
> 
> Timing[data3 = 
>    Flatten[Outer[{#1, #2, xx, yy} &, Range[n], Range[10, m, 10]], 
>     1];]
> 
> {0.00092,Null}
> 
> data1 == data2 == data3
> 
> True
> 
> 
> Bob Hanlon
> 
> ---- AES <siegman at stanford.edu> wrote: 
> 
> =============
> Trying to think of a "two-variable iterator" approach that will let one 
> produce the same results as
> 
> data = { };
> Do[
>    Do[
>       data = AppendTo[data, {aa, bb, xx, yy}], 
>       {bb, 10, 30, 10}], 
>    {aa, 1, 2}];  
> Print[data // TableForm];
> 
> without using those universally deprecated (and even worse, nested) 
> Do[ ] loops, not to mention the equally deprecated AppendTo[].
> 
> Somewhat to my surprise, the construction
> 
> Table[{aa = kk[[1]];  bb = kk[[2]];  aa, bb, xx, yy}, 
>           {kk,  {{1, 10},  {1, 20},  {1, 30},  {2, 10},  {2, 20}, 
> {2,30}}}
>           ] // TableForm
> 
> using a two-dimensional iterator iterator variable kk, actually does 
> this, though the "iterator list" is obviously a bit messy.
> 
> Simpler way of defining an iterator to do this?  In more than two 
> iterator dimensions (aa, bb, cc,...)?  (with no functional dependence 
> between the values of aa, bb, cc).
> 
> Is the concept of a multi-dimensional iterator variable kk mentioned in 
> the M6 documentation anywhere?  (A search on "Iterate" or "iteration" 
> brings up 193 hits at 10 per screen.)
> 
> 
> --
> 
> Bob Hanlon
> 
> 
A quick comment on the above - never time any process for less than (at 
least) a few seconds - otherwise the result is meaningless.

This is not aimed at you, Bob, but at the original author of the question.

David Bailey
http://www.dbaileyconsultancy.co.uk


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