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Re: reducing the time of constructing a List


On May 31, 2005, at 5:00 AM, marloo3 at mail15.com wrote:

> please, what is the best way to reduce the timing of the following  
> lines, the
> source of data is:
> dat = Table[Random[Integer, {0, 10}], {100000}];
> and the purpose is to construct the List lst in the following way:
> For[i = 1, i <= 100000,
>       If [dat[[i]] == 1,
>         lst = Join[lst, {1}], lst = Join[lst, {0}]]; i++]; // Timing
>
> Out[]= {226.07 Second, Null}
> on the P4 celeron 2 Ghz , memory 382 MB ram
> thanks

Here's one way:

Timing[dat /. {0 -> 0, 1 ->
       1, 2 -> 0, 3 -> 0, 4 -> 0, 5 -> 0, 6 -> 0, 7 -> 0, 8 -> 0, 9 - 
 > 0,
          10 -> 0};]

which results in

{0.065167 Second, Null}

on a 2.7GHz G5 PowerMac.

Later,

Ssezi


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