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Speed of Join and Append. Was Re: Simple list question

  • To: mathgroup at smc.vnet.net
  • Subject: [mg132596] Speed of Join and Append. Was Re: Simple list question
  • From: Richard Fateman <fateman at cs.berkeley.edu>
  • Date: Wed, 16 Apr 2014 03:40:06 -0400 (EDT)
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On 4/14/2014 8:02 PM, Bill Rowe wrote:
> On 4/14/14 at 5:28 AM, stevebg at roadrunner.com (Ste[hen Gray) wrote:
>
>> pcin = 2;
>> pcex = 3;   (*pcin and pcex can have various small values *)
>>
>> coin = Table[{10 RandomReal[], 10 RandomReal[]}, {pcin}]
>> coex = Table[{3 RandomReal[], 3 RandomReal[]}, {pcex}]
>> cobo = Append[coin, coex]
>> pcin = 2;
>> pcex = 3;   (*pcin and pcex can have various small values *)
>>
>> coin = Table[{10 RandomReal[], 10 RandomReal[]}, {pcin}]
>> coex = Table[{3 RandomReal[], 3 RandomReal[]}, {pcex}]
>> cobo = Append[coin, coex]
>
> <snip>
>
>> What I want is a series of x,y coordinates with no extra nesting:
>
> Instead of Append, use Join, i.e.
>
> cobo = Join[coin, coex]
>
> In fact, Append is something to generally avoid using
> particularly for large arrays due to the computing resources
> used. It will require more memory and is slower. Won't matter
> for small arrays like this. But for large arrays, Append is very
> much a problem and better replaced with other commands such as Join.
>
>

You are, I think, confusing the conventional implementation of lists,
append and cons from Lisp, and the peculiar implementation in 
Mathematica of Append and Join.

I expect that in Mathematica they are equally wasteful, because
lists are represented by arrays and must be copied whether to
join or append.





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