Re: Table[] Why such a hog?

• To: mathgroup at smc.vnet.net
• Subject: [mg3586] Re: Table[] Why such a hog?
• From: wagner at bullwinkle.cs.Colorado.EDU (Dave Wagner)
• Date: Wed, 27 Mar 1996 03:26:25 -0500
• Organization: University of Colorado, Boulder
• Sender: owner-wri-mathgroup at wolfram.com

```In article <4iqpo1\$puj at dragonfly.wolfram.com>,
>Seems like the Table[] function for building arrays takes way longer than it ought to.  I hope someone can
>tell me why:
>
>The following expression takes around twenty minutes to complete on my RS/6000 workstation:
>
>Table[Exp[N[2 Pi I ((i-32)^2 + (j-32)^2)/64]], {i,.001,63.876,.125},{j,.001,63.876,.125}]
>
>Why should this take so much resource?  Is there a better way to generate an array of function values?

Try compiling the expression that you are iterating.  First, for
comparison here is the uncompiled version on my machine (note the much
abbreviated limits of iteration):

(Local) In[3]:=
Table[Exp[N[2 Pi I ((i-32)^2 + (j-32)^2)/64]],
{i,.001,3.876,.125},{j,.001,3.876,.125}]; // Timing
(Local) Out[3]=
{2.4 Second, Null}

Now for the compiled version:

(Local) In[11]:=
f = Compile[{i, j},
Exp[2 Pi I ((i-32)^2 + (j-32)^2)/64]]
(Local) Out[11]=
CompiledFunction[{i, j},

2           2
2 Pi I ((i - 32)  + (j - 32) )
Exp[------------------------------], -CompiledCode-]
64

(Local) In[12]:=
Table[f[i,j],
{i,.001,3.876,.125},{j,.001,3.876,.125}]; // Timing
(Local) Out[12]=
{0.616667 Second, Null}

By the way, I hope you're not trying to print the result of your Table
command in the front-end.  The amount of time necessary to format the result
and transmit it to the front-end considerably outweighs the computation time.

Dave Wagner
Principia Consulting
(303) 786-8371
dbwagner at princon.com
http://www.princon.com/princon

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