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Again, How to speed up this calculation? and more
*To*: mathgroup at smc.vnet.net
*Subject*: [mg37246] Again, How to speed up this calculation? and more
*From*: Cheng Liu <cliu at lanl.gov>
*Date*: Fri, 18 Oct 2002 05:17:22 -0400 (EDT)
*Sender*: owner-wri-mathgroup at wolfram.com
Dear group,
Thank you all for the responses. I might not make my question
very clear. Suppose that I have a list of points {p1,p2, ..., pn}, I try
to find all possible pairs of them. The pairs may include {pi,pi}, but
{pi,pj} and {pj,pi} are considered the same and only one is counted.
That said. After some try and error, I came to the following way:
h=4;w=5;
points=Flatten[Outer[List,Range[w],Range[h]],1];
pairs=Flatten[Map[Outer[List,{#},Drop[points,Position[points,#][[1,1]]-1],1][[1]]&,points],1];
The speed of the above calculation is reasonably fast. But I run into the
memory problem. For example, for h=64 and w=64, the length of the list
pairs will be w*h (w*h+1)/2 = 8,390,656. In my case, The numbers for h and
w will be a lot larger than 64. How can I get around this memory problem
or that's the dead end for my calculation (I do have 1 GB physical mem in
my machine)? Thanks a lot.
Cheng
At 12:26 PM 10/16/2002, you wrote:
>Dear group,
>
> I have the following question regarding a lengthy calculation
>using Mathematica:
>
>For given w points in x direction and h points in y direction, I can
>construct all the points using
>
> h=10; w=8;
> points=Flatten[Transpose[Outer[List,Range[w],Range[h]]],1]
>
>Next, I need to find all the possible pairs of point including points
>themselves, i.e., pair AA. I can use
>
> pairs=Outer[List,points,points,1]
>
>Then, I have to clear those pairs that repeat themselves, i.e., pair AB and
>pair BA. Also, when w and h are of the order of 1000s, the computation
>takes a very long time. Is there a better way of doing the second part of
>the computation? Thanks in advance.
>
>Sincerely
>
>Cheng
>
>
>====================================================
>Cheng Liu, Ph.D.
>MST-8, Structure/Property Relations
>Materials Science and Technology Division
>Los Alamos National Laboratory
>Los Alamos, New Mexico 87545
>
>Phone: 505-665-6892 (office), 505-667-9950 (lab)
>Fax: 505-667-8021
>email: cliu at lanl.gov
>====================================================
====================================================
Cheng Liu, Ph.D.
MST-8, Structure/Property Relations
Materials Science and Technology Division
Los Alamos National Laboratory
Los Alamos, New Mexico 87545
Phone: 505-665-6892 (office), 505-667-9950 (lab)
Fax: 505-667-8021
email: cliu at lanl.gov
====================================================
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