       Re: How to speed up this calculation?

• To: mathgroup at smc.vnet.net
• Subject: [mg37242] Re: How to speed up this calculation?
• From: "Kevin J. McCann" <kmccann at umbc.edu>
• Date: Fri, 18 Oct 2002 05:17:03 -0400 (EDT)
• Organization: University of Maryland, Baltimore County
• References: <aokc15\$agm\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Here is a way with symbolic lists.

(* Here are some sample lists*)

lst1={A,B};

lst2={a,b,c};

(*Join into one list*)

list1=Join[lst1,lst2];

(*Do the outer product to get all possible ordered pairs*)

list2=Partition[Flatten[Outer[List,list1,list1]],2]

{{A, A}, {A, B}, {A, a},

{A, b}, {A, c}, {B, A},

{B, B}, {B, a}, {B, b},

{B, c}, {a, A}, {a, B},

{a, a}, {a, b}, {a, c},

{b, A}, {b, B}, {b, a},

{b, b}, {b, c}, {c, A},

{c, B}, {c, a}, {c, b},

{c, c}}

(*Turn the pairs into products*)

list3=list2/.{x_,y_}®{x y}

{{A^2}, {A*B}, {a*A}, {A*b},

{A*c}, {A*B}, {B^2}, {a*B},

{b*B}, {B*c}, {a*A}, {a*B},

{a^2}, {a*b}, {a*c}, {A*b},

{b*B}, {a*b}, {b^2}, {b*c},

{A*c}, {B*c}, {a*c}, {b*c},

{c^2}}

(*Union weeds out repeats*)

list4=Union[list3,list3]

{{a^2}, {a*A}, {A^2}, {a*b},

{A*b}, {b^2}, {a*B}, {A*B},

{b*B}, {B^2}, {a*c}, {A*c},

{b*c}, {B*c}, {c^2}}

(*Now turn the products back into pairs*)

(*This is the step the requires symbols*)

list5=list4/.{x_^2}®{x,x}/.{x_*y_}®{x,y}

{{a, a}, {a, A}, {A, A},

{a, b}, {A, b}, {b, b},

{a, B}, {A, B}, {b, B},

{B, B}, {a, c}, {A, c},

{b, c}, {B, c}, {c, c}}

(*List it out*)

(*If you have numbers, you can now use a Rule to replace*)

Sort[lst]//TableForm

--
Kevin J. McCann
Joint Center for Earth Systems Technology (JCET)
Department of Physics
UMBC
Baltimore MD 21250

"Cheng Liu" <cliu at lanl.gov> wrote in message
news:aokc15\$agm\$1 at smc.vnet.net...
> 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
> ====================================================
>
>

```

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