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Re: How to map a list on function

  • To: mathgroup at smc.vnet.net
  • Subject: [mg97567] Re: How to map a list on function
  • From: Ray Koopman <koopman at sfu.ca>
  • Date: Sun, 15 Mar 2009 05:29:57 -0500 (EST)
  • References: <gpg18t$ceh$1@smc.vnet.net>

On Mar 14, 3:35 am, buts <mange... at yahoo.com> wrote:
> Hello,
>
> Could anyone explain me how to do the following:
> I have a long list of integer numbers in groups of four:
>
> list= {{10,3,5,7},{4,6,8,9},{0,8,3,6}, ...... }
>
> or its Flatten version.
>
> How to write a fast function g[list] which does this:
>
> g[list] =x^10*y^3*z^5*u^7 + x^4*y^6*z^8*u^9 + y^8*z^3*u^6+ ...
>
> It should be done many times (say, 10^4-6) on lists with
> length > 1000, so taking parts is not a good idea.
> How to use Map or similar ?
> Thanks.

Here are five ways to do it. For lists of 1000 and 2000 4-tuples,
g4 and g5 are best and about equally fast, but for longer lists
g5 is fastest.

list = {{10,3,5,7},{4,6,8,9},{0,8,3,6}};
g1[list_] := Tr[Times@@({x,y,z,u}^#)& /@ list];
g2[list_] := Tr[Inner[Power,{x,y,z,u},#,Times]& /@ list];
g3[list_] := Tr@Inner[#2^#1&,list,{x,y,z,u},Times];
g4[list_] := Tr[Times@@({x,y,z,u}^Transpose@list)];
g5[list_] := Tr@Inner[Power,{x,y,z,u},Transpose@list,Times];
SameQ @@ (#@list& /@ {g1,g2,g3,g4,g5})
g5[list]//InputForm

True
u^6*y^8*z^3 + u^7*x^10*y^3*z^5 + u^9*x^4*y^6*z^8

try[length_,reps_] := Block[{list = Table[Random[Integer,9],
{length},{4}]}, {length, First/@{Timing@Do[g1[list],{reps}],
Timing@Do[g2[list],{reps}], Timing@Do[g3[list],{reps}],
Timing@Do[g4[list],{reps}], Timing@Do[g5[list],{reps}]}/.Second->1}]

try[1000#,64/#]&/@{1,2,4,8}

{{1000, {1.93, 1.56, 1.50, 1.21, 1.23}},
 {2000, {2.10, 1.75, 1.73, 1.46, 1.47}},
 {4000, {2.45, 2.13, 2.16, 1.95, 1.88}},
 {8000, {2.72, 2.44, 2.42, 2.36, 2.17}}}


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