Timing puzzle

*To*: mathgroup at smc.vnet.net*Subject*: [mg130424] Timing puzzle*From*: carlos%colorado.edu at gtempaccount.com*Date*: Wed, 10 Apr 2013 00:53:07 -0400 (EDT)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*Delivered-to*: l-mathgroup@wolfram.com*Delivered-to*: mathgroup-newout@smc.vnet.net*Delivered-to*: mathgroup-newsend@smc.vnet.net

I am writing a graphics package that often creates objects with thousands of polygons, possibly up to 10^5. Out of curiosity I tested 5 ways of dynamically creating a plot list, using AppendTo, Append, Join, etc., and did the following timing test of 5 ways to do it: ClearAll[poly,p,n]; poly={}; n00; p[arg_]:= {mygraphic[mycolor[Random[],Random[],Random[]]], mygraphic[mypoly[{{Random[],Random[]}, {Random[],Random[]},{Random[],Random[]}}]]}; Print[Timing[Do[AppendTo[poly,p[i]],{i,1,n}]][[1]]]; ClearAll[poly]; poly={}; Print[Timing[Do[poly=Append[poly,p[i]],{i,1,n}]][[1]]]; ClearAll[poly]; poly={}; Print[Timing[Do[poly=Join[poly,{p[i]}],{i,1,n}]][[1]]]; ClearAll[poly]; poly={}; Print[Timing[Do[poly={poly,{p[i]}},{i,1,n}];poly=Flatten[poly]][[1]]]; ClearAll[poly]; poly=Table[0,{n}]; Print[Timing[Do[poly[[i]]=p[i],{i,1,n}]][[1]]]; Running with n00 on a MacPro under Mac OSX 10.6.8 gives these times: 0.911327 Second 0.891656 Second 0.927267 Second 0.504454 Second 0.009575 Second Question: why is the last method much faster? I thought that appending an object to a list should take about the same time as storing an array entry. When I worked with linked lists several decades ago (using assembly code on a CDC 7600) all I had to do is retrieve the object address, manipulate registers, store in a pointer array, and presto! it was done.

**Follow-Ups**:**Re: Timing puzzle***From:*Bob Hanlon <hanlonr357@gmail.com>