Re: List concatenation speed

*To*: mathgroup at smc.vnet.net*Subject*: [mg87581] Re: List concatenation speed*From*: Szabolcs Horvát <szhorvat at gmail.com>*Date*: Mon, 14 Apr 2008 05:39:31 -0400 (EDT)*Organization*: University of Bergen*References*: <ftsd1e$bba$1@smc.vnet.net>

carlos at colorado.edu wrote: > I am building mesh plots that require concatenation of thousands of > Graphics objects into one list for Show[]. This is done by appending > objects as they are created, and there are several ways to do that. > Tested 3 of them for speed using a simple object: > > p=Table[x,{50}]; n=10000; > ClearAll[poly]; poly={}; If you set poly = {}, then ClearAll is not necessary here. > Print[Timing[Do[AppendTo[poly,p],{i,1,n}]]]; Print[] is not necessary here. Do not use the semicolon at the end. > ClearAll[poly]; poly={}; > Print[Timing[Do[poly=Append[poly,p],{i,1,n}]]]; > ClearAll[poly]; poly={}; > Print[Timing[Do[poly=Join[poly,{p}],{i,1,n}]]]; > > {5.8395 Second,Null} > {5.7206 Second,Null} > {6.29728 Second,Null} > > Tests were run on version 5.2 on a G5 desktop Mac. I expected Join to > win, > but it didnt. Is there a faster way? > Yes, there is, but it involves using a slightly different algorithm. The problem is that you are thinking in a procedural way here: you assign a value to a variable, and change its value in a loop. Assignment and appending to lists are slow in Mathematica because they involve copying the whole data structure. Try to avoid this. I'd use In[1]:= p = Table[x, {50}]; n = 10000; Table[p, {n}]; // Timing Out[2]= {0., Null} which is much-much faster. It is not even necessary to use Join or Append. If you need the sublists joined together, then use Join @@ Table[p, {n}] This will produce a flat list of 'x' symbols. Generally, you can build up a list of lists first, and Join[] or Flatten[] them in the end. I hope this helps, Szabolcs