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MathGroup Archive 2005

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Re: four approaches to do a simple sum

  • To: mathgroup at smc.vnet.net
  • Subject: [mg57212] Re: [mg57171] four approaches to do a simple sum
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Sat, 21 May 2005 02:39:25 -0400 (EDT)
  • Reply-to: hanlonr at cox.net
  • Sender: owner-wri-mathgroup at wolfram.com

I don't know the answer to your question but Tr is even faster.

longlist=Table[Random[],{1000000}];

Timing[Plus@@longlist]

{1.39 Second,499953.}

Timing[Fold[Plus,0,longlist]]

{0.53 Second,499953.}

Timing[Tr[longlist]]

{0.03 Second,499953.}


Bob Hanlon

> 
> From: Hui Fang <fangh73 at xmu.edu.cn>
To: mathgroup at smc.vnet.net
> Date: 2005/05/20 Fri AM 04:43:04 EDT
> Subject: [mg57212] [mg57171] four approaches to do a simple sum
> 
> Here I have a long list, length of 1 million, and I used 4 ways to get 
> the sum of all elements.
> 
> In[1] = longlist=Table[Random[], {1000000}];
> 
> Method 1:
> In[2] = Timing[sum=0; For[i=1,i<=Length[longlist],sum+=longlist[[i]]]; 
sum]
> Out[2] = {6.219 Second, 500358}
> 
> Method 2:
> In[3] = Sum[longlist[[i]],{i,1,1000000}]
> Out[3] = {1.718 Second, 500358}
> 
> Method 3:
> In[4] = Timing[Plus@@longlist]
> Out[4] = {0.407 Second, 500358}
> 
> Method 4:
> In[5] = Fold[Plus,0,longlist]
> Out[5] = {0.156 Second, 500358}
> 
> The computing time gets shorter and shorter from top to bottom. It's 
> easy to understand why the first two methods are slow because they 
> involved an extra variable i for loop control and basically violates the 
> principle for list manipulation "Never take a list apart".
> What I don't understand is why method 4 is faster than method 3.
> Any explanation?Or do you have an even faster method?
> Thanks a lot!
> 
> Hui Fang
> 
> 


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