/edit: See here for an interesting discussion of the topic. Thanks @Dan
Using a(m,n) = 0 appears to be faster, depending of the size of matrix a, than a = zeros(m,n). Are both variants the same when it comes to pre-allocation before a loop?
- You're right, I seem to have searched for the wrong keywords. Thank you. Will edit my main post in a few minutes, I'm on mobile right now.Fred S– Fred S2013-08-13 10:11:51 +00:00Commented Aug 13, 2013 at 10:11
- See also this for more on preallocation.horchler– horchler2013-08-13 13:58:51 +00:00Commented Aug 13, 2013 at 13:58
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2 Answers
They are definately not the same.
Though there are ways to beat the performance of a=zeros(m,n), simply doing a(m,n) = 0 is not a safe way to do it. If any entries in a already exist they will keep existing.
See this for some nice options, also consider doing the loop backwards if you don't mind the risk.
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Fred S
Looping backwards seems interesting, I will try that. Apart from when
a already exists, why else would a(m,n) not be safe? Am I safe to use clear a; a(m,n) = 0; for preallocation (ignoring the fact that using clear makes it slower again)?I think it depends on your m and n. You can check the time for yourself
tic; b(2000,2000) = 0; toc; Elapsed time is 0.004719 seconds. tic; a = zeros(2000,2000); toc; Elapsed time is 0.004399 seconds. tic; a = zeros(2,2); toc; Elapsed time is 0.000030 seconds. tic; b(2,2) = 0; toc; Elapsed time is 0.000023 seconds. 
6 Comments
Fred S
Hmm that's a much smaller difference than what I experienced. I may have made a mistake in my benchmark. Will check once I'm back in my office.
High Performance Mark
I wouldn't bet the farm on any timings measured in milliseconds on a PC.
Pavan K
As m and n increase you can see the difference. This is just an example.
Fred S
I have indeed found a small mistake in my benchmarks, but I can reproduce what you wrote. It appears to be highly dependent on my matrices size. Dan posted a link to an interesting discussion with more in depth benchmarks. Thank you for your answer, Pavan.
horchler
zeros may depend on the number of elements and for sufficiently large allocations how many threads Matlab supports on your computer. |