optimising matlab for loop

Question

I have this following loop for a code (that computes the histograms). I am writing in Matlab. As I am new to Matlab I don't know of any faster methods to do this. I am currently writing

for i=1:size(b)
    a(b(i)) = a(b(i)) + 1;
end

Are there any faster methods to do this, preferably those which do not require the for loop?

general comment: size isn't the right function for what you are doing now, rather use length or numel. — Gunther Struyf
– Gunther Struyf, Commented Sep 28, 2012 at 17:45
The idea that for-loops are inefficient/slow and should be replaced if possible is unsubstantiated. Even though you could replace this code with something that is faster, you would probably save only a fraction of a second. This is not the level of code optimization you should be worried about. — anon
– anon, Commented Sep 28, 2012 at 17:58
@Kavka Concerning this type of for loop, I would agree. A more complex for loop that manipulates large matrices would be worth vectorizing, right? Matlab was designed to work with Matrices -- all of its matrix operations are supposed to be optimized. — Derek
– Derek, Commented Sep 28, 2012 at 18:32
See also mathworks.com/matlabcentral/newsreader/view_thread/10055 — Ben Voigt
– Ben Voigt, Commented Sep 28, 2012 at 18:47

Community · Accepted Answer · 2017-05-23 11:56:17Z

9

You can simply vectorize it by a(b) = a(b) + 1. Check the following:

>> a = [1 2 3 4];
>> b = [2 4]; %# indices to modify. Be sure that they are in bounds.
>> a(b) = a(b) + 1

a =

     1     3     3     5

If you use some indices multiple times, then accumarray will help as follows:

>> a = [1 2 3 4];
>> b = [2 4 2];
>> a = accumarray([1:numel(a) b].',[a ones(size(b))])'

a =

     1     4     3     5

Alternatively, you can use:

>> a = [1 2 3 4];
>> b = [2 4 2];
>> b = accumarray(b.',ones(size(b)));
>> a(nzIndex) = a(nzIndex) + b(nzIndex)'

a =

     1     4     3     5

See this nice answer here for further details.

edited May 23, 2017 at 11:56

CommunityBot

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answered Sep 28, 2012 at 17:40

petrichor

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1 Comment

anirudh Over a year ago

In my case b may be [2 4 2], so that I need to increment a(2) twice. This method updates a(2) only once.

anon · Accepted Answer · 2012-09-28 17:43:45Z

1

If both a and b are vector, this should work.

a = 1:100;
b = 100:-1:1;

a(b) = a(b) + 1;

answered Sep 28, 2012 at 17:43

anon

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