Avoid for loops with if inside

Hello everyone,

I have a code as follows:

for i=1:10000
  for j=1:5000
     if (A(1,i)-B(1,j)<5)
       C(i,j)=C(i,j)+1;
     end
  end
end

Can anyone show me how I can avoid these for loops, so that my run can be much faster? Any help is highly appreciated. Thanks!

Answer 1

[Matt Fig]

idx = bsxfun(@minus,A(1,:).',B(1,:))<5;
C(idx) = C(idx) + 1;

It would help if you specified the sizes of A, B and C. I assume A and B are matrices, but are they vectors or what?

Here is how I tested the above code:

A = round(rand(1,10000)*10);
B = round(rand(1,5000)*10);
C = round(rand(10000,5000)*10);
C2 = C;

tic
for i=1:size(A,2)
  for j=1:size(B,2)
     if (A(1,i)-B(1,j)<5)
       C(i,j)=C(i,j)+1;
     end
  end
end
toc

tic
idx = bsxfun(@minus,A(1,:).',B(1,:))<5;
C2(idx) = C2(idx) + 1;
toc

isequal(C,C2)  % Yes...  And the loops take MUCH longer!

[Azzi Abdelmalek]

how much longer?

[Oleg Komarov]

Elapsed time is 45.431542 seconds.

Elapsed time is 1.081703 seconds.

[Azzi Abdelmalek]

Thanks Oleg

[Matt Fig]

Wow, Oleg! What machine are you using, may I ask? On my machine the times are:

Elapsed time is 201.455394 seconds.
Elapsed time is 4.744238 seconds.

[Matt Fig]

Incidentally, this is nearly as fast as full vectorization:

for ii=1:size(A,2)
    idx = A(1,ii)-B(1,:)<5;
    C3(ii,idx) = C3(ii,idx)+1;
end

[Azzi Abdelmalek]

Thanks Matt, it seems Oleg's machine is MUCH faster then yours

[Matt Fig]

I agree, that's why I asked what he is using ;-).

[Alireza]

Thank you very much for your reply!

Well, my actual code is as follows, which is quite different from my original question:

counter=1;
for i=1:size(A,1)
  for j=counter:size(B,1)
     if D(1,B(j,1))>0
        if B(j,2)-A(i,2)<5
           C(A(i,1),B(j,1))=C(A(i,1),B(j,1))+1;
           D(1,B(j,1))=D(1,B(j,1))-1;
           B(j,3)=1;
           counter=j+1;
        end
        break; 
     end
  end
end

• A is an 100*3 matrix
• B is an 1000*3 matrix
• C is an 100*1000 matrix
• D is an 1*1000 matrix

So, can it be written by much less computational effort? Any comment is very appreciated! Thanks!

[Matt Fig]

Now why would you ask a question, have people spend time on it, then tell them that it isn't the real question? Please close this question then ask your new question - and give the real question the first time...

[Walter Roberson]

Reformatted for you. Please read http://www.mathworks.com/matlabcentral/answers/13205-tutorial-how-to-format-your-question-with-markup

[Alireza]

Sorry! But I really didn't mean to bother anyone. That was a misunderstanding since I was not very familiar with guidelines. I will ask my question as a new one. Thanks!

[Oleg Komarov]

@Matt: R2012a win7 64 on i7-2600 3.40GHz