How to vectorize a find

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Matlab2010 2012-12-11

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x = zeros(1000,1);
x(76) = 1;
x(100) = 1;
x(200) = 1;

I can do:

first = find(x ==1, 1, 'first'); %the answer =76

but this is slow. I would like to speed this up. How? thanks

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Matlab2010 2013-1-15

编辑：Matlab2010 2013-1-15

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just going to put up some code which might help others looking for v fast finds in future:

Method 3 is incredibly fast (credit to Matt J!)

%METHOD 1  
lastTimeValue_SLOW =  arrayfun(@(x,y) find(data(x:end,2) <= y, 1, 'last'), trades, tradeTimePlusDt); %works but v slow!
%METHOD 2
  lastTimeValue = NaN(length(trades),1);
  for z = 1 : length(trades)
      p=data(trades(z):end,2);
      for kk = 1:length(p)
          if(tradeTimePlusDt(z) <= p(kk))
              lastTimeValue(z) = kk-1;
              break;
          end
      end    
  end
%METHOD 3
  %http://www.mathworks.co.uk/matlabcentral/newsreader/view_thread/304895
  lastTimeValue_TEST = NaN(length(trades),1);
  for z = 1 : length(trades)
     I = histc(data(trades(z):end,2),[-inf;tradeTimePlusDt] );
      temp = cumsum(I(1:z));
      lastTimeValue_TEST(z) = temp(end);
  end

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Answer 1

Matt Fig 2012-12-11

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编辑：Matt Fig 2012-12-11

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The last statement you show is vectorized, so the question is ill-posed. I think what you are asking for is a way to speed up the process.

Also, just how fast do you think it should be??

x = zeros(1000,1);
x(76) = 1;
x(100) = 1;
x(200) = 1;
tic,first = find(x ==1, 1, 'first');toc
Elapsed time is 0.000023 seconds.  % That is slow???

.

EDIT

I did find a way to do it faster, but it will only work if you have a binary matrix as in your example. You can avoid the comparison as it is not necessary.

tic,first = find(x);first = first(1);toc
Elapsed time is 0.000012 seconds.

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Matt Fig 2012-12-11

编辑：Matt Fig 2012-12-11

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There is no find.m in MATLAB.

which find
built-in (C:\Program Files\MATLAB\R2011b\toolbox\matlab\elmat\@logical\find)  % logical method

Matt Fig 2012-12-11

Also, see my edit above for a faster method if you have a binary array.

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Answer 2

Sean de Wolski 2012-12-11

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If you only have zeros and ones and you are positive there is atleast one one, then you can use the second output from max().

[~,first] = max(x);

I don't know if this will be faster or not.

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Answer 3

Mark Whirdy 2012-12-11

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编辑：Mark Whirdy 2012-12-11

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a temporary vector of row numbers, then use your vector-of-interest & a logicsl statement to index this row-number vector
    a =[1:size(x,1)]'; % row numbers
    b=a(x==1); % logical indexing the populated rows
    b(1) % first instance