Vectorizing the creation of an offset vector

Question

Matthias 2013-5-22

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/76604-vectorizing-the-creation-of-an-offset-vector

Is there any possibility to vectorize this piece of code:

BIN_1 = 7;
C = randi(10,1000,1);
offset_vec = zeros(size(C));
delete = [];
o = 0;
for i = 1:size(C)
    if C(i) == BIN_1
        o = o + 1;
        delete = [delete, i];
    end
    offset_vec(i) = o;
end

It creates an offset vector, which increases whenever the corresponding element in C equals 7. The for-loop seem very slow.

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Answer 1

Andrei Bobrov 2013-5-22

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https://ww2.mathworks.cn/matlabcentral/answers/76604-vectorizing-the-creation-of-an-offset-vector#answer_86256

编辑：Andrei Bobrov 2013-5-22

在 MATLAB Online 中打开

ii = find(C == BIN_1);
offset_vec = zeros(size(C));
offset_vec(ii) = 1;
offset_vec = cumsum(offset_vec);

or (ADD)

t = C == BIN_1;
offset_vec = cumsum(t);
ii = find(t); % analog of 'delete'

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Jan 2013-5-22

Slightly faster: Omit the find() and use logical indexing.

Andrei Bobrov 2013-5-22

Hi Jan! I agree with you. Added.

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Answer 2

Jan 2013-5-22

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/76604-vectorizing-the-creation-of-an-offset-vector#answer_86278

在 MATLAB Online 中打开

Some comments:

1. Letting a vector grow iteratively is a severe waste of time. Note, that if delete is a 1x1000 vector finally, the iterative construction demands for reverving sum(1:1000) elements any copying almost the same number of elements. Better:

delete = false(1, numel(C));
o = 0;
for i = 1:numel(C)
  if C(i) == BIN_1
      delete(i) = true;
  ...

2. size(C) replies a vector. Therefore for i = 1:size(C) might perform unexpected things. Better use (as shown in the code above already) numel(C) or size(C, 1) (or what ever you need exactly).

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Matthias 2013-5-22

I know that growing Arrays are pretty slow, but the Finale size of the Array is somewhere around 100, while the for loop has several million iterations. That's why I believe it doesn't really matter in this specific case.

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