Count instances of subarray inside array

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Mitsu
Mitsu 2021-11-4
编辑： Turlough Hughes 2021-11-7
For an array
A = [2,2,2,2];
I would like to count how many times [2,2] appears in A, using each value only in one possible pair.
For example,
numel(strfind(A,[2,2]))
returns 3.
But if each "2" is used only once, then we have only 2 instances, instead of 3.
A is not necessarily of all same elements or even-number size, it could be e.g. A = [1,2,3].
Is there a simple way to do that?
  2 个评论
KSSV
KSSV 2021-11-4
Try reshape into n*2 array and use ismember with rows option.
Mitsu
Mitsu 2021-11-4
I mean for this approach to work regardless of the size and elements, such as A = [1,2,3]. I edited the question to clarify.

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回答（1 个）

Turlough Hughes
Turlough Hughes 2021-11-4
You could do the following:
b = A == 2; % binary indicating where A equal to 2
count = 0;
ii = 1;
while ii < numel(A)
if b(ii+1) && b(ii)
count = count + 1;
ii = ii + 1;
end
ii = ii + 1;
end
When there is a match, the loop variable, ii, increments an extra time so that each "2" is only used once.
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Mitsu
Mitsu 2021-11-4
编辑：Mitsu 2021-11-4
Thank you. There might be no other way around but to do it following the logic in your answer. For now I will wait to see if there are other answers with simpler, maybe one-line, approach.
Turlough Hughes
Turlough Hughes 2021-11-4
编辑：Turlough Hughes 2021-11-7
Another way would be to get the length of each group of twos (based on this answer by @Image Analyst) and then round down each group's length to the nearest whole number divisible by two:
measurements = regionprops(A == 2, 'Area');
allLengths = [measurements.Area];
count = sum(floor(allLengths/2))
(if you have the image processing toolbox)

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