How to filter an array?

조회 수: 10 (최근 30일)
Meshooo
Meshooo 2016년 1월 6일
댓글: Guillaume 2016년 1월 7일
Dear all,
I have this array
A = [0;0;0;0;0;0;1;1;1;0;0;0;0;0;0;1;0;0;0;0];
I want only one representative for each group of ones. So how to make
A = [0;0;0;0;0;0;0;1;0;0;0;0;0;0;0;1;0;0;0;0];
Any help will be appreciated.
Best, Meshoo
  댓글 수: 3
Stephen23
Stephen23 2016년 1월 6일
And what if there are an even number of ones?
A = [0;1;1;0]
what output do you want?
Meshooo
Meshooo 2016년 1월 6일
I don't care which right or left to take in case of even number of ones.
A = [0;0;1;0]
or
A = [0;1;0;0]
Any answer is OK.

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채택된 답변

Guillaume
Guillaume 2016년 1월 6일
It's very simple:
diff([0; A]) == 1
simply detects all transition from 0 to 1 and return them as 1.
  댓글 수: 1
Guillaume
Guillaume 2016년 1월 7일
Note that this keeps just the first 1 of a block as opposed to the middle 1 of a block, which the other solutions do.

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추가 답변 (3개)

goerk
goerk 2016년 1월 6일
A = [0;0;0;0;0;0;1;1;1;0;0;0;0;0;0;1;0;0;0;0];
% A = [0;1;1;0;1;1;1];
dA = diff(A);
ind = 1:length(A);
startInds = ind(dA>0)+1;
endInds = ind(dA<0);
if length(endInds)<length(startInds) %last value is 1
endInds(end+1) = length(A);
end
midInd = floor((startInds+endInds)/2); % when even choose left
% midInd = ceil((startInds+endInds)/2); % when even choose right
B = zeros(size(A));
B(midInd) = 1;
[A B] % show result, to check input and output
  댓글 수: 2
Guillaume
Guillaume 2016년 1월 6일
Your answer will fail if the first element of A is 1. And if it starts and ends with 1, the start and end offsets will be completely wrong due to the way you detect that the last value is 1.
The best way to solve both is to prepend and append A with 0 before the diff:
dA = diff([0 A 0]); %guarantees you have the same number of starts and ends.
midInd = floor((find(dA > 0) + find(dA < 0) - 1) / 2); %much simpler way of calculating mid indices
goerk
goerk 2016년 1월 6일
You are right, for a 1 at the first position my will fail. Thanks for your very nice and short solution. With the correct concatenation it works fine.
dA = diff([0; A; 0]);

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Stephen23
Stephen23 2016년 1월 6일
>> A = [0;0;0;0;0;0;1;1;1;0;0;0;0;0;0;1;0;0;0;0];
>> X = find(diff([0;A;0]));
>> Z = zeros(size(A));
>> Z(fix((X(1:2:end)+X(2:2:end)-1)/2)) = 1
Z =
0
0
0
0
0
0
0
1
0
0
0
0
0
0
0
1
0
0
0
0

Meshooo
Meshooo 2016년 1월 7일
Thank you all for the kind help and suggestions.

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