How to keep rows whose values follow a certain sequence?

Question

Lu Da Silva 2022-2-3

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https://ww2.mathworks.cn/matlabcentral/answers/1642520-how-to-keep-rows-whose-values-follow-a-certain-sequence

评论： Stephen23 2022-2-3

I have a matrix composed of two columns A and B, I need to only keep the rows which contain the sequence 90-180-270:

A      B                     A      B  
--------                    ----------
    1                     90      1
   9                    180      9
   2                    270      2
    0            ->       90      0
   3                    180      4
    0                    270      6
   4                     
   6                     

How can I implement this into a Matlab code?

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Stephen23 2022-2-3

编辑：Stephen23 2022-2-3

"It only includes three possible numbers: 180, 270 and 90. (I wrote 1 2 3 for simplicity reasons)."

It is more complex and slower. Because solutions that work with 1-2-3 might not work with other values. But because you gave us incorrect information we all spend our time developing solutions that might not suit your needs at all.

You should tell us the exact values and the exact order.

Lu Da Silva 2022-2-3

编辑：Lu Da Silva 2022-2-3

Yes, the actual sequence is 90, 180, 270. But that shouldn't matter, the code should work for any given sequence.

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Answer 1

Stephen23 2022-2-3

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编辑：Stephen23 2022-2-3

在 MATLAB Online 中打开

Here is an old MATLAB trick that you can use, which still works today:

https://blogs.mathworks.com/loren/2008/09/08/finding-patterns-in-arrays

Where M is your matrix and S is the desired sequence:

S = [90,180,270]; % the required sequence
M = [90,1;180,9;270,2;90,0;270,3;90,0;180,4;270,6]
M = 8×2
    90     1
   180     9
   270     2
    90     0
   270     3
    90     0
   180     4
   270     6
X = strfind(M(:,1).',S);
Y = X-1+(1:numel(S)).';
Z = M(Y,:)
Z = 6×2
    90     1
   180     9
   270     2
    90     0
   180     4
   270     6

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David Hill 2022-2-3

Very tricky

Stephen23 2022-2-3

@David Hill: just two steps: STRFIND and some simple indexing :)

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Answer 2

David Hill 2022-2-3

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https://ww2.mathworks.cn/matlabcentral/answers/1642520-how-to-keep-rows-whose-values-follow-a-certain-sequence#answer_888440

在 MATLAB Online 中打开

s=strfind(num2str(yourMatrix(:,1))','123');
newMatrix=[];
for k=1:length(s)
  newMatrix=[newMatrix;yourMatrix(s(k):s(k)+2,:)];
end

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Lu Da Silva 2022-2-3

在 MATLAB Online 中打开

I tried your suggestion but I get an error:

Error using strfind
First argument must be text

David Hill 2022-2-3

在 MATLAB Online 中打开

This should work.

r=yourMatrix(:,1);
r(r==90)=1;r(r==180)=2;r(r==270)=3;%change based on sequence desired
f=strfind(num2str(r)','123');
newMatrix=[];
for k=1:length(f)
  newMatrix=[newMatrix;yourMatrix(f(k):f(k)+2,:)];
end