How to keep rows whose values follow a certain sequence?

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

How can I implement this into a Matlab code?

[Stephen23]

What are the possible values in column A ? Can it also include 4, 5, 9, 123, or does it include only 1, 2, & 3 ?

[Lu Da Silva]

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

[David Hill]

Do you want 90,180,270 sequence?

[Stephen23]

"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]

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

Answer 1

[Stephen23]

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

[Lu Da Silva]

This works perfectly! Thank you very much Stephen!

[David Hill]

Very tricky

[Stephen23]

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

Answer 2

[David Hill]

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

[Lu Da Silva]

I tried your suggestion but I get an error:

Error using strfind
First argument must be text

[David Hill]

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