finding sequences

Question

Robert 2011-10-27

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https://ww2.mathworks.cn/matlabcentral/answers/19590-finding-sequences

I am trying to find sequences of increasing or decreasing values from a set of data.

For each sequence I will have to find the length of the sequence and the slope for each sequence.

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Doug Hull 2011-10-27

You are going to need to describe the goal in more detail than this. Give an example input and expected output.

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Answer 1

Walter Roberson 2011-10-27

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https://ww2.mathworks.cn/matlabcentral/answers/19590-finding-sequences#answer_25927

sign(diff()) will be positive for the duration of each increasing sequence, and negative for the duration of each decreasing sequence.

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Answer 2

Dr. Seis 2011-10-27

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在 MATLAB Online 中打开

Here is an example that will get you on your way:

t = 0:0.1:10;
f = sin(2*pi/3*t);
p = zeros(size(t));
s = ones(size(t));
num_seq = 1;
if f(2) - f(1) > 0
  inc_or_dec = 'inc';
  p(1:2) = 1;
else
  inc_or_dec = 'dec';
  p(1:2) = -1;
end
for i = 3 : length(t)
  if f(i) - f(i-1) > 0
      if isequal(inc_or_dec,'dec')
          inc_or_dec = 'inc';
          num_seq = num_seq+1;
      end
      p(i) = 1;
  else
      if isequal(inc_or_dec,'inc')
          inc_or_dec = 'dec';
          num_seq = num_seq+1;
      end
      p(i) = -1;
  end
  s(i) = num_seq;
end
figure;
subplot(2,1,1);
plot(t,f,'b-',t(p>0),f(p>0),'go',t(p<0),f(p<0),'ro');
legend('data','increasing data','decreasing data');
subplot(2,1,2);
plot(t,s,'b-'); 
legend('sequence number'); ylim([0 num_seq+1]);

It basically checks each sample in your data ("f" in my example) to see if it is increasing or decreasing. Each time it detects a change from "inc" to "dec" (or "dec" to "inc") it groups the subsequent data points in a different sequence. The values in "s" denote which sequence those data points belong to. I will leave it up to you to figure out how to determine the length of the sequence and the slope (hint: look at the way I plot points that follow an increasing and decreasing trend to select data points within a specific sequence).