Scanning Matrix For Changes In Polarity

Just a bit of background: I have a very large matrix (106740x5). I need to process this into smaller files with a certain format to be compatible with other software. I have figured out how to produce the files in the right format but I've had another problem.

Basically, I need to break the files up so that if the changes in one of the column values changes direction (for example:

• 1
• 5
• 12
• 2 - 2 is less than 12 which contradicts the pattern of the previous values being bigger). When a direction change does occur, I need to break the matrix at this point and output it to a text file (which I know how to do).

Breaking the problem down, I can think of 2 main things that I need to know what to do to solve this problem:

• Identifying how many times that the column changes direction
• Identifying where these changes occur

There are other issues which I may come back to but I think that if I can figure out these parts, I will be able to sort things out.

Answer 1

[Robert]

You could use diff and sign to create a logical array, then cumsum to convert that to a group index.

% where x is your column of data
ddx = diff(sign(diff(x)))~=0;
group = cumsum([1;0;ddx])
for ii = 1:group(end)
    isinthisgroup = group == ii;
    % your processing, file writing
end

[Harry Eggo]

Thank you, Robert! This is exactly what I was looking for.