How to iterate through the rows of array and grab data?

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Shayma Al Ali
Shayma Al Ali 2023-1-25
回答： Amit Dhakite 2023-3-13
I have a structure of data that contains photon heights, 'track1.ph', in the ocean as well as the lat, lon, time, etc. I also have a variable, 'track1.surf_type' thats a flag that lets me know which data is in the open ocean. However, 'track1.surf_type' flags each segment instead of the individual photon heights. I don't know which photons belong to each segment, however I have a variable 'track1.seg_ph' that lets me know how many photons are in each segment. I created a for loop that iterates through the 'track1.seg_ph' to use the number of photons in each segment as an index in the 'track1.ph' array. I will post the code below. The only thing is that it takes a while for the code to run due to the amount of data thats in each file. Is there any possible way that I can make my code much more efficient and run more quickly? I will post the code and some data below.
function [ph, ph_id]=getOceanData(track)
%use code to identify ocean data
%variable track1.surf_type to find ocean data (2nd row is ocean, segement==1
%track1.seg_ph to know the photon count of each segement
%use photon count as an index to see which segment each photon belongs to
ph=[]; ph_id=[];
for i=1:length(track.seg_ph)
if i==1
l= 1:track.seg_ph(i); %get segment photon count for first segment
if l ~= 0
k=track.ph(1:track.seg_ph(i)); %get the photons assosciated with first segment
if track.surf_type(2,i) == 1 %see if segment is ocean data
ph=[ph; k];
ph_id=[ph_id; l'];
ffl=ph_id(end);
else
ffl=track.seg_ph(i);
end
else
ffl=0;
end
else
if track.seg_ph(i) ~= 0 %if statement to make sure segment isn't empty
f=ffl; %find the index for the last photon of the last segment
ffl=f+track.seg_ph(i); %add the next segment photon count to this segment to get index
l=f:ffl; %index of photons for this segment
if track.surf_type(2,i) == 1 %see if segment is ocean data
ph_id=[ph_id; l']; %save index of photons
ph_id=unique(ph_id);
%ph=[ph; track.ph(l)]; %save photons that contain ocean data
end
end
end
end

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回答（1 个）

Amit Dhakite
Amit Dhakite 2023-3-13
Hi Shayma,
There are several ways you can make your code efficient and faster:
  1. Please note that as an alternative you can use Vectorization which is more concise and less prone to errors. For more information you can refer to the following link: https://www.mathworks.com/help/matlab/matlab_prog/vectorization.html
  2. Further, you can preallocate ph and ph_id, instead of resizing them in each iteration.
  3. Also, consider extracting the if block utilized for the first segment check within each iteration, out of the for loop.
For further information, kindly go through the following link: https://www.mathworks.com/help/matlab/matlab_prog/techniques-for-improving-performance.html

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