how to calculate the elapsed time between rows

Hi, I have a 11619x6 matrix of the form [No.Data, YY, mm, dd, hour, sec]

1   1968   1    1   7    50   50.76
2   1968   1    4   11   37    8.190
3   1968   1    11  23   17   0.1400
4   1968   1    14  21   21   59.06
5   1968   1    17  19   34   55.70
6   1968   1    18  21   22   32.22

etc...

and I want to calculate the elapsed time between the 1st row and the 2nd, and then between the 1st row and the 3rd, then between the 1st row and the 4th and so on...

My thought was to do something simple without using a for loop

T= x(1:end-1,2:7);
Tn= x(2:end,2:7);
dt=etime(Tn,T);

but it didn't work. Can anyone give me an idea how to solve this?

Thanks!

Answer 1

[Star Strider]

This works:

M = [1 1968 1 1 7 50 50.76
2 1968 1 4 11 37 8.190
3 1968 1 11 23 17 0.1400
4 1968 1 14 21 21 59.06
5 1968 1 17 19 34 55.70
6 1968 1 18 21 22 32.22];

MT = M(:,2:7);                              % Time Vectors

for k1 = 2:size(MT,1)
    MET(k1,:) = etime(MT(k1,:), MT(1,:));   % Vector of Elapsed Times
end

You can do it without a loop, but you have to provide a matrix of MT(1,:) values equal in length to the rest of the time vectors:

MT1 = repmat(MT(1,:), size(MT,1)-1, 1);     % Matrix of ‘MT(1,:)’ Values
MET = etime(MT(2:end,:), MT1);

The loop takes about 8% longer than the repmat version in this example.

[Angeliki Efstathiou]

Thank you very much!

[Star Strider]

My pleasure!

Answer 2

[dpb]

Can't just subtract times in hr,min,sec form since there's rollover. Convert to datenums and subtract them then convert back to whatever units desired...

>> dn=datenum(t(:,2:end));  % convert to datenums
>> dt=dn(2:end)-dn(1)         % compute since first (days.fractions_thereof)
dt =
  3.1571
 10.6432
 13.5633
 16.4889
 17.5637
>> dt*24   % and difference in hours
ans =
 75.7715
255.4359
325.5190
395.7347
421.5282
>>

[Angeliki Efstathiou]

Thnx a lot! I will give it a try! :)