Why slow conversion from date in Matlab time to day of year?

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K E
K E 2015-5-18
评论： Peter Perkins 2015-6-3
I have a vector of dates in Matlab time convention which I need to convert to day of the year. The following code is slow (9 seconds) even though it is using builtin Matlab functions. What do you recommend to speed it up?
timeGmt = datenum(1996,1,1):1/24:now; % Long vector of dates covering many years
timeGmt = timeGmt'; % Change from row vector to column vector for compatibility in next line
yearDay = timeGmt - ...
datenum(str2num(datestr(timeGmt, 10)), 1, 1) ... % Subtract off first day of this year
+ 1; % Add a day so that first day of year is day 1

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James Tursa
James Tursa 2015-5-18
编辑：James Tursa 2015-5-18
Try this variation using datevec instead of datestr:
vecGmt = datevec(floor(timeGmt)); % Set hms to 0's using floor
vecGmt(:,2:3) = 1; % Set month and day to Jan 1
yearDay = timeGmt - datenum(vecGmt) + 1;

更多回答（1 个）

Peter Perkins
Peter Perkins 2015-5-18
编辑：Walter Roberson 2015-5-19
In R2014b or later, use datetime. On a not particularly fast Win7 machine:
>> timeGmt = '1-Jan-1996':hours(1):datetime('now');
>> tic, dayOfYear = day(timeGmt,'dayofyear'); toc
Elapsed time is 0.041145 seconds.
  2 个评论
K E
K E 2015-5-26
编辑：K E 2015-5-26
I don't seem to have the function day.m but it may be part of the Financial toolbox .
Peter Perkins
Peter Perkins 2015-6-3
If you have R2014b or later, you have the day function. It is a method of the datetime class, which was introduced in R2014b.

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