How to make datenum more efficient for large arrays?

2013 10 29
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更新时间：2013 10 30
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I am using textscan() to create an matrix that is 3,081,680 rows long. The very first column, A{1}, is the timestamp column, and it's elements look like this:
"2011-01-23 18:38:30"
I have tried using datenum() to convert this, but it is either taking too long or not working at all (longer than ten minutes). Is there a way I can make this more efficient?
Thanks!

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Cedric
Cedric 2013-10-29
编辑：Cedric 2013-10-29

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What is the purpose ultimately? Do you need an accurate time stamp which accounts for the date and time? If you were computing daily averages for 2011 only, for example, we could find a way to generate month/day IDs probably faster than DATENUM.
Otherwise, there are FEX contributions, e.g. this one by Jan Simon: http://www.mathworks.com/matlabcentral/fileexchange/28093-datestr2num
as well as a few relevant threads, among which: http://undocumentedmatlab.com/blog/datenum-performance/

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Marian
Marian 2013-10-30
The data I have is a text file containing meteorological data over a series of months. For this script in particular, I need to average one-minute data and output it into a new text file. The datenum has been the only issue I'm having.
Marian
Marian 2013-10-30
Sorry, I meant to add ... one-minute data for December 2011 only. So this text file contains multiple months over two years, and I will need to have the correct year, month, day, hour, minute, AND second. I will look into the links you posted, thank you.
Cedric
Cedric 2013-10-30
Ok, Kelley's suggestion is the first thing to try; if it's not fast enough, Jan's function on the FEX is certainly the second. I am unsure how you are binning the data then with the date serial number; if I had to do it, I would probably use DATEVEC, build a minuteID using year, month, day, hour, minute, and then use ACCUMARRAY to compute the average per minuteID.
Marian
Marian 2013-10-30
Ah, datevec! I have been teaching myself MATLAB and did not know of this function, but I believe that I needed datevec to read arrays, as datenum or datestr2num would not work for type "cell". Thanks so much!
Cedric
Cedric 2013-10-30
You're welcome. Let me know if you go for an ACCUMARRAY approach for binning and need help with that.

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Kelly Kearney
Kelly Kearney 2013-10-29

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Are you specifying the date format? Particularly for large arrays,
datenum(A{1}, 'yyyy-mm-dd HH:MM:SS')
will be much faster than
datenum(A{1})

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Cedric
Cedric 2013-10-29
+1 this is definitely the first thing to try.
Marian
Marian 2013-10-30
Error using datenum (line 179) DATENUM failed.
Error in onemin (line 9) time=datenum(A{1}, 'yyyy-mm-dd HH:MM:SS');
Caused by: Error using dtstr2dtnummx Failed on converting date string to date number.
This is the error I get using that, and I believe it is because of the double quotations around it.
Cedric
Cedric 2013-10-30
Use the following format: '"yyyy-mm-dd HH:MM:SS"'

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