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How Can I avoid a loop for in a table?

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Adriano on 22 Dec 2016
Commented: KSSV on 23 Dec 2016
Hi everyone,
I have a table called 'export' with more than 2000000 rows. One of its column, called ' Expiration', is a dates vector. I woul like to avoid the loop for in this code:
for i = 1:size(export,1)
if weekday(export.Expiration(i,1))==7
export.Expiration(i,1) = cellstr(datetime(datenum(export.Expiration(i,1))-1,'ConvertFrom','datenum','Format','MM/dd/yyyy'));
Can somebody help me? Many thanks.
Guillaume on 22 Dec 2016
Indeed an example would have made things clearer, however the format can be inferred from the given code. export.Expiration is a column of date strings in the form 'MM/dd/yyyy'
The purpose of the loop is to change the date to the day before when it is the last day of the week.

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Accepted Answer

Guillaume on 22 Dec 2016
Edited: Guillaume on 22 Dec 2016
An example of the data and an explanation of what you want to achieve would indeed have been useful. From your code, it would appear that the dates are stored as date strings and that if the date is the last day of the week, you want to make it the day before.
That is trivial to achieve. The exact equivalent of your code without a loop is simply:
wday = weekday(export.Expiration);
export.Expiration(wday == 7) = cellstr(datetime(datenum(export.Expiration(wday == 7) - 1, 'ConvertFrom', 'datenum', 'Format', 'MM/dd/yyyy'));
However, I would recommend that you actually store your date as datetime, which would make the date manipulation much easier and avoid this triple conversion to date number -> datetime -> date string:
export.Expiration = datetime(export.Expiration, 'InputFormat', 'MM/dd/yyyy', 'Format', 'MM/dd/yyyy');
islastday = day(export.Expiration, 'dayofweek') == 7;
export.Expiration(islastday) = export.Expiration(islastday) - 1;
I assume you're aware that in matlab, the last day of the week is Saturday. Otherwise you'll have to change the 7 to 1, if you meant to find the Sundays.
  1 Comment
Peter Perkins
Peter Perkins on 22 Dec 2016
One small tweak:
export.Expiration(islastday) = export.Expiration(islastday) - caldays(1);
With no time zone, it won't matter, but it is more readable, and it will continue work correctly if time zones are added and the day happens to fall on a Daylight Saving shift.
Another possibility would be
export.Expiration(islastday) = dateshift(export.Expiration(islastday),'dayofweek','saturday','previous');

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More Answers (1)

KSSV on 22 Dec 2016
You can convert your table into cell using table2cell and access complete row/column you want.
KSSV on 23 Dec 2016
That's true....but if any head lines of the table present, they do come along with row/ column. Isn't it?

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