MATLAB: How can i change the precision using datenum

datasetdatenumintersectprecision

Im using datenum on some datasets to try to see the intersection between the data (using intersect function), the problem is that im getting different times with the same time (the last few numbers are a little bit different). I already saw some people using a different aproach but i don't know how can i do it with my data. Im posting the code im using:
    horas = datestr(horas);
    datas = datestr(datas);
    timesteps = [num2str(datas) num2str(horas(:,12:17))];
    timesteps = datenum(timesteps,'dd-mmm-yyyy HH:MM');
    [c,ia,ib]=intersect(timesteps,time_interval);
    hm0(ib) = hm0;
It should work but since it's not getting all the times, it fails and tells me "In an assignment A(:) = B, the number of elements in A and B must be the same.". But if i can solve the early problem it will work since i already used this function before. Thank you

Best Answer

The basic problem is that intersect tests for exact equivalency of floating point numbers (which is all that datenum's are). This should be avoided, for reasons that have been discussed thousands of times before (see the link in my comment). Here are some proposals for how to deal with this:
  • Round the datenum to some number of decimal places. This sounds simple, but it can introduce artifacts into the data, and requires picking an appropriate number of digits to round to. A bit fiddly.
  • Download and use my FEX submission dateround, which rounds to the nearest second, minute, hour, etc.
  • Convert to datevec, pick the units/column you want to use (e.g. years to minutes), and then use intersect with the 'rows' option.
Personally I prefer the last solution: neat, intuitive, and does not involve comparing floating point numbers. Here is a complete working example, which demonstrates how differences in the seconds can be ignored:
>> C1 = {'2018-10-29 12:34:56','2018-10-29 12:00:00'};
>> C2 = {'2018-10-29 12:34:55','2018-10-29 12:01:00'};
>> V1 = datevec(C1(:))
V1 =
   2018     10     29     12     34     56
   2018     10     29     12      0      0
>> V2 = datevec(C2(:))
V2 =
   2018     10     29     12     34     55
   2018     10     29     12      1      0
>> intersect(V1(:,1:5),V2(:,1:5),'rows')
ans =
   2018     10     29     12     34
Note that this is equivalent to doing a floor operation on the seconds column: to round the seconds up/down to the nearest minute you could either use dateround or do the rounding yourself.
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