MATLAB: How to extract rows by date

extract data with same variables

say I have a table A
18-Dec-2015 42.3000   29.9000    7.3000
18-Dec-2015 37.3000   30.1000   13.4000
18-Dec-2015 39.1000   30.0300    6.5000
18-Dec-2015 42.3000   29.9000    7.3000
19-Dec-2015 37.3000   30.1000   13.4000
19-Dec-2015 39.1000   30.0300    6.5000
20-Dec-2015 42.3000   29.9000    7.3000
20-Dec-2015 39.1000   30.0300    6.5000
20-Dec-2015 37.3000   30.1000   13.4000
I have almost 3000 days to do. How could I use a loop to extract rows by time and do some calculation separately? Thanks in advance!!! I tried ismember and the following one but neither works.
for i = (:,1)
if A(i,1)=A(i+1,1)
C=A(i,:)
...

Best Answer

It's not clear what you want to do, but, as Star-san mentioned, retime function would be your help. Let me start assuming your table A has 4 columns and the 1st column is stored as datetime, like:
>> A
A =
9×4 table
     Time       Var1    Var2     Var3
  __________    ____    _____    ____
  2015/12/18    42.3     29.9     7.3
  2015/12/18    37.3     30.1    13.4
  2015/12/18    39.1    30.03     6.5
  2015/12/18    42.3     29.9     7.3
  2015/12/19    37.3     30.1    13.4
  2015/12/19    39.1    30.03     6.5
  2015/12/20    42.3     29.9     7.3
  2015/12/20    39.1    30.03     6.5
  2015/12/20    37.3     30.1    13.4
To use retime function, first you should convert your table into "timetable" by using table2timetable function.
>> A = table2timetable(A);
>> A
A =
9×3 timetable
     Time       Var1    Var2     Var3
  __________    ____    _____    ____
  2015/12/18    42.3     29.9     7.3
  2015/12/18    37.3     30.1    13.4
  2015/12/18    39.1    30.03     6.5
  2015/12/18    42.3     29.9     7.3
  2015/12/19    37.3     30.1    13.4
  2015/12/19    39.1    30.03     6.5
  2015/12/20    42.3     29.9     7.3
  2015/12/20    39.1    30.03     6.5
  2015/12/20    37.3     30.1    13.4
Now you can use retime function to do some calculation separately for each day. For example, you can calculate daily average for each variable (Var1~Var3) in 1 line, like:
>> retime(A,'daily','mean')
ans =
3×3 timetable
     Time        Var1      Var2      Var3 
  __________    ______    ______    ______
  2015/12/18     40.25    29.983     8.625
  2015/12/19      38.2    30.065      9.95
  2015/12/20    39.567     30.01    9.0667
I hope my post somehow helps you !
Related Question