# Locate indices of datetime from one table in another?

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BR on 4 Mar 2022
Edited: BR on 7 Mar 2022
Hello,
I have to loacte indices in the date column in a table that looks like this (Table 1)
from a table that looks like (Table 2)
Given that the Table 2 (day_time_date) has categorical values whereas the Table 1 has datetime values. The reason it is categorical is because I have used
groupsummary
to summarise the dates from Table 1 which removed the timestamps as well. Thanks

Peter Perkins on 4 Mar 2022
1) It looks like you must have use "day" as an input to groupsummary, which returns a categorical as the group values:
>> tt = timetable(datetime(2022,3,randi([1 3],10,1)),rand(10,1))
tt =
10×1 timetable
Time Var1
____________________ ________
01-Mar-2022 00:09:43 0.45054
01-Mar-2022 00:47:39 0.083821
02-Mar-2022 00:18:40 0.22898
03-Mar-2022 00:31:42 0.91334
03-Mar-2022 00:09:56 0.15238
01-Mar-2022 00:36:07 0.82582
02-Mar-2022 00:15:46 0.53834
02-Mar-2022 00:39:14 0.99613
01-Mar-2022 00:41:21 0.078176
02-Mar-2022 00:44:53 0.44268
>> groupsummary(tt,"Time","day","mean")
ans =
3×3 table
day_Time GroupCount mean_Var1
___________ __________ _________
01-Mar-2022 4 0.35959
02-Mar-2022 4 0.55153
03-Mar-2022 2 0.53286
>> class(ans.day_Time)
ans =
'categorical'
Categorical is what those "convenience" flags like "day" do. But you don't have to use that. The simplest way to get the group values as datetimes is like this:
>> tt.Day = dateshift(tt.Time,'start','day')
tt =
10×2 timetable
Time Var1 Day
____________________ ________ ___________
01-Mar-2022 00:09:43 0.45054 01-Mar-2022
01-Mar-2022 00:47:39 0.083821 01-Mar-2022
02-Mar-2022 00:18:40 0.22898 02-Mar-2022
03-Mar-2022 00:31:42 0.91334 03-Mar-2022
03-Mar-2022 00:09:56 0.15238 03-Mar-2022
01-Mar-2022 00:36:07 0.82582 01-Mar-2022
02-Mar-2022 00:15:46 0.53834 02-Mar-2022
02-Mar-2022 00:39:14 0.99613 02-Mar-2022
01-Mar-2022 00:41:21 0.078176 01-Mar-2022
02-Mar-2022 00:44:53 0.44268 02-Mar-2022
>> groupsummary(tt,"Day","mean")
ans =
3×3 table
Day GroupCount mean_Var1
___________ __________ _________
01-Mar-2022 4 0.35959
02-Mar-2022 4 0.55153
03-Mar-2022 2 0.53286
>> class(ans.Day)
ans =
'datetime'
2) I'm confused. None of the "dates" in day_time_date appear in time_date.
Your question might be, "which rows of time_date correspond to each row of day_time_date?" That's easy, once you have tt.Day:
ismember(tt.Day,day_time_date)
BR on 7 Mar 2022
Edited: BR on 7 Mar 2022
Incredible, very keen insights. Much appreciated and I do apologise for my late gratefulness. :-)
To answer your confusion, I agree they aren't very clear but first few dates are there from day_time_date that appear in time_date (08-Jun-2020/ 09-Jun-2020). But, as you clearly shown me the solution in your first point saves the day using ismember then. Thanks

KSSV on 4 Mar 2022
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KSSV on 4 Mar 2022
You can convert them into same class..whats the problem?