how to find the index of same datetime value

Question

uzzi 2022-11-7

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评论： Lei Hou 2022-11-18

采纳的回答： Khushboo

I have two tables of different sizes of datetime

t = 1251x1 datetime

v = 115x1 datetime with the format of YYYY-MM-DD hh:mmm:ss.SSS as attached below.

I'd like to compare datetime (hh:mmm:ss) without considering .SSS and find the index of the same compared values. Can someone help me with this?

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Stephen23 2022-11-10

编辑：Stephen23 2022-11-10

"I'd like to compare datetime (hh:mmm:ss) without considering .SSS and find the index of the same compared values."

Then use DATESHIFT and ISMEMBER. No loops are required.

DO NOT convert to text and perform text comparisons in complicated nested loops.

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Answer 1

Khushboo 2022-11-7

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Hello,

You can use datestr to convert the datetime to a string (it handles the .SSS part). I am not sure if there is any existing function that can compare different rows of different tables to find rows with identical value in a column. So this is what I have done, I ma sure more efficient solutions might exist:

%assuming the two tables are A,B with first column having the datetime value
A.t = datestr(A.t); 
B.t = datestr(B.t);
h1 = height(A);
h2 = height(B);
for i = 1:h1
    for j = 1:h2
        if strcmp(A{i,1},B{j,1}) == 1
            i,j
            break
        end
    end
end

Hope this helps!

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uzzi 2022-11-7

Hello Khushboo,

I'll give you more details about it tmr.

Lei Hou 2022-11-18

dateset is now in discourage use. You can use [h,m,s] = hms(datetime) to extract time components and then call floor(s) to remove .SSS data.

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Answer 2

Steven Lord 2022-11-7

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Truncate the datetime to the previous whole second then compare.

d = datetime('now');
d.Format = d.Format + ".SSSSSS"
d = datetime
   07-Nov-2022 13:52:16.687821
d2 = dateshift(d, 'start', 'second')
d2 = datetime
   07-Nov-2022 13:52:16.000000
d3 = dateshift(d, 'start', 'minute')
d3 = datetime
   07-Nov-2022 13:52:00.000000

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uzzi 2022-11-7

I am sorry. I don't understand your answer. Can you elaborate more, please?

Steven Lord 2022-11-7

在 MATLAB Online 中打开

Let's say I want to check if right now is 6:13 PM (and some number of seconds.) In the example below even though d has those "some number of seconds" d3 trims off those seconds. d is not equal to d4, but d3 is.

d = datetime('now');
d.Format = d.Format + ".SSSSSS"
d = datetime
   07-Nov-2022 18:13:15.396629
d2 = dateshift(d, 'start', 'second')
d2 = datetime
   07-Nov-2022 18:13:15.000000
d3 = dateshift(d, 'start', 'minute')
d3 = datetime
   07-Nov-2022 18:13:00.000000
d4 = datetime(2022, 11, 7, 18, 13, 0)
d4 = datetime
   07-Nov-2022 18:13:00
isequal(d4, d3)
ans = logical
   1
isequal(d4, d)
ans = logical
   0