How to locate the index of a certain date for a date-time array?

Hello,

I have a date-time array, 'dt' that is recorded hourly for a whole year.

The first three data points are as follows, 01-Jan-2017 00:00:00, 01-Jan-2017 01:00:00, 01-Jan-2017 02:00:00.

I would like to find the index of every Monday(at 00:00:00) present in the data set . Below is what I have used, this is not feasable as I would need to know the exact date of every Monday, type it out row by row and save it to my matrix m.

m=zeros(52,1)

m(1)=find(datetime=='02-Oct-2017 00:00:00')

m(2)=find(datetime=='09-Oct-2017 00:00:00')

...

m(52)=find(datetime=='25-Dec-2017 00:00:00')

Is there a better way to do this?

What I am trying to achieve with this is to create new arrays that start on a Monday and last for a whole week, and do this for the span of a whole year.

I tried to use

[week,edges]=discretize(dt,"week")

but it does not start on a Monday, so I was not able to get it to work.

Thank you

Answer 1

[Stephen23]

You are working with datetimes, so do not compare text!

dt = datetime(2017,1,1,(0:1:123).',0,0, 'Format','u MM dd eee HHmmss')

dt = 124×1 datetime array
   2017 01 01 Sun 000000
   2017 01 01 Sun 010000
   2017 01 01 Sun 020000
   2017 01 01 Sun 030000
   2017 01 01 Sun 040000
   2017 01 01 Sun 050000
   2017 01 01 Sun 060000
   2017 01 01 Sun 070000
   2017 01 01 Sun 080000
   2017 01 01 Sun 090000
   2017 01 01 Sun 100000
   2017 01 01 Sun 110000
   2017 01 01 Sun 120000
   2017 01 01 Sun 130000
   2017 01 01 Sun 140000
   2017 01 01 Sun 150000
   2017 01 01 Sun 160000
   2017 01 01 Sun 170000
   2017 01 01 Sun 180000
   2017 01 01 Sun 190000
   2017 01 01 Sun 200000
   2017 01 01 Sun 210000
   2017 01 01 Sun 220000
   2017 01 01 Sun 230000
   2017 01 02 Mon 000000
   2017 01 02 Mon 010000
   2017 01 02 Mon 020000
   2017 01 02 Mon 030000
   2017 01 02 Mon 040000
   2017 01 02 Mon 050000

ix = timeofday(dt)==0 & weekday(dt)==2

ix = 124×1 logical array
   0
   0
   0
   0
   0
   0
   0
   0
   0
   0
   0
   0
   0
   0
   0
   0
   0
   0
   0
   0
   0
   0
   0
   0
   1
   0
   0
   0
   0
   0
   0
   0
   0
   0
   0
   0
   0
   0
   0
   0
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   0
   0

md = dt(ix)

md = datetime
   2017 01 02 Mon 000000

[Stephen23]

"What I am trying to achieve with this is to create new arrays that start on a Monday and last for a whole week, and do this for the span of a whole year."

I would have answerd that, but your explanation is incomplete. It sounds as if you want to split your data into lots of separate arrays, but doing so is

• not a good use of MATLAB,
• means that you cannot use the inbuilt mehods for grouping and processing groups of data:

https://www.mathworks.com/help/matlab/matlab_prog/perform-calculations-by-group-in-table.html

https://www.mathworks.com/help/matlab/matlab_prog/grouping-variables-for-splitting-data.html

https://www.mathworks.com/help/matlab/matlab_prog/split-data-into-groups-and-calculate-statistics.html

Instead of asking us about how to split data into lots of arrays, you should actaully ask us about your actual goal or task with that data:

https://xyproblem.info/

https://meta.stackexchange.com/questions/66377/what-is-the-xy-problem

[Sevde Tavasli]

Thank you for your help, I thought I knew what I needed to do to analyse my data but turns out I did not.

The method you stated above worked to solve what I was trying to do in the first part of my question, but you are right, from reading the attached help links what I really want to do is perform calcualtions by using groups in my table.

The links you attached were very helpful and have meant that I have been able to look at the data and perform calculations based on the day of the week as well as weeks!

Answer 2

[Arif Hoq]

I don't have your data. try this one:

% creating datetime data from 1st January 2017 to 31 December 2017
firstdate = '01-01-2017 00:00:00';
t1 = datetime(firstdate,'InputFormat','dd-MM-yyyy HH:mm:ss');
lastdate='31-12-2017 23:00:00';
t2 = datetime(lastdate,'InputFormat','dd-MM-yyyy HH:mm:ss');
oneyeardata=t1:+hours(1):t2;

realdate=oneyeardata(:);
dayname=day(realdate,'name');
strname=string(dayname);
dateday=table(realdate,strname);
findmonday=find(strcmp(string(table2cell(dateday(:,2))),"Monday"));
idxmonday=dateday(findmonday,1);
aa=string(table2cell(idxmonday));
bb=split(aa," ");
cc=find(strcmp(bb(:,2),'00:00:00'));
Monday_Only=idxmonday(cc,1);