how can i select specific rows and sum the values later

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MAHMOUD ALZIOUD 2017-9-20

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评论： MAHMOUD ALZIOUD 2017-9-20

Hi all; i have a problem to select every row. for eg: i have matrix 2600x25. one of the columns is the day of data, 1 to 31 representing each day in the month, and each number is repeated around 95 times, i want to select data for example for Monday from this month, how can matlab know that this number represents Monday of the month in 2007? and how can i choose the whole Mondays rows for example in this month, the expected number of rows will be 4*95 = around 400 rows representing that specific day .

thanks a lot in advance if somebody can help me.

regards

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John Chilleri 2017-9-20

You might find the day function useful (can see documentation here). Otherwise, you can probably just identify the first Monday of 2007 and keep adding 7 using modulus for the months (brute force solution).

MAHMOUD ALZIOUD 2017-9-20

thank you very much i will give it a try now

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Answer 1

Walter Roberson 2017-9-20

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The following code assumes that you have a column that is just the day-of-month, with no column explicitly indicating month number or year number, and assumes that the way you know you have switched months is that the day number got smaller (e.g, 30 then 1)

   %initialize
   day_of_month_colnum = 7;    %which column holds the day number?
   startyear = 2007;           %year being implicitly discussed
   startmonth = 1;             %which month number does the data start with?
   %invent some data
   test_day_of_month = reshape( [repelem(1:31,3),repelem(1:28,3),repelem(1:31,3)], [], 1);
   YourData = rand(length(test_day_of_month), 25);
   YourData(:,day_of_month_colnum) = test_day_of_month;
   %start the work
   DoMcol = YourData(:, day_of_month_colnum);
   %calculate the relative month number for each entry
   relmonth = cumsum([0;diff(DoMcol)<0]);
   %build datevec information
   entry_date = zeros(length(relmonth), 3);
   entry_date(:,1) = startyear;
   entry_date(:,2) = startmonth + relmonth;
   entry_date(:,3) = DoMcol;
   %turn datevec into weekday number
   day_of_week = weekday( datenum(entry_date) );
   %now select the Monday data out of the original data. Weeks start with Sunday
   monday_data = YourData(day_of_week == 2, :);