How to pull specific indices out of a character array in a loop?

I have an array that contains multiple dates in the format yyyymmdd, stored as a 50x1 double. I am trying to pull out the year,month, and day so I can use datenum to assign each date a serial number.

Indexing an individual date, converting the using str2num, then indexing and pulling the appropriate values works fine, but when I try to loop through the list of dates it doesn't work- only variations of the number 2 are returned.

dates = [20180910; 20180920; 20181012; 20181027; 20181103; 20181130; 20181225];

% version1
datesnums=num2str(dates); % dates is a list of dates stored as 
integers

for i=1:length(datesnums)
    pullyy=str2num(datesnums(1:4));
    pullmm=str2num(datesnums(5:6));
    pulldd=str2num(datesnums(7:8));
end

As well as

%version2
datesnums=num2str(dates,'%d')

for i = 1:length(datesnums)
    dd=datenum(str2num(datesnums(i(1:4))),str2num(datesnums(i(5:6))), 
    str2num(datesnums(i(7:8))));
end

I'm trying to generate a new array that is just the serial numbers of the input dates. In the examples shown, I am only getting single integer values, which I know is because the loop is incorrect and I get errors that say "Index exceeds the number of array elements (1)." for version 1. When I've gotten it to successfully loop through everything, the outputs are just '2222','22,'22' for every single date which is incorrect. What am I doing wrong? Do I need to incorporate a cell array?

Solution

To get all the years, month, and days in a loop:

datesnums=num2str(dates);

for i=1:size(datesnums, 1)
    pullyy(i) = str2num(datesnums(i,1:4));
    pullmm(i) = str2num(datesnums(i,5:6));
    pulldd(i) = str2num(datesnums(i,7:8));
end

Actually, you can do this without a loop:

pullyy = str2num(datesnums(:,1:4));
pullmm = str2num(datesnums(:,5:6));
pulldd = str2num(datesnums(:,7:8));

Explanation:

If for example the dates vector is a [6x1] array:

dates =[...
    20190901
    20170124
    20191215
    20130609
    20141104
    20190328];

Than datesnums=num2str(dates); creates a char matrix of size [6x8] where each row corresponds to one element in dates:

datesnums =

  6×8 char array

    '20190901'
    '20170124'
    '20191215'
    '20160609'
    '20191104'
    '20190328'

So in the loop you need to refer to the row index for each date and and the column indices to extract the years, month, and days.