Re: Positions of earliest dates for each month in a list of dates

• To: mathgroup at smc.vnet.net
• Subject: [mg106416] Re: Positions of earliest dates for each month in a list of dates
• From: Bill Rowe <readnews at sbcglobal.net>
• Date: Tue, 12 Jan 2010 04:47:10 -0500 (EST)

```On 1/11/10 at 6:52 PM, warsaw95826 at mypacks.net (Garapata) wrote:

>I have a list of 20+ years of dates (from earliest to latest) in
>DateList format:

>A subset of the list should suffice to illustrate:

>myDates = {{1984,12,7,0,0,0}, {1985,1,3,0,0,0},{1985,1,4,0,0,0},
>{1985,1,7,0,0,0}
>,{1985,2,1,0,0,0},{1985,2,4,0,0,0},{1985,2,5,0,0,0},{1985,3,4,0,0,0}
>, {1985,3,5,0,0,0}, {1985,3,6,0,0,0},...};

>The list does not include all dates, weekends and it may not include
>some holidays or other seemingly arbitrary days.

>I need to find out the positions of the the first date appearing in
>each month.

>So, from the sample of myDates above I need to get:

>{1, 2, 5, 8}

>Corresponding to the positions for the dates:

>{{1984,12,7,0,0,0}, {1985,1,3,0,0,0}, {1985,2,1,0,0,0},
>{1985,3,4,0,0,0}}

>I've tried working with Position, Split, SplitBy, Ordering, Sort,
>and Sortby, but haven't made much progress.

Here is one way to achieve what you want using Position, Split
and Ordering

In[1]:= myDates = {{1984, 12, 7, 0, 0, 0}, {1985, 1, 3, 0, 0,
0}, {1985, 1, 4, 0, 0, 0}, {1985, 1, 7, 0, 0, 0}, {1985, 2,
1, 0,
0, 0}, {1985, 2, 4, 0, 0, 0}, {1985, 2, 5, 0, 0, 0}, {1985,
3, 4,
0, 0, 0}, {1985, 3, 5, 0, 0, 0}, {1985, 3, 6, 0, 0, 0}};

In[2]:= firstDays =
First /@ Split[myDates[[Ordering[AbsoluteTime /@ myDates]]],
AbsoluteTime[#1[[;; 2]]] == AbsoluteTime[#2[[;; 2]]] &]

Out[2]= {{1984, 12, 7, 0, 0, 0}, {1985, 1, 3, 0, 0, 0}, {1985,
2, 1,
0, 0, 0}, {1985, 3, 4, 0, 0, 0}}

an alternative to what I've done above would be:

firstDays=First/@ Split[SortBy[myDates, AbsoluteTime],
AbsoluteTime[#1[[;; 2]]] == AbsoluteTime[#2[[;; 2]]] &]

Once you have the first day for each month, finding the position
in the original list is easily done by:

In[3]:= Flatten[Position[myDates, #] & /@ firstDays]

Out[3]= {1,2,5,8}

```

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