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

• To: mathgroup at smc.vnet.net
• Subject: [mg106417] Re: [mg106404] Positions of earliest dates for each month in a list
• From: DrMajorBob <btreat1 at austin.rr.com>
• Date: Tue, 12 Jan 2010 04:47:21 -0500 (EST)
• References: <201001112352.SAA21004@smc.vnet.net>
• Reply-to: drmajorbob at yahoo.com

If myDates is sorted:

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}};
Accumulate@Prepend[Most@Tally[myDates[[All, {1, 2}]]][[All, 2]], 1]

{1, 2, 5, 8}

Otherwise, sort it first.

Bobby

On Mon, 11 Jan 2010 17:52:55 -0600, Garapata <warsaw95826 at mypacks.net>  
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.
>
> I've also tried creating an index for the dates like this:
>
> SplitBy[Transpose[Join[{myDates}, {Range[1, Length[myDates]]}]]];
>
> and hoped to use its output to sort or split the list into sublists of
> each month then just use Part to take the first date in each month
> with its paired index, but I still can't figure out how to do it.
>
> I've got the feeling I've missed a simple solution to this, any help
> much appreciated.
>


-- 
DrMajorBob at yahoo.com

• References:
  • Positions of earliest dates for each month in a list of dates
    • From: Garapata <warsaw95826@mypacks.net>