# hpr1143 :: The N Days of Christmas? Intro to Recreational Math

### Intro to Recreational Math Part Zero

Hosted by Charles in NJ on 2012-12-19 is flagged as Explicit and is released under a CC-BY-SA license.
Tags: mathematics,"calendar counting".
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```Hacker Public Radio: 206 203 5729

The N Days of Christmas? Intro to Recreational Math
Part Zero: Calendar Counting

First episode of HPR that contains a direct discussion of a math topic.
- Episode 479 Ohio Linux Fest, Klaatu interviews DWick about math
software for Linux

- Episode 523 Using Petunia software to teach math

Inspired by a traditional song that is proof that some songs do not
need to be recorded by William Shatner to be annoying.
- Repetitive and formulaic
- Involves a lot of counting, and that's our focus here.

What is the 12 Days of Christmas?
- Starts on Christmas Day, runs through the day before the next Season
- Hint: That's 'Epiphany', which starts January 6.
- Counting calendar days comes hard, so we tend to use our fingers
- Turns out that using our fingers is quite mathematical. Here's why.

Finger Counting: How do I count Twelve Days?
- Let's start easy, with the fingers on one hand. My hands have five.
- To name the Five Days of New Years is easy: January 1-5 <done>

- What about the Five Days of Christmas?
Physical way                   General way
* Christmas Day gets 1 (thumb)     Dec 25 is one day after Dec 24
* Dec 26 gets 2 (index)            26 - 24 = 2 days
* Dec 27 gets 3 (salute finger)    27 - 24 = 3 days
* Dec 28 gets 4 (ring)             28 - 24 = 4 days
* Dec 29 gets 5 (pinky)            29 - 24 = 5 days

- Notice that counting 5 days, starting with Dec 25, is the same
as numbering the days after Dec 24 (Christmas Eve).
* In math, we call this "1-1 correspondence with natural numbers"
* Math can give you the same certainty as using your fingers.
* But it handles larger problems, because you don't run out.

- Example: I'm booked to speak on Day 4 of a 5-day conference
* Starts on the 25th of the month
* When do I have to show up?
- Wrong: Add 4 to first day (25), and arrive a day late.
- Correct: Add 4 to date of pre-registration cocktail party (24),
and arrive on time.

- OK. Back to Twelve Days of Christmas.
* The labeling approach tells us that December can hold only the
first seven of the Twelve Days of Christmas,
* December 31 - December 24 gives me 7 days.

- How do we handle the case where we go into the next month?
- Key insight: Running out of December days for the Twelve Days is
like running out of fingers on one hand when we count to 8.
- We are so good at counting on our fingers that we don't recognize
the act of partitioning the number 8 between our two hands.
*  Left hand gets 1, 2, 3, 4 and 5.
*  Right hand picks up 6, 7 and 8 by mapping them to fingers 1,2,3.

- To count even higher, we could:
1) keep borrowing other people's hands, or
2) track the number of times we reuse our two hands as we go

* First method mirrors calendar math ("Annexing" hands, or months)
* Second is positional notation ("base 10" and all that)

Back to the Twelve Days
- I have Twelve Days: 1, 2, ... 12 to assign to dates, even though I
may only be interested in the first and last dates right now.
* Start: How many can I fit into December?
* December 31st is last. It gets assigned 31 - 24, or 7.
* By "finger math", that means I have mapped 7 of the Twelve Days
* That leaves 12 - 7, or 5 days into January.

- Who can tell me which days are assigned in January? Anyone?
* That's right, Ken.  January 1, 2, 3, 4 and 5.
* So the Twelve Days of Christmas runs 25 December to 5 January

Question: What if there were 72 Days of Christmas?  When would it end?
- Note: Don't worry.  This is purely hypothetical.

- Let's attack this with finger math, with partitioning and annexing
* December, as we have seen, accounts for 7 days: 25 through 31
* That leaves 72 - 7, or 65 days
* January easily picks up 31 days: 1 to 31, leaving 65 - 31 = 34 days
* February can handle either 28 days, or 29 on a leap year.
* This leaves us either 5 or 6 days into March

- Final Answer: 72 Days of Christmas would run from Christmas until the
following March 5 (leap year), or March 6 (all other years).
* On Day 73, everyone would enter treatment for Christmas overdose.

Let's check the answer: Day 72 would end ten weeks and 2 days after
the opening cocktail party (Monday). So Day 72 should be Wednesday.
* Next year is not a leap year, so last day is March 6.
* By the Doomsday perpetual calendar method, Feb 28 is Thursday.

Doomsday method: http://en.wikipedia.org/wiki/Doomsday_rule

* So March 7 is Thursday, and March 6 is Wednesday.
* It worked.

Why should I bother with Calendar Math?
- I learn to look for ways to partition hard problems into easier ones.
- I learn the same skills that I'll need to debug "off-by-one" errors
and other boundary violations, which kill you in C programs.
- I will never miss a speaking engagement, as long as I count my
Conference Days from the cocktail party, not from the Keynote.

Next episode: Part One
Counting partridges and gold rings with Pascal
- Warning: There will be two semi-magic formulas at the end.
- I'll show you an easy way to do running sums in a spreadsheet.
- You can skip the formulas, and I'll never know.
- Since this is HPR, not school.  We can look up the formulas.

Contact: Charles in NJ
Email: catintp@yahoo.com

Charlie + Alpha + Tango + India + November + Tango + Papa.

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