## Friday Puzzle #7

20 10 2011

A train approaches an observer at the speed of 100 km/h.  The observer tosses a tennis ball at the speed of 100 km/h towards the train.  What will be the speed and direction of the tennis ball after it has bounced off of the collision with the train.  All idealizations apply (bouncing consumes no kinetic energy; front of train is flat and vertical; friction is ignored; train/ball weight ratio is practically infinite; etc.)

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## Solution To Friday Puzzle #6

9 10 2011

Let black squares have value 1 and white squares have value -1. The entire chessboard has now turned into a base -1 multiplication table of its first row and first column. For example, if a white first row square (having value -1) contains a pawn, and another first column square that is black (having value 1) contains a pawn, then the square on the intersectiong row and column will have value 1*-1 = -1 and hence will be white. Try it with other color combinations as well!

Creating the diagonally symmetric square pattern described in the puzzle is therefore equivalent to computing the square – the 2nd power – of the number designated by the first row. If the first row has 2 blacks and 4 whites then its value is 2*1 + 4*-1 = -2 , the square of which is 4 .  Effecticely this means that four more black squares (+1) are covered than white square (-1) by the complete pattern. Note that the 2nd power can never be negative which means that there can never be more white squares covered than black squares.

So the question of the smallest board containing a difference that is a prime numer is equivalent to asking for the smallest 2nd power of an integer that is a prime, which obviously does not exist. Hence my intended answer was that there is no solution.

However, one reader pointed out to me offline that a square with 1 pawn in (0,0) should qualify as an answer. Intriguing! Some googling quickly revealed that there have been different definitions historically of what is a prime number. The best explanation is provided by Wikipedia:

Most early Greeks did not not even consider 1 to be a number, so clearly did not consider it a prime. In the 19th century however, many mathematicians did consider the number 1 a prime. For example, Derrick Norman Lehmer’s list of primes up to 10,006,721, reprinted as late as 1956, started with 1 as its first prime. Henri Lebesgue is said to be the last professional mathematician to call 1 prime. Although a large body of mathematical work is also valid when calling 1 a prime, the fundamental theorem of arithmetic does not hold as stated. For example, the number 15 can be factored as 3 · 5 or 1 · 3 · 5. If 1 were admitted as a prime, these two presentations would be considered different factorizations of 15 into prime numbers, so the statement of that theorem would have to be modified.

For the reason stated in the last sentence of the above quotation (i.e. preserving unique factoring) the contemporary definition of a prime number explicitly excludes 1 from the set of primes. But because I failed to specify the definition of primes relevant to this puzzle, I feel compelled to accept her solution as a correct one.

## Friday Puzzle #6

6 10 2011

A chess board has 8*8 squares with alternating black/white colors. The rows are labeled 1-8 and the columns are labeled A-H. The square A1 is black.

However, for the purpose of this week’s friday puzzle, let’s re-label the rows and columns with numbers starting from zero. Then, square (0,0) is black, as well as (1,1), (2,2), etc. Also, the chess board is not restricted to 8*8 squares, it can be arbitrarily large (yet finite), or N*N in size.

Then place pawns on the board according to the following rules:

• Square (0,0) contains a pawn
• If and only if square (0,X) contains a pawn, then square (X,0) contains a pawn
• If and only if both squares (R,0) and (0,S) contain a pawn, then square (R,S) contains a pawn

When the pawns are correctly configured the total number of pawns will be a square number, and they will form a diagonally symmetric pattern. Some pawns will occupy a white square and others will occupy a black square. Call their difference the target.

The puzzle: What is the smallest board where a pattern can be created whose target is a prime number?

## Solution To Friday Puzzle #5

3 10 2011

In short, the answer is Yes.  A 2*164 box can contain 329 bottles. I won’t dig into the math, but instead I’ll offer a link to Wolfram Demonstrations where the solution is nicely visualized.

Happy work week everyone!

## Friday Puzzle #5

29 09 2011

A bottle has diameter of 1 unit.  A box having size 2*2 units can contain four bottles at most.  A box having size 2*3 units can contain six bottles at most. A box having size 2*4 units can contain eight bottles at most. Et cetera…

Is there a box having size 2*N units that can contain more than 2*N bottles? Can you suggest an upper limit for N? Can you compute and prove (or at least justify) a minimum value for N?

## Factoring Integers In O(1) Time

27 09 2011

They say that factoring integers is hard. Well, what do they know – this article presents a program which takes an integer (at compile time) and prints its factors in O(1) time. If the number has multiple factors then just one pair will be printed. If the number is prime, then the number itself, times one, will be printed.

Naturally it is impossible to compute factors of an integer in constant time. The trick used here is called Template Metaprogramming. Template metaprogramming is essentially creating a set of C++ templates which encode a program. Algorithms can be represented by a handful of template metaprogramming idioms – there are idioms for conditionals, recursion, expression evaluation, data passing, and more – and if the template is instantiated anywhere in the code, the metaprogram is executed by the compiler at compile time. At this point it is worth noting that template metaprograms cannot contain mutable values; therefore it is impossible to metaprogram regular index-based loops, but recursion must be used instead. This means that trees can be more economical data structures than linear arrays or lists, because the latter needs much more “stack space” (=simultaneously instantiated template objects) than e.g. tree. Anyway, in order to create a program which apparently factors integers in constant time, a vastly greater compilation time must have passed to compute the actual result, but the result is eventually stored into the program code as a plain old constant value.

The most canonical factoring algorithm is trial division. However, in this article I’ve decided to use the Unusual Recursive Factoring Algorithm instead because it uses more economical tree traversal, as opposed to indexed loop needed in trial division. Alas, my fear was unfounded: it seems that the modern compilers (I used MS Visual C++ Express 2010) no longer have limitations to the number and/or depth of template instantiations which they had when I first tried this code for real.

Anyway, here’s the code.

## Solution To Friday Puzzle #4

26 09 2011

Reader yrpe was the first to post the correct answer to Friday Puzzle #4.  This is their answer, slightly paraphrased:

Given any two cards there is exactly one card that competes the set, so the answer is 1/79 .

## Friday Puzzle #4

22 09 2011

Set is a card game requiring good perception and quick thinking. It is played with a deck of 81 special cards. Each card has four properties, and every property has three different values. There is exactly one card for every combination of property values, hence the size of the deck is 3^4 = 81. A “set” is three cards for which all four properties have either the same value, or all have a different value. A more elaborate description of the game can be found here.

What is the probability that three randomly chosen Set cards form a “set”?

## Swapping Without Temporary Storage

21 09 2011

Swapping two variables typically uses a temporary variable. But how do you swap if you are short of storage space? For instance, suppose you have your home keys in your left hand, and a grocery bag in your right hand. If you’re right-handed you’ll need to swap the keys to the other hand in order to open the door, but how do you do that with no temporary storage around?

The following algorithm is perhaps not the most efficient one but it fits the bill perfectly!

template <typename Thing> void Swap( Thing &oneHand,Thing &otherHand )
{
try
{
throw oneHand;
}
catch( Thing falling )
{
oneHand = otherHand;
otherHand = falling;
}
}

Again, the complete code can be found here.

N.B. This article was written tongue firmly in cheek. If you read the code like a story it should appear humorous, but in real world I do not advocate using exceptions as a mechamism for program flow control.

## Solution To Friday Puzzle #3

19 09 2011

John: “Hi, Mary, how’s it going?”

Mary: “Fine, thanks. Have you been thinking of that spinner problem someone blogged about on Friday?”

John: “Tell you what, I was just going to ask you the same. You wanna try solving it using the Monte Carlo method?”

Mary: “Monte who?

John: “Heehee…  Monte Carlo method is an experimental way to get approximate solutions to problems about probabilities. You repeat an experiment over and over and over, and make notes about the outcomes. When you have repeated the experiment a great many times you will have a pretty good idea of the probability you are trying to figure out. For instance, if you toss a coin ten times you may get 7 heads and 3 tails, but if you toss the same coin 10,000 times you may get 5,014 heads and 4,986 tails. The ratio will converge to 1/2 just like it should if the coin is good.”

Mary: “All right, let’s try it out. Hey, wait a minute, we don’t have any axles or blades to make spinners out of. How you gonna experiment with a device with… no device?”

John: “Mind you, I’ve thought of that, too! See, here are three pencils. Two of them are green and one is red. If the axles are called A and B, and the point at which the blades are pointing to is X, then the distance |AB| =1 must be greater than both of the distances |AX| and |BX| or else the blades can’t overlap.  In other words, the simulated blades will overlap if and only if the length |AB| is the longest side of the triangle ABX.”

Mary: “A-ha, I see now: I’ll toss the pencils on the floor, the pencils will define three straight lines, and those three lines together will define a random triangle.”

John: “Exactly! And next consider the red pen as the base between the axles and the green pens as the blades, then the experiment is successful every time when the red pen defines the longest side of the resultant random triangle. Count successes and divide by total numer of experimentations and voila, Monte Carlo method in action! Let’s start, toss the pencils, Mary.”

Mary: “But…”

John: “Yes I know we have a long task ahead of us so lets just start.”

Mary: “But…”

John: “Please, Mary, the longer we argue the longer it takes to find out the approximate solution. You do want to find it out, don’t you?”

Mary sighs and tosses the pencils.

John: “Well?”

Mary: “Well…”

John: “Well?”

Mary: “Well… ummm… errr… I am sorry. I was trying to tell you that I am color blind. I can’t tell apart red and green, but I can say that because there are three pencils, the red pencil defines the longest side of the triangle with probability 1/3.”

John is at loss of words in face of Mary’s spontaneous insight.

So the answer to Friday Puzzle #3 is that the blades overlap with probability P=1/3. Also, my apologies for failing to supply the solution by Sunday like promised.