# Probability and Coin Tossing

Updated 0 Tallied Votes 2K Views

Here's an interesting little scenario.

You get challenged to a game of coin toss. Both players pick a sequence of coin toss results (heads or tails). The coin is repeatedly tossed until the sequence for one of the players occurs. The loser pays the winner a dollar. You'd assume that you each have an equal chance of winning. However, using two little "tricks" you can tip the odds in your favour. The first trick is to always choose second. If you can manage this then your strategy is as follows:

Your first choice should be the opposite of your opponent's second choice.
Your second and third choices should be the same as your opponents first two choices.

The top three buttons are for player A, the bottom three for player B. You can set the choices manually by clicking the buttons. If you click on "Choose B for me" then player B will be set to the choices based on the above strategy. Each time you click "Start" a new game is played with the coin tosses and game results displayed in the listbox.

For the GUI, the top three buttons are named `Button1`, `Button2` and `Button3`. The bottom three are `Button4`, `Button5`, and `Button6`. The remaining three controls are (buttons) `btnChoose`, `btnStart`, and (listbox) `lbxResults`.

``````Public Class Form1

Private aWon As Integer = 0     'number of games won by player A
Private bWon As Integer = 0     'number of games won by player B

'Assign common handler to toggle choice buttons

For i As Integer = 1 To 6
Dim btn As Button = Me.Controls("Button" & i)
Next

End Sub

Private Sub btnStart_Click(sender As System.Object, e As System.EventArgs) Handles btnStart.Click

'Repeat coin tosses until one player wins

Dim rnd = New Random()
Dim ht() As Char = {"H", "T"}
Dim res(2) As Char

'Seed with three initial tosses

For i As Integer = 0 To 2
res(i) = ht(rnd.Next(0, 2))
Next

Dim done As Boolean = False

Do Until done

lbxResults.Items.Add(res(0) & "-" & res(1) & "-" & res(2))

Select Case True
Case CheckWin(1, res)
aWon += 1
done = True
Case CheckWin(4, res)
bWon += 1
done = True
End Select

res(0) = res(1)
res(1) = res(2)
res(2) = ht(rnd.Next(0, 2))

'Keep most recent results in view (scroll to bottom)

lbxResults.SelectedIndex = lbxResults.Items.Count - 1

Loop

Me.Text = "A - " & aWon & " games    " & "B - " & bWon & " games"

End Sub

Private Sub Button_Click(sender As System.Object, e As System.EventArgs)

'Toggle H or T in a choice button

Dim btn As Button = sender
btn.Text = IIf(btn.Text = "H", "T", "H")

End Sub

Private Sub btnChoose_Click(sender As System.Object, e As System.EventArgs) Handles btnChoose.Click

'Set player B to optimal choices

Button4.Text = IIf(Button2.Text = "H", "T", "H")    'Opposite of player A 2nd choice - Feb 22 corrected typo Button3 -> Button2
Button5.Text = Button1.Text                         'Same as player A 1st choice
Button6.Text = Button2.Text                         'Same as player A 2nd choice

End Sub

Private Function CheckWin(start As Integer, seq() As Char) As Boolean

'Check if the next 3 buttons match the given sequence

For i As Integer = 0 To 2
Dim btn As Button = Me.Controls("Button" & i + start)
If btn.Text <> seq(i) Then Return False
Next

Return True

End Function

End Class``````
Teme64 215

When tossing a coin we can (and will) make following assumptions:
1. the result will allways only be tails or heads
2. the coin is fair i.e. the probability of tails is the same as heads, P(T) <=> P(H)
3. the coin tossing is stateless operation i.e. the coin does not and can not "remember" last result
4. from the previous assumptions follows that given any sequence of coin tossing results, the next toss has the probability P(T) <=> P(H)

Let's say that player A chooses a sequence T, T and T. Tossing this sequence has, according classic probability theory, likelihood P(T) * P(T) * P(T), (or 1/2 * 1/2 * 1/2 = 1/(2^3) = 1/8 because P(T) <=> P(H) and the coin is fair as stated above).

Using the given strategy player B chooses a sequence: H, T and T. Tossing this sequence has the probability
P(H) * P(T) * P(T) or 1/2 * 1/2 * 1/2 = 1/(2^3) = 1/8 which is exactly the same as the probability of the player A's sequence.

From this follows that
1. using the strategy, how to choose the sequence, does not increase the probability of that sequence
2. to always choose second does not help i.e. increase the probability of the sequence

Am I right?

Reverend Jim 3,956

I don't know the math, but by running the code you can see that player B will win either by a little or by a lot. You can modify the code, for example, so that each click on Start plays 100 or 1000 games and checks the results.

Reverend Jim 3,956

It's one of those problems where the "obvious" solution is not the correct solution. Take the following problem, for example. Let's assume, for the moment, that the Earth is perfectly round and flat. Imagine that you could build a solid ring that touches the surface at every point along the equator. The ring would be (if Google is correct) 40,075 km long. Now let's imagine that you want to enlarge the ring so that it is one meter above the surface at every point. Without doing the calculation, approximately how much material (length) would you have to add to the ring? would you guess 100 meters?, 1000 meters?, 10,000 meters? The actual answer is just over 6 meters. It doesn't seem logical but that's the answer you get if you do the math. Your common sense tells you one answer and the math, another.

Picture it this way. Imagine the Earth is a cube whose perimiter is 40,075 km, If you cut the ring at the corners and lift the four segments one meter up, all you would have to add is an arc of material at the four corners. The arc has a radius of one meter and the four corners form a circle of diameter 2 meters. The circumference of that circle is 2 * 3.1415 or just over 6 meters. But the earth isn't a cube. Well then, make it an octagon (in profile). The segments you have to add still combine to make a circle. This is true no matter how many sides you use.

JamesCherrill 4,577

Being deeply sceptical about this, and agreeing with Teme64, I hacked together a little Java version of the logic (no GUI needed), ran 10000 iterations with all 8 possible "A" choices and guess what - absolutely no consistent or significant difference in the number of wins. Here's the code in case I made a mistake...

``````public class CoinTosser {

public static void main(String[] args) {

ThreeTosses first, second;

for (int i = 0; i <= 7; i++) { // try all 8 possible choices
first = new ThreeTosses((i & 1) == 1, (i & 2) == 2, (i & 4) == 4);

second = first.optimumResponse();

int firstWins = 0, secondWins = 0;

for (int j = 0; j < 10000; j++) {
ThreeTosses random = ThreeTosses.newRandomResult();
if (random.equals(first)) {
firstWins++;
}
if (random.equals(second)) {
secondWins++;
}
}
System.out.print(first + " vs " + second);
System.out.println(" : " + firstWins + "  " + secondWins);
}
}

}

class ThreeTosses {

private static java.util.Random r = new java.util.Random();

final public boolean a, b, c;

ThreeTosses(boolean a, boolean b, boolean c) {
this.a = a;
this.b = b;
this.c = c;
}

public static ThreeTosses newRandomResult() {
return new ThreeTosses(r.nextBoolean(), r.nextBoolean(), r.nextBoolean());
}

public ThreeTosses optimumResponse() {
return new ThreeTosses(!b, a, b);
}

public boolean equals(ThreeTosses other) {
return a == other.a && b == other.b && c == other.c;
}

@Override
public String toString() {
return a + "," + b + "," + c;
}
}
``````
Reverend Jim 3,956

There was a small typo in my code (corrected in the original post). In the button_Choose handler the line should be

``````Button4.Text = IIf(Button2.Text = "H", "T", "H")    'Opposite of player A 2nd choice
``````

I had the comment right but made a typo in the code. `Button3` should have been `Button2`.

@James - try your code again with the corrected line. My results show an advantage for player B. Running a thousand trials with each configuration I get the following results

`````` A   B  AWIN BWIN
HHH THH  232  768
HHT THH  348  652
HTH HHT  401  599
HTT HHT  444  556
THH TTH  378  622
THT TTH  377  623
TTH HTT  325  675
TTT HTT  235  765
``````
JamesCherrill 4,577

I don't think I had that particular bug in my code anyway, and here are my reuslts for 1000 trials of each

false,false,false vs true,false,false = 120 vs 122
true,false,false vs true,true,false = 125 vs 126
false,true,false vs false,false,true = 126 vs 137
true,true,false vs false,true,true = 133 vs 106
false,false,true vs true,false,false = 130 vs 146
true,false,true vs true,true,false = 122 vs 123
false,true,true vs false,false,true = 141 vs 114
true,true,true vs false,true,true = 109 vs 124

With 3 coins you would expect a 1/8 hit rate against any given choice, as seen above.
Your results show vastly too many hits. No amount of majic will make three tosses come up THH 768/1000 times. Sorry, but I'm sure you have a bug somewhere.
Anyone else try this?

<EDIT>

Sorry - I misunderstood your definition of "trial". Here's a typical run usig your def'n

``````T,T,T vs H,T,T = 504 vs 496
H,T,T vs H,H,T = 483 vs 517
T,H,T vs T,T,H = 513 vs 487
H,H,T vs T,H,H = 477 vs 523
T,T,H vs H,T,T = 519 vs 481
H,T,H vs H,H,T = 490 vs 510
T,H,H vs T,T,H = 495 vs 505
H,H,H vs T,H,H = 516 vs 484
``````

supporting code (re-written) follows

``````public class CoinTosser {

public static void main(String[] args) {

ThreeTosses first, second;

for (int i = 0; i <= 7; i++) { // try all 8 possible choices
first = new ThreeTosses((i & 1) > 0 ? Coin.H : Coin.T,
(i & 2) > 0 ? Coin.H : Coin.T,
(i & 4) > 0 ? Coin.H : Coin.T);

second = first.optimumResponse();

int firstWins = 0, secondWins = 0;

while (firstWins + secondWins < 1000) {
ThreeTosses random = ThreeTosses.nextRandom();
if (random.equals(first)) {
firstWins++;
}
if (random.equals(second)) {
secondWins++;
}
}
System.out.print(first + " vs " + second);
System.out.println(" = " + firstWins + " vs " + secondWins);
}
}

enum Coin {

H, T;
private static java.util.Random r = new java.util.Random();

public static Coin nextRandom() {
return r.nextBoolean() ? H : T;
}

public Coin flip() {
return (this == H) ? T : H;
}
}

static class ThreeTosses {

final public Coin a, b, c;

ThreeTosses(Coin a, Coin b, Coin c) {
this.a = a;
this.b = b;
this.c = c;
}

public static ThreeTosses nextRandom() {
return new ThreeTosses(Coin.nextRandom(), Coin.nextRandom(), Coin.nextRandom());
}

public ThreeTosses optimumResponse() {
return new ThreeTosses(b.flip(), a, b);
}

public boolean equals(ThreeTosses other) {
return a == other.a && b == other.b && c == other.c;
}

@Override
public String toString() {
return a + "," + b + "," + c;
}
}
}
``````
Reverend Jim 3,956

I don't speak java (luddite, I suppose). In case there is a misunderstanding about the tosses, when I do the tosses I don't generate three new tosses at each pass. i throw out the oldest toss and generate one new toss. I start by seeding three tosses (1,2,3) and compare those to both players. Then I generate a new toss (2,3,4) and compare those, then (3,4,5), (4,5,6), etc.

It looks to me that you generate three new tosses on every turn which would signficantly change the parameters of the game.

The idea (I suppose, from a street grifter point of view) is to present a game of chance that appears to have even odds but which is weighted in favour of one player.

JamesCherrill 4,577

Ah, yes. I was generating 3 new random each time. I;ll try shifting them as you described and report back...

JamesCherrill 4,577

OK, I tried generating each trial as (sorry, did some renaming since last code posted)

``````        ThreeCoins nextIncrementalRandom() {
return new ThreeCoins(b, c, Face.nextRandom());
}
``````

and got the same results - no statistical difference at all.

We desparately need some whone who speaks both languages so they can spot what we're doing differently.

JamesCherrill 4,577

OK, got the answer. You have to rotate the throws as you describe until someone wins, then throw a complete new set of 3. With those rules I get

``````T,T,T vs H,T,T = 122 vs 878
H,T,T vs H,H,T = 309 vs 691
T,H,T vs T,T,H = 336 vs 664
H,H,T vs T,H,H = 275 vs 725
T,T,H vs H,T,T = 263 vs 737
H,T,H vs H,H,T = 354 vs 646
T,H,H vs T,T,H = 349 vs 651
H,H,H vs T,H,H = 144 vs 856
Final score: A won 2152, B won5848

public class CoinTosser {

public static void main(String[] args) {

int aTotal = 0, bTotal = 0;

for (ThreeCoins playerA : ThreeCoins.allPossibleCombinations()) {

ThreeCoins playerB = playerA.optimumResponse();

int aWins = 0, bWins = 0;

ThreeCoins random = ThreeCoins.nextRandom();

boolean weHaveAWin = false;
while (aWins + bWins < 1000) {
random = random.nextIncrementalRandom();
if (random.equals(playerA)) {
weHaveAWin = true;
aWins++;
}
if (random.equals(playerB)) {
weHaveAWin = true;
bWins++;
}
if (weHaveAWin) {
random = ThreeCoins.nextRandom();
weHaveAWin = false;
}
}
System.out.print(playerA + " vs " + playerB);
System.out.println(" = " + aWins + " vs " + bWins);
aTotal += aWins;
bTotal += bWins;
}
System.out.println("Final score: A won " + aTotal + ", B won" + bTotal);
}

enum Face {

H, T;

private static java.util.Random r = new java.util.Random();

static Face nextRandom() {
return r.nextBoolean() ? H : T;
}

Face flip() {
return (this == H) ? T : H;
}
}

static class ThreeCoins {

final Face a, b, c; // immutable class

ThreeCoins(Face a, Face b, Face c) {
this.a = a;
this.b = b;
this.c = c;
}

static ThreeCoins[] allPossibleCombinations() {
ThreeCoins[] all = new ThreeCoins;
for (int i = 0; i <= 7; i++) {
all[i] = new ThreeCoins((i & 1) > 0 ? H : T,
(i & 2) > 0 ? H : T,
(i & 4) > 0 ? H : T);
}
return all;
}

static ThreeCoins nextRandom() {
return new ThreeCoins(Face.nextRandom(), Face.nextRandom(), Face.nextRandom());
}

ThreeCoins nextIncrementalRandom() {
return new ThreeCoins(b, c, Face.nextRandom());
}

ThreeCoins optimumResponse() {
return new ThreeCoins(b.flip(), a, b);
}

boolean equals(ThreeCoins other) {
return a == other.a && b == other.b && c == other.c;
}

@Override
public String toString() {
return a + "," + b + "," + c;
}
}
}
``````
JamesCherrill 4,577

I think I know why as well....

A is looking for a,b,c
B is looking for -b,a,b
the trials are rotating as described.

For A to win you need to get a,b,c
The previous try will have been x,a,b - which B has a 50/50 chance of matching.
Ie for any sequence that is a match for A, B has a 50/50 chence of matching it on the previous turn.

Conversely for B to win you need to get -b,a,b
The previous try will have been x,-b,a - which A has zero chance of winning.

Its the rolling "random" rolls, combined with A= a,b,x and B= y,a,b that gives B the edge in matching on the turn before A can match, but not ViceVersa.

Very neat.

Teme64 215

The origin of the problem is the Random object itself. Suppose our Random object outputs only 0's and 1's (just like tossing a coin gives only heads and tails). Ideally Random object would somehow output a sequence that meets the conditions

1. probability of 0 is equal to the probability of 1 ("fair coin")
2. given any output sequence of 0's and 1's the probability of getting next a '0' is equal to the probability of getting a '1'. That is, knowing any number of previously outputted 0's and 1's does not change the predictability of the next output. On the other words, Random object does neither "remember" what the previous output was nor does it use any previous output to generate next output. This would make Random object stateless (like a coin).

So, how does this Random object "produce" these random 0's and 1's? The answer is: it does not produce anything random. It produces pseudo-random 0's and 1's.

The computers are deterministic like they should be. Algorithms ("computer programs") are deterministic by definition. This is why our Random object is also deterministic and not "random" in statistical sense. When someone wrote the actual code for our Random object, he or she chose an algorith that "mimics" randomness i.e. the outputted values have as uniform distribution as possible and knowing any number of previously outputted values keeps the predictability of the next output as small as possible.

This applies to C#, Java or any programming language which has some sort of Random object or Random function. The best you can get is pseudo-random numbers.

Choosing our three heads and tails, like in the first post, may give a slightly better chance to win. But that is the result of the pseudo-randomness, the algorith used in the Random object or there may even be a bug in the Random object's code. Changing the code to Java simply changes "unfair coin" to another "unfair coin".

JamesCherrill 4,577

I'm sorry, but the "random" functions in major languages are not so terrible that they would give such significant non-random results. That's a complete red herring.

Please re-read my previous post. The first triple is random but the following sequence of triples is very highly corrolated. Any triple a,b,c will be followed by a triple b,c,x. This scam works by exploiting the correlation between successive trys. Because of the carefully chosen difference between A's and B's triples player B has a 50/50 chance of matching A's sequence one turn before A sees it, but A can never match B's sequence before B sees it.

So here are the expected odds:
A's sequence will appear 1 in 8 tries, but half of those will be captured by B in the previous round, so that's 1/16 for A and 1/16 for B
B's sequence will appear 1 in 8 tries, but all of these will be captured by B

So we would expect A to win 1/16 of the time, amd B to win 3/16.
Now here's my result from a run of 1,600,000 tries (200,000 each of A's possible sequences)

``````T,T,T vs H,T,T = 4999 vs 35027
H,T,T vs H,H,T = 20068 vs 40281
T,H,T vs T,T,H = 16418 vs 33462
H,H,T vs T,H,H = 11256 vs 33399
T,T,H vs H,T,T = 10956 vs 33051
H,T,H vs H,H,T = 16633 vs 33187
T,H,H vs T,T,H = 20056 vs 39915
H,H,H vs T,H,H = 5077 vs 35144
Final score: 1600000 tries, A won 105463, B won283466
``````

That's very slightly better for A than my analysis, but the reason is probably that my analysis doesn't apply to the first one or two tries after a win (because then there's a new random triple, and the odds are 1/8 for both players).

Reverend Jim 3,956

When I test the random number generation (for 0, 1) against ten million tosses I compare the number of each result and see only (approximately) a 0.02% difference. In any case, as long as neither player is aware of any inbuilt bias toward either a heads or tails then both players are playing by the same rules. Therefore the difference in the probability of a win for A or B is not due to a problem with the random number generation.

@James - thanks for the analysis

The generation of random numbers is too important to be left to chance.
—Robert R. Coveyou

xrjf 213

I think the non random result is due to the fact that the random is defined inside a sub procedure and not a class level. The only way to create a random sequence is to define the random variable just once in all the analysis. Then you may play 100, 1000 times but all of the time with the same random seed.

JamesCherrill 4,577

No. In all the programs above there is only one instance of Random per run.
Even if there were multiple instances the reults would only be non-random if the instances were created with explicitly the same seed.

This is a known scam, see my earlier post for a full description on how it works, but in brief: the individual tosses are random, but the consecutive sequences abc, bcd, cde etc are highly correlated and that's the basis of the non-random behaviour.

xrjf 213

Defining more than one instance has probability non-zero to repeat an earlier seed from a previous instance. Let's say there are 100 diferent possible seeds, every 200 tosses -more or less- there'll be a great probability to repeat every seed twice -more or less. For me this escenario is not a random sequence.

JamesCherrill 4,577

100 possible seeds? Where did you get that? Java.util.Random uses a `long` value for its seed - something like 10^20 possible seeds.
The API doc for the no-seed constructor says, with typical mathematical care

This constructor sets the seed of the random number generator to a value very likely to be distinct from any other invocation of this constructor.

(Having said that, at least one version of the API source code that I looked at used the system time in mSec as the seed - which on some hardware could be the same value for up to 1/60 sec. In which case I agree there is a very significant risk of two instances in the same run sharing a seed.)

We agree that having more than one instance is a bad idea. But in any case. all the code samples in this thread do use a single instance for each run, so any debate about multiple instances, although interesting, is irrelevant.

AssertNull 1,094

Your first choice should be the opposite of your opponent's second choice.

Not sure if that was part of the "typo", but the description does not match the graphic in post 1: "THT" (graphic) in response to "HTH". By the description, you'd pick "HHT" in response to "HTH".

I imagine randomly generating trials is counterproductive here. This is more of a Combinatorics/Infinite Series problem where you need to find the patterns. That's my take from at least partially pencil-and-papering. Randomness tands to hide the actual ratios involved with certain strategies.

If you go the Computer Science route, I think you'd do it as a state machine. Again, no random numbers: Game start, H, T, HH, HT, TH, TT, Player 1 win, player 2 win. Game start can go to H or T, H can go to HH or HT, T can go to TH or TT, and then for each two letter state (representing the last two flip results), it can either go to a winning/end state or another two letter state, possibly including itself. Given that you are in a certain state, you can figure out the probability of ending up in any other state. For "HHH" vs. "THH", it's quckly apparent that if you're ever in "T" or "HT" or "TH", player 2 is guaranteed to win. I imagine once you start looking at the graphs, no program is needed as there are very few states to keep track of.

Teme64 was correct in his first explanation of the probabilty, so one mark up. The theory has been proven an can be assumed to be true. What then follows is that any computer program that tries to mimick reality has to come as close as can be to that reality. So if this is not achieved, the programming is faulty or the random number generator is not good enough. In this case I reckon that the program is not good enough because each coin toss is independent of the previous result and also independent of the following result.

JamesCherrill 4,577

Teme's first post was based on an incorrect understanding of the process being modelled here.
There's nothing wrong with the programs, or the pseudo-random number generators.
This is simply a trick, based on the fact that each set of three throws contains two results from the previous set. It uses that knowledge to skew the results. See my posts or AssertNull's for more details.

xrjf 213

The unique problem is a typo in line#42 where it says `Case CheckWin(4, res)` it should say `Case CheckWin(3, res)`, because button3 is the first button for player B, isn't it?

xrjf 213

Sorry, I was wrong `Case CheckWin(4,res)` is right. Any way, I can't see why player B after several hits approches a probability p=7/40, when one could expect p=1/8=1/2 x 1/2 x 1/2 < 7/40.

AssertNull 1,094

As this is a programming forum, it may be useful to consider the problem in terms of regular expressions containing h's and t's. A monkey is typing on a two key keyboard. Player 1 and player 2 each come up with a no wildcard length 3 regular expression to find the first match of the monkey's writing. Earliest match wins. As with all games, player 2 has to decide whether to make his play independent of player 1 or can he exploit the knowledge of player 1's play, a potential advantage player 1 does not have? Consider that last sentence. It's key.

Thinking about it this way gives it a Computer Science rather than Math feel. There are many ways to do the problem, but I threw Reg Exp out there for the Comp Sci folks who don't like Game Theory or Graph Theory.

xrjf 213

Splitting up `btnStart_Click()` as shown surprisingly the probability for both players tends to 1/8. Now, the difference is in that when a player wins `res()` is not initialized again.

``````    Private Sub btnStart_Click(sender As System.Object, e As System.EventArgs) Handles btnStart.Click
Dim res(2) As Char

'Seed with three initial tosses
'
For i As Integer = 0 To 2
res(i) = ht(rnd.Next(0, 2))
Next

Dim sRepeat As String = InputBox("# of turns: ", "", 100)
If IsNumeric(sRepeat) Then
For i = 0 To Int32.Parse(sRepeat)
play(res)
Next
End If
End Sub
Function ht(r As Int32) As Char
Static ht1() As Char = {"H", "T"}
Return ht1(r)
End Function
Sub play(res() As Char)

'Repeat coin tosses until one player wins
'

Dim done As Boolean = False

Do Until done

'lbxResults.Items.Add(res(0) & "-" & res(1) & "-" & res(2))

If CheckWin(1, res) Then
aWon += 1
done = True
ElseIf CheckWin(4, res) Then
bWon += 1
done = True
End If

total += 1

res(0) = res(1)
res(1) = res(2)
res(2) = ht(rnd.Next(0, 2))

'Keep most recent results in view (scroll to bottom)

'lbxResults.SelectedIndex = lbxResults.Items.Count - 1

Loop

Me.Text = "A - " & aWon & " games" & (aWon / total) & "    " & "B - " & bWon & " " & (bWon / total)

End Sub``````
JamesCherrill 4,577

@xrj: Because, as I have already explained, only one of the tosses is random. The other 2 are brought forward from the previous pass and are known

O.K than we go back to part of the initial statement:
"You get challenged to a game of coin toss. Both players pick a sequence of coin toss results (heads or tails). The coin is repeatedly tossed until the sequence for one of the players occurs."

Here it states that a coin is reapetedly tossed which means that for each toss a random number must be newly initiated. Since Jim's code doesn't do this means the programming is not refleting the requirement as stated and hence the code is wrong not addressing the requirement.

Maybe the initial statement should have been, " Use my code for the game and follow the instructions and you will always win"

xrjf 213
``````If player A plays THH then player B choose is TTH.
If first toss is T-c2-c3 let's see what happens depending on c2, c3 and the
next toss c4.

T(H=c2) | (H=c3) A player wins | H=c4 none wins (although this will
|                      | not happen because res() will be
|                      | initialized)
|                      |
|                      | T=c4 none wins (same as before)
|                      |
| (T=c3) no winner

T(T=c2) | (H=c3) B wins        | H=c4 (A wins, combination=THH but
|                      | this win is sidestep because B won
|                      | and it'll never arrive here)
|                      | T=c4 (THT, no winner)
|                      |
| (T=c3) no winner     | H=c4  (TTH, B wins)
|                      | T=c4  (TTT, no winner)

So, B wins two times while A player wins once.``````
AssertNull 1,094

Minimalist,

Your posting style strikes me as unnecessarily strident and doesn't add to what should be the goal of the thread: everyone walks away with a better understanding and everyone's happy. If you choose to be snarky and/or use the "You're wrong" throw-down, at least make sure that YOU are right and YOU understand the problem. You clearly don't. Teme64 and James Cherrill didn't initially understand the problem, but they expressed themselves in a way that invited conversation, clarification, and mutual understanding. Your posts seem designed to shut conversation down.

1. to always choose second does not help

Teme64 was incorrect here. Things were clarified. That's the way it's supposed to work. Teme64 laid out his understanding of the problem in a way that was easy for everyone to check if they had the same understanding and conclusions and ended with "Am I right?". I thought it was a well-written post.

He wasn't right. Not a problem. But compare his posting style to yours, a year later, AFTER some clarification occurred and some tests run that, if they didn't PROVE that the second person had an advantage, at least gave strong evidence that he/she did. You'd have to have a TERRIBLE RNG or an extreme anomaly to get the results JC got and not believe the second player has an advantage.

Teme64 was correct in his first explanation of the probabilty, so one mark up. The theory has been proven an can be assumed to be true. What then follows is that any computer program that tries to mimick reality has to come as close as can be to that reality. So if this is not achieved, the programming is faulty or the random number generator is not good enough. In this case I reckon that the program is not good enough because each coin toss is independent of the previous result and also independent of the following result.

You don't leave a lot of room for anyone to explain how you are misinterpreting the problem and your "not good enough" phrasing comes off (to me) as in-your-face as opposed to "RJ, I think there is a problem here in your code". You don't have the "Am I right?" and "I'm skeptical" tone that Teme64 and JC had in their early posts. Those invite clarification and reconsidering from RJ and leave open the possibility that RJ MIGHT actually be right. Your wording does not, and suggests that you're not open to the possibility that YOU'RE the one who might not get it.

JC pointed out that Teme64 misunderstood the probblem originally. That's not a dig at Teme64. Just a fact. You doubled down on your earlier attitude. In particular, this line...

Maybe the initial statement should have been, " Use my code for the game and follow the instructions and you will always win"

NO ONE, especially the OP, thinks or ever thought that anyone will "always win the game". No one could interpret RJ's post, with the acknowldged error (no big deal, everyone makes them. You correct them and move on) as saying that. Why are you writing this?

I haven't run RJ's code, but it sure looks like he has a Random Number Generator in there and is generating random numbers? If you want to argue that the RNG is not a good one, make that argument. If there's a bug in his code where th RNG is being bypassed on accident, by all means point that out.

Are you confident you understand this problem? Are you open to the idea that you do not? How do you want people to interpret your posts?

From my point of view, I'm still trying to determine the optimal strategy and to prove that it's the optimal stratgegy, as well as generalize it and derive the exact odds. Will post if I have time and actually "solve" it.

Be a part of the DaniWeb community

We're a friendly, industry-focused community of developers, IT pros, digital marketers, and technology enthusiasts meeting, learning, and sharing knowledge.