Sunday evening, stupid games…

April 1, 2012

(This article was first published on Freakonometrics - Tag - R-english, and kindly contributed to R-bloggers)

This evening, while I was about to wash the dishes, I heard my elders starting a game (call them Him and Her)
Him: “I have picked – in my head – a number, lower than 50. Try to guess…
Her: “No way, too difficult…
Him: “You can try five different numbers…
Her: “.,. um … No, no way…
Me: “Wait… each time we suggest a number, you tell us if yours is either above, or below ?
You can see me coming clearly, can’t you ? Using a simple subdivision
rule, we have a fast algorithm (and indeed, if I have to choose between washing the dishes and
playing with the kids…)
Him: “um…. ok
Her: “Daddy, are you sure we will win ?
Me: “Well… I cannot promise that we will win… but I am rather sure [sic] that we will win quite frequently: more gains than losses” (I guess).
Her: “Great ! I am playing with daddy…

Him: “um….
wait, is it one of you trick, again ? I don’t to play anymore… Do you
want to see the books we’ve chosen at the library ?

Her: “Sure…
Me: “What ? no one wants to see if I was right ? that we have indeed more than 50% chances to win
Him and her: “No !” 
The point of that story ? If we listen to kids, science will not go
forward, trust me. But I am curious… I want to see if my intuition
was correct. Actually, the intuition was based on the fact that

> 2^5
[1] 32 
> 2^6
[1] 64

so in 5 or 6 steps the algorithm of subdivision should converge. I guess… I mean, I do not know for sure, since 50 is not a power of 2, so it might be difficult, each time, to split in two: we have to deal only with integers here…
To be sure, let us substitute my laptop to my son… to pick up numbers, randomly (yes, sometimes I feel like I am Doctor Tenma, 天馬博士).
The algorithm is simple: there are bounds, and at each stop I should suggest the
middle of the interval. If the middle is not an integer, I suggest
either the integer below or the integer above (with equal probabilities).

if(m %% 1 == 0){m=m}
if(m %% 1 != 0){m=sample(c(m-.5,m+.5),size=1)}

The following functions runs 10,000 simulations, and tells us how many times, out of 5 numbers suggested, we got the good one.

for(simul in 1:NS){
for(i in 1:tries){

It looks like the probability that we got the good number is higher than 60%,

> winning()
[1] 0.61801

Which is not bad. And if the upper limit was not 50, but something
else, the probability of winning would have been the following.


Actually, after losing a couple of times, I am rather sure that my son would have to us that we can suggest only four numbers. In that case, the probability would have been close to 30%, as shown on the blue curve below (where four numbers only can be suggested)

Anyway, as intuited, with five possible suggestions, we were quite likely to win frequently. Actually with a probability of almost 2 out of 3…and 1 out of 3 if my son had
decided to pick an number between 1 and 100, or only 4 possible suggestions… Those are quite large
actually, when we think about it. It reminds me that McGyver story I mentioned a few months ago… Anyway, calculating probabilities is nice, but I still have to wash the dishes…

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