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        riddles >> medium >> Random numbers
        
(Message started by: Altamira_64 on Jul 29^th, 2013, 6:39am)

Title: Random numbers
Post by Altamira_64 on Jul 29^th, 2013, 6:39am

Get two random numbers between 0 and 1. Subtract the smaller from the bigger. What is the probability that the result is <0.3? Is it bigger than the probability it is > or equal to 0.3?
For what number the probability for both cases is the same?

Title: Re: Random numbers
Post by towr on Jul 29^th, 2013, 8:56am

Nicely chosen parameters. But easy enough when you draw it.
[hide]You have a strip along the diagonal of the square where the difference is less than 0.3, leaving two triangles with side 0.7 at opposite corners, which together add to .7² = .49 chance that the difference is greater than 0.3 So the chance of <0.3 is only just greater than >0.3, by 2%[/hide]

Title: Re: Random numbers
Post by Altamira_64 on Jul 29^th, 2013, 11:06am

Can you draw this?

Title: Re: Random numbers
Post by towr on Jul 29^th, 2013, 11:27am

See attachment.

Title: Re: Random numbers
Post by Altamira_64 on Jul 29^th, 2013, 11:59am

Great, thanks!

Title: Re: Random numbers
Post by EdwardSmith on Jul 16^th, 2014, 2:16am

begin
for i := 1 to bignumber do
begin
a:= random(1);
b:= random(1);
if a > b then
c:= c+(a-b)
else
c :=c+(b-a);
end;
average := := c/bignumber;
if average < 0.3 then
writeln("yes")
end;

*** error unknown variable on line 2 ***

Title: Re: Random numbers
Post by towr on Jul 16^th, 2014, 9:04am

Is that pascal?

In any case, the answer should be a probability, not "yes" or nothing.

Title: Re: Random numbers
Post by Grimbal on Jul 16^th, 2014, 9:34am

A pascal program should start with "program <name>;". But maybe that is optional.

I think it is meant as pseudo-code. The variables are not declared, c is not initialized and there is a suspicious double :=.

Anyway, the program computes the average distance between the 2 random numbers, not how often that difference is larger than 0.3. So it definitely doesn't answer to the question.

Title: Re: Random numbers
Post by towr on Jul 16^th, 2014, 10:31am

on 07/16/14 at 09:34:59, Grimbal wrote:

I think it is meant as pseudo-code.

Yeah, but then what's up with the "*** error unknown variable on line 2 *** ", that makes me think some compiler or interpreter threw an error.

Quote:

Anyway, the program computes the average distance between the 2 random numbers, not how often that difference is larger than 0.3. So it definitely doesn't answer to the question.

Maybe if you also had the variance. :P

Title: Re: Random numbers
Post by Grimbal on Jul 17^th, 2014, 3:12am

Yes, the error message also bothered me.
It looks like Edward used an IDE to type it but did not go as far as making it a valid program.