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Topic: Random numbers
Replies: 64   Last Post: Dec 24, 2007 1:04 PM

 Messages: [ Previous | Next ]
 quasi Posts: 12,067 Registered: 7/15/05
Re: Random numbers
Posted: Dec 21, 2007 10:46 AM

On Fri, 21 Dec 2007 09:17:44 -0500, quasi <quasi@null.set> wrote:

>On Fri, 21 Dec 2007 08:13:37 -0500, quasi <quasi@null.set> wrote:
>

>>On 21 Dec 2007 14:53:38 +0200, Phil Carmody
>><thefatphil_demunged@yahoo.co.uk> wrote:
>>

>>>quasi <quasi@null.set> writes:
>>>> On 21 Dec 2007 14:33:08 +0200, Phil Carmody
>>>> <thefatphil_demunged@yahoo.co.uk> wrote:
>>>>

>>>> >John <iamachamp@gmail.com> writes:
>>>> >> Given a function that returns a random number between 1-5, write one
>>>> >> that returns a random number between 1-7 for the case when it should
>>>> >> be integer and for the case it can be real.

>>>> >
>>>> >int rand1to7i()
>>>> >{
>>>> > int r1=rand1to5i();
>>>> > if(r1==5) { r1+=rand1to5i()/2; }
>>>> > return r1;
>>>> >}

>>>>
>>>> Fatally flawed.

>>>
>>>Wrong. Care to back up your claim? Between you and me, I
>>>reckon you've not thought about my response enough.

>>
>>Yes, do a simulation, and then _you_ think about it.
>>

>>>> >double rand1to7d()
>>>> >{
>>>> > return rand1to5d()+(rand1to5d()-1)/2;
>>>> >}

>>>>
>>>> Inefficient -- uses 2 RNG calls, instead of 1.

>>>
>>>Wrong. If you stretch the numbers out, you'll leave gaps
>>>between them, these aren't actual reals, you did realise
>>>that, right? You *need* 2 calls to ensure that you leave
>>>no gaps.

>>
>>Forget gaps -- your method is biased!

>
>Ok, your method for the real case is not biased -- sorry.

No, I take it back -- it _is_ biased -- the geometry in R^2 makes that
very clear.

Of course, as you now claim, you didn't care about bias.

But yet you were careful to avoid inflating the gaps!

Bottom line -- both of the methods you proposed were biased. If you
really intended that (which is very doubtful), you should have at
least disclosed that fact.

quasi

Date Subject Author
12/21/07 Champ
12/21/07 quasi
12/21/07 quasi
12/21/07 Phil Carmody
12/21/07 quasi
12/21/07 quasi
12/21/07 Phil Carmody
12/21/07 quasi
12/21/07 Phil Carmody
12/21/07 Phil Carmody
12/21/07 quasi
12/21/07 quasi
12/21/07 Phil Carmody
12/21/07 quasi
12/21/07 Phil Carmody
12/21/07 Phil Carmody
12/21/07 quasi
12/21/07 Phil Carmody
12/21/07 quasi
12/21/07 Marshall
12/21/07 Phil Carmody
12/21/07 quasi
12/21/07 Phil Carmody
12/21/07 Marshall
12/21/07 briggs@encompasserve.org
12/21/07 William Elliot
12/21/07 quasi
12/22/07 William Elliot
12/21/07 Pubkeybreaker
12/21/07 b92057@yahoo.com
12/22/07 quasi
12/21/07 simple.popeye@gmail.com
12/21/07 simple.popeye@gmail.com
12/22/07 quasi
12/22/07 Gib Bogle
12/22/07 quasi
12/21/07 Marshall
12/22/07 simple.popeye@gmail.com
12/22/07 quasi
12/22/07 simple.popeye@gmail.com
12/22/07 quasi
12/22/07 quasi
12/22/07 quasi
12/22/07 simple.popeye@gmail.com
12/22/07 quasi
12/23/07 simple.popeye@gmail.com
12/23/07 simple.popeye@gmail.com
12/23/07 simple.popeye@gmail.com
12/23/07 simple.popeye@gmail.com
12/23/07 simple.popeye@gmail.com
12/22/07 simple.popeye@gmail.com
12/22/07 Herman Rubin
12/22/07 b92057@yahoo.com
12/22/07 quasi
12/23/07 b92057@yahoo.com
12/23/07 quasi
12/23/07 b92057@yahoo.com
12/24/07 quasi
12/24/07 quasi