[Gllug] Algorithm question for your Friday morning

[salsaman]

salsaman wrote:

> Adrian McMenamin wrote:
>
>> Inspired by an article in the latest edition of Linux Journal (NB you
>> really should subscribe - exchange rate makes it about 50% cheaper than
>> buying in on the news stand) on top (1)
>>
>> I have some code that currently takes an arithmetic mean of people's
>> preferences/votes. But it occurs to me that it would be better to 
>> replace
>> that with some sort of algorithm that weights historical votes less than
>> more recent votes.
>>
>> (I believe top(1) uses an exponential decay function)
>>
>> Is there a simple way to do this when the only record I currently 
>> have of
>> historic votes is the arithmetic mean?
>>
>>  
>>
> The algorithm is not difficult. Basically you currently calculate the 
> mean value as:
>
> (a0+a1+a2+a3+a4+...an) / n = mean
>
> What you want to do is to weight current votes more than past votes.
>
> You can do this on a daily basis for example.
>
> From the equation above:
>
> mean * n = (a0+a1+a2+...an)
>
> Simply halving n will give today's votes twice as much effect as 
> yesterday's votes, four times as much as the day before, etc.
>
> Of course, halving n might be too much, you could use some other 
> factor, e.g.
>
> 1/sqrt(2) will give today's votes twice as much effect as those from 2 
> days ago and 4 times as much as from 4 days ago. Etc.
>
>
> Gabriel.
>
Hmmm...maybe not so easy, because then the range is wrong.


More thought is needed.

Gabriel.

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