Not generating the same random number twice....

[Ewoud Dronkert]

Re: Not generating the same random number twice....

On Tue, 14 Jun 2005 08:35:44 GMT, nemo wrote:[color=blue]
> All these much more sophisticated solutions make me wonder where I went
> wrong in the early days.
> $Query="select image_filename from Tablename order by rand() limit 5";[/color]

1. His image filenames are not in a db :)
2. Order by rand potentially causes enormous overhead. See for example
http://jan.kneschke.de/projects/mysql/order-by-rand/ (performance data
at bottom of page).


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[Geoff Berrow]

Re: Not generating the same random number twice....

I noticed that Message-ID: <av4ta1h08up98h ikop6q8jldgsiho bqkpn@4ax.com>
from Ewoud Dronkert contained the following:
[color=blue]
>On Mon, 13 Jun 2005 23:54:08 +0100, Geoff Berrow wrote:[color=green]
>> for($i=1;$i<50; $i++){
>> $balls[]=$i;
>> }[/color]
>
>See http://php.net/range[/color]

Code is from a course I run and it's a good idea to get the students
used to using loops[color=blue]
>[color=green]
>> for($i=0;$i<6;$ i++){
>> $key=array_rand ($balls);
>> $ballschosen[]=$balls[$key];
>> unset($balls[$key]);
>> }[/color]
>
>See http://php.net/shuffle[/color]

For some reason I've never been able to get shuffle to work properly.[color=blue]
>[color=green]
>> asort($ballscho sen);[/color]
>
>Why use asort when you don't use the keys? See http://php.net/asort[/color]

Seemed like a good idea at the time :)
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Geoff Berrow (put thecat out to email)
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[Dominik Susmel]

Re: Not generating the same random number twice....

Ewoud Dronkert wrote:[color=blue]
> On Mon, 13 Jun 2005 23:58:32 +0200, Dominik Susmel wrote:[color=green]
>> - seed the rand[/color]
>
> Not needed as of PHP4.2.0[/color]

true...I don't know what he has
[color=blue][color=green]
>> - put nubers in array with array_rand[/color]
>
> That is not how array_rand() works, please see
> http://php.net/array-rand[/color]

duh...he said he has numbers? thus, populate the array and randomize it and
trim it with _rand.. sorry, I thought it was obvious
[color=blue][color=green]
>> - use array_unique to initialize duplicate entries with 0[/color]
>
> Duplicate entries?! Bad algorithm. Also, that is not how
> array_unique() works, please see http://php.net/array-unique[/color]

that is how it works - check for yourself.. duplicates are not removed (if
he has any - and shouldn't if he populates from db..)
if you go through the array with it, keys are not removed - they are simply
reinitialized ..look at your link that you've posted by yourself
[color=blue]
>[color=green]
>> - do some magic to 0 numbers in contrast to existing stuff in array[/color]
>
> This is really helpful.[/color]

yes, indeed it isn't - but that's only if he gets duplicate id's from db (if
I remember correctly - he needs to randomize sequence from db?)...and if he
gets duplicates from this..then magic is what is needed.. some form of
deletion operations

--
--
Dominik Susmel | art director
www.vongestern.com | vonGestern art company . Zagreb, Croatia

[Dominik Susmel]

Re: Not generating the same random number twice....

Ewoud Dronkert wrote:[color=blue]
> On Mon, 13 Jun 2005 23:58:32 +0200, Dominik Susmel wrote:[color=green]
>> - seed the rand[/color]
>
> Not needed as of PHP4.2.0
>[color=green]
>> - put nubers in array with array_rand[/color]
>[/color]

ok, I see he doesn't have image file names in db... so forget about key, val
combo ;)

--
Dominik Susmel | art director
www.vongestern.com | vonGestern art company . Zagreb, Croatia