Long running loop and set_time_limit

kanuj

I'm supposed to pick out a winner from a list of members who are entered into a database. The thing is, that this thing is supposed to be fair, or something. The person i'm making this for, wants a random fraction between 0 and 1, multiplied by the highest ID in the members table. Whatever. Anyway, he wants the winner selection function to be called as long as a unique winner ID is selected. I've did that, and there's a very less likelihood of a unique ID being selected in the normal time limit of PHP execution, 30 seconds. So I used set_time_limit to increase it upto 5 minutes. Now the script won't even run. Here's the code:


<?php

include('connect.php');

set_time_limit(3000);


function SelHighID() {

$sqlSel="SELECT aff_id FROM ipod_member WHERE cstatus='1' AND wstatus='0' ORDER BY `aff_id` DESC LIMIT 0 , 1";

$resultSel=mysql_query($sqlSel);
$arrSel=mysql_fetch_array($resultSel);

$high_id=$arrSel[aff_id];

$upperBound=1;
$lowerBound=0;

$range = $upperBound-$lowerBound; 
$num = $lowerBound + $range * mt_rand(0, 32767)/32767;

$var=$num*$high_id;

$var2=intval($var);


$sqlWin="SELECT * FROM ipod_member WHERE aff_id='$var2' AND cstatus='1' AND wstatus='0'";
$resultWin=mysql_query($sqlWin);
$rowWin=mysql_num_rows($resultWin);
$arrWin=mysql_fetch_array($resultWin);

return $arrWin[aff_id];
 }

SelHighID();

$var3 = SelHighID(); 

while ($var3 != "") {
SelHighID(); }

echo $var3;




?>

The default time limit would get exceeded every time, and the script would shut down. Any comments? Thanks.

etully

Wait... let me see if I understand correctly. You have a database of users... let's say their numbers are 1, 2, and 3. So to pick a random user, you set up a loop, pick a random fraction (let's say it's .25) which you multiply by the highest id # (which is 3). So 1/4 of 3 is .75 so you then check the database to see if there is an id # .75 - and of course, there is not, since the numbers are 1, 2, and 3. Is that right? So you are forcing your poor CPU to pick trillions of random numbers waiting until you get exactly .33333, .66666 or 1.000000. Do I have that right?

Ok. That's the wrong way to do it. Instead, read the ID#'s into an array like this:

$id[0] = the first id #
$id[1] = the second id #
$id[2] = the third id #
etc.

Then pick a random number between 0 and the number of elements in the array like this:

$r = rand(0,count($id));

Then the lucky winner is $id[$r]

So you only have to pick ONE random number instead of a trillion. Your poor CPU doesn't overheat trying to pick a winner. You get an answer in 1/100th of a second instead of 3 minutes or more.

Many people would suggest a simpler method. They would put the random function inside the SQL statement instead of reading out all the ID #'s into an array. And that's pretty smart. Then, instead of reading 32700 numbers into an array, you only have to pick one item from the MySQL table. That's probably the most efficient way.

kanuj

Yes, etully you've got it right to some extent. See, I'm making this script for someone else, and I've to go the way he has described. I know this is'nt the most efficient way, but what can you do, huh? Also, when we come up with a fraction, we round it to an integer. That's why I had to put the intval() in there. That's the ID. Well, hopefully. The thing is, when I keep it at the default time limit, the script hits the time limit without turning the unique ID up. When I increase the time limit, the script halts. I'm screwing up the loop, are'nt I?

etully

Well, the client gets to tell you what the script must accomplish. It's your job as a programmer to make sure that your script is efficient and correct. When I pay someone to build a house for me, I can't say, "The roof can't leak. And it must be made of bean sprouts." In the case of your script, there's no reason to pick lots of random numbers (possibly trillions of them) just to find a winner. That doesn't increase the randomness - it just wastes CPU and time.

I think the error in your script, by the way, is that you need single or double quotes around aff_id in the line:
return $arrWin[aff_id];
That's why it's taking 3+ minutes. Increasing the time limit won't fix the problem.

But trust me, the right way to do this is to pick ONE random number and that's the winner. It's 100% fair and random. And you'll get a winner on the first try in one line of code instead of looping through some unknown number of attempts. I guess the reason I'm so insistent here is that if you get into bad habits now, they will stick. You won't learn the right way to do it and you'll be screwed when you have a database with a million records and it's taking three hours for the computer to pick a random record.

Weedpacket

etully wrote:
I think the error in your script, by the way, is that you need single or double quotes around aff_id in the line:
return $arrWin[aff_id]; That's why it's taking 3+ minutes.

Not really: it does need quotes (and someone has been developing with error reporting turned off again....), but the real killer is the selection process (the habit of using "SELECT *" when only one field is wanted doesn't help either) - two database queries in the inner loop? So that if there are a thousand records there are on average a thousand queries to be made before succeeding (half of them identical with identical results - sorted, no less)? Ouch.

etully wrote:
if there is an id # .75 - and of course, there is not, since the numbers are 1, 2, and 3. Is that right?

Well, there's an intval() in there, so .75 would get rounded down to 0. The loop ends when there are no records returned for the second query (so it makes no sense to return $arrWin['aff_id'] in that case anyway).

If you really do need to have a roof made of bean sprouts, at least put the set_time_limit inside the loop so that it keeps getting reset until there is a success (if there ever is one....)