Percentage chances on winning multiple prizes - php

Question

I'm making a script that handles the percentage chances of winning when the user clicks a button.
With the help of this topic php - Chance of winning. I used the following code:
function winningChance($percentage) {
if($percentage < 1 || $percentage > 100) throw new Exception('Invalid percentage');
global $result;
if(rand(1, 100) <= $percentage) {
$result = 'won';
} else {
$result = 'lost';
}
return $result;
}
echo "you have ".winningChance(50).'!';
Once this script has run, it registers the user name / last name / email and a field called winner with the $result in a SQL database
This works great, however I would like to handle multiple prizes with different percentages of winning chances.
Lets say prize1 have 20% chances of being won,
prize2 30% chances
and prize3 50% chances.
If I use winningChance(20), winningChance(30), winningChance(50) the user will have more chances of winning. How can I handle it so, the win/lost process happens in the same function for multiple prizes?

Answer 1

If I understand you correctly, the chance of winning depends on the price.
function getChanceOfWinning($price)
{
$chances = array(
10 => 20,
20 => 30,
30 => 50
);
if (isset($chances[$price])) {
return $chances[$price];
}
return 0; // <-- default chance
}
function calculateWinningChance($price)
{
$chance = getChanceOfWinning($price);
$calc = rand(1, 100);
if ($calc <= $chance) {
return true;
}
return false;
}
function calculateWinningChances(array $prices)
{
$results = array();
foreach($prices as $price) {
$results[$price] = calculateWinningChance($price);
}
return $results;
}
var_dump(calculateWinningChances(array(10,20, 30,40,700)));

Answer 2

How about this solution ?
function roll( $iChance ) {
$iCursor = rand( 0,99 );
$aModel = array();
while ( count( $aModel ) != $iChance ) {
$iRandValue = rand( 0,99 );
if ( !in_array( $iRandValue, $aModel ) ) {
$aModel[] = $iRandValue;
}
}
return in_array( $iCursor, $aModel );
}
Edit: More perfomance:
function roll( $iChance ) {
$iChance = ceil( ( $iChance > 100 ) ? 100 : (int)$iChance);
$iCursor = rand( 0, 99 );
$aModel = range( 0, 99 );
shuffle( $aModel );
return in_array( $iCursor, array_slice( $aModel, 0, $iChance ) );
}

Answer 3

If I understood correctly, you want to have multiple winning calculations for each user at the same time, independent of one another. There are numerous ways to do this. Modify your function so that you can pass an associative array as an argument, for example.
The array would be a map of price=>percentage values and then you do the calculations for each pair.
You'd also need to modify your result variable in an array, and on each pass just push the result of the calculation into it. You can also use an associative array here to show price=>won/lost. After you looped through all the pairs and filled up the result variable with the results, just return the variable.
Based on your last comment, this is what you need:
function winningChance($percentage) {
foreach($percentage as $p) {
if($p < 1 || $p > 100)
throw new Exception('Invalid percentage');
}
if (count($percentage) != 3)
throw new Exception('Three prizes need to be defined');
$rand = rand(1, 100); // generate the random chance only once!
if ($rand <= $percentage[0])
$result = 'won first prize';
elseif ($rand <= $percentage[1])
$result = 'won second prize';
elseif ($rand <= $percentage[2])
$result = 'won third prize';
else
$result = 'lost';
return $result;
}
And call the function like this:
//the array contains probability percentages for the first, second and third place respectively
$res = winningChance( array(20, 30, 50) );
echo "You have $res!";
// write $res to the db here

Answer 4

tweak your function code like,
instead of,
if(rand(1, 100) <= $percentage) {
$result = 'won';
} else {
$result = 'lost';
}
to,
if(rand(1, 100) >= 50) {
$result = 'won third price';
} else if(rand(1, 100) >= 30) {
$result = 'won second price';
}
else if(rand(1, 100) >= 20) {
$result = 'won first price';
}
else
{
$result='lost';
}