Shuffling Array in the same way, according to number

Question

I am running a quiz making website. I wish to show the answers to a question to a user in a shuffled order.

I'm trying to avoid storing the order that answers were presented to the user, if I were to shuffle them randomly.

I want to shuffle the answers predictably, so that I can repeat the shuffle in the same way later (when displaying results).

I've thought that I could shuffle the list of answers by a certain number (either using the number within the sort, or having multiple types of sorts identifiable by an ID number. This way I can simply store the number that they were shuffled by and recall that number to reshuffle them again to the same order.

Here's the skeleton of what I have so far, but I don't have any logic to put the answers back in the $shuffled_array in the shuffled order.

<?php

function SortQuestions($answers, $sort_id)
{
    // Blank array for newly shuffled answer order
    $shuffled_answers = array();

    // Get the number of answers to sort
    $answer_count = count($questions);

    // Loop through each answer and put them into the array by the $sort_id
    foreach ($answers AS $answer_id => $answer)
    {
        // Logic here for sorting answers, by the $sort_id

        // Putting the result in to $shuffled_answers
    }

    // Return the shuffled answers
    return $shuffled_answers;
}


// Define an array of answers and their ID numbers as the key
$answers = array("1" => "A1", "2" => "A2", "3" => "A3", "4" => "A4", "5" => "A5");

// Do the sort by the number 5
SortQuestions($answers, 5);

?>

Is there technique that I can use to shuffle the answers by the number passed into the function?

Answer 1

PHP's shuffle function uses the random seed given with srand, so you can set a specific random seed for this.

Also, the shuffle method change's the array keys, but this is probably not the best outcome for you, so you may use a different shuffle function:

function shuffle_assoc(&$array, $random_seed) {
    srand($random_seed);

    $keys = array_keys($array);

    shuffle($keys);

    foreach($keys as $key) {
        $new[$key] = $array[$key];
    }

    $array = $new;

    return true;
}

This function will keep the original keys, but with a different order.

Answer 2

You could rotate the array by a factor.

$factor = 5;
$numbers = array(1,2,3,4);
for ( $i = 0; $i < $factor; $i++ ) {
    array_push($numbers, array_shift($numbers));
}
print_r($numbers);

The factor can be randomized, and a function can switch the array back in place, by rotation the other way around.