How to compare every element of array with one another?

Question

I want to compare every element of array with one another.

 $char=array();    
 for($i=0;$i<=10;$i++)
 {
        $char[$i]=rand(0,35);
 }

I want to compare every element of $char array. If there is any value repeated than it should change value and select another random value which should be unique in array..

Answer 1

In this particular example, where the range of possible values is very small, it would be better to do this another way:

$allPossible = range(0, 35);
shuffle($allPossible);

// Guaranteed exactly 10 unique numbers in the range [0, 35]
$char = array_slice($allPossible, 0, 10);

Or with the equivalent version using array_rand :

$allPossible = range(0, 35);
$char = array_rand(array_flip($allPossible), 10);

If the range of values were larger then this approach would be very wasteful and you should go with checking for uniqueness on each iteration:

$char = array();
for ($i = 0; $i < 10; ++$i) {
    $value = null;

    // Try random numbers until you find one that does not already exist
    while($value === null || in_array($value, $char)) {
        $value = rand(0, 35);
    }

    $char[] = $value;
}

However, this is a probabilistic approach and may take a lot of time to finish depending on what the output of rand happens to be (it's going to be especially bad if the number of values you want is close to the number of all possible values).

Additionally, if the number of values you want to pick is largish (let's say more than 50 or so) then in_array can prove to be a bottleneck. In that case it should be faster to use array keys to check for uniqueness instead of values, since searching for the existence of a key is constant time instead of linear:

$char = array();
for ($i = 0; $i < 100; ++$i) {
    $value = null;

    // Try random numbers until you find one that does not already exist
    while($value === null || array_key_exists($char, $value)) {
        $value = rand(0, 1000);
    }

    $char[$value] = $value;
}

$char = array_values($char); // reindex keys to start from 0

Answer 2

To change any repeated value for a random one, you should loop through the array twice:

$cont= 0;
foreach($char as $c){
    foreach($char as $d){
        if($c == $d){
            //updating the value
            $char[$cont] = rand(0,35);
        }
    }
    $cont++;
}

But what I don't know is if the random value can also be repeated. In that case it would not be so simple.

Answer 3

I've taken this code from the PHP Manual page for rand()

<?php
 function uniqueRand($n, $min = 0, $max = null)
 {
  if($max === null)
   $max = getrandmax();
  $array = range($min, $max);
  $return = array();
  $keys = array_rand($array, $n);
  foreach($keys as $key)
   $return[] = $array[$key];
  return $return;
 }
?>

This function generates an array which has a size of $n and you can set the min and max values like in rand.

So you could make use of it like

uniqueRand(10, 0, 35);

Answer 4

Use array_count_values() first on the $char array.

Afterwards you can just loop all entries with more than 1 and randomize them. You have to keep checking until all counts are 1 tho. As even the random might remake a duplicate again.

Answer 5

I sugggest two option to make random array:

    <?php
    $questions = array(1, 2, 3, 4, 5, ..., 34, 35);
    $questions = shuffle($questions);
    ?>

after that you choose the top 10 elements.

Answer 6

you can try this code to replace any repeated value.

for ($i = 0; $i < count($char); $i++) {
    for ($n = 0; $n < count($char); $n++) {
        if($char[$i] == $char[$n]){
            $char[$i] = rand(0,35);
        }
    }
}

Answer 7

The function array_unique() obtains all unique values from an array, keyed by their first occurrence.

The function array_diff() allows to remove values from one array that are inside another array.

Depending on how you need to have (or not have) the result keyed or the order of keys preserved you need to do multiple steps. Generally it works as I outline in the following paragraphs (with PHP code-examples):

In an array you've got N elements of which Nu are unique.

$N  = array(...);
$Nu = array_unique($N);

The number of random elements r you need then to replace the duplicates are the count of N minus the count of Nu . As the count of N is generally a useful value, I also assign it to nc :

$nc = count($N);
$r  = $nc - count($Nu);

That makes r an integer ranging from 0 to count(N) - 1 :

           0 : no duplicate values / all values are unique
           1 : one duplicate value / all but one value are unique
            ...
count(N) - 1 : all duplicate values / no unique value

So in case you you need zero random values ( $r === 0 ) the input $N is the result. This boundary condition is the second most simple result (the first simple result is an input array with no members).

For all other cases you need r random unique values. In your question you write from 0 to 35. However this can not be the full story. Imagine your input array has got 36 duplicated values, each number in the range from 0 to 35 is duplicated once. Adding random numbers from the range 0 to 35 again to the array would create duplicates again - guaranteed.

Instead I've read your question that you are just looking for unique values that are not yet part of the input array.

So you not only you need r random values ( Nr ), but they also must not be part of N or Nu so far.

To achieve that you only need to create count(N) unique values, remove the unique values Nu from these to ensure nothing duplicates values in Nu . As this the theoretical maximum and not the exact number needed, that array is used to obtain the slice of exactly r elements from:

$Nr = array_slice(array_diff(range(0, $nc - 1), $Nu), 0, $r);

If you also want to have these new values to be added shuffled as range(0, $nc - 1) is ordered, you can do the following:

shuffle($Nr);

That should bring the randomness you seem to ask for in your question back into the answer.

That now leaves you with the unique parts of the original array $Nu and r new values in $Nr . Merging both these arrays will give you a result array which ignores key => value relations (the array is re-index):

array_merge($Nu, $Nr);

For example with an exemplary array(3, 4, 2, 1, 4, 0, 5, 0, 3, 5) for $N , the result this gives is:

Array
(
    [0] => 3
    [1] => 4
    [2] => 2
    [3] => 1
    [4] => 0
    [5] => 5
    [6] => 7
    [7] => 9
    [8] => 6
    [9] => 8
)

As you can see all the unique values (0-5) are at the beginning followed by the new values (6-9). Original keys are not preserved, eg the key of value 5 was 6 originally, now it is 5 .

The relation or key => value are not retained because of array_merge() , it does re-index number keys. Also next to unique numbers in Nr keys also need to be unique in an array. So for every new number that is added to the unqique existing numbers, a key needs to be used that was a key of a duplicate number. To obtain all keys of duplicate numbers the set of keys in the original array is reduced by the set of keys of all for matches of the duplicate numbers (keys in the "unique array" $Nu ):

$Kr  = array_keys(array_diff_assoc($N, $Nu));

The existing result $Nr can now be keyed with with these keys. A function in PHP to set all keys for an array is to use the function array_combine() :

$Nr = array_combine($Kr, $Nr);

This allows to obtain the result with key => value relations preserved by using the array union operator ( + ) :

$Nu + $Nr;

For example with the $N from the last example, the result this gives is:

Array
(
    [0] => 3
    [1] => 4
    [2] => 2
    [3] => 1
    [5] => 0
    [6] => 5
    [4] => 8
    [7] => 6
    [8] => 9
    [9] => 7
)

As you can now see for the value 5 it's key 6 has been preserved as well as for the value 0 which had the key 5 in the original array and now as well in the output instead of the key 4 as in the previous example.

However as now the keys have been preserved for the first occurrences of the original values, the order is still changed: First all previously unique values and then all new values. However you might want to add the new values in place. To do that, you need to obtain the order of the original keys for the new values. That can be done by mapping the order by key and the using array_multisort() to sort based on that order.

Because this requires passing return values via parameters, this requires additional, temporary variables which I've chosen to introduce starting with the letter V :

// the original array defines the order of keys:
$orderMap = array_flip(array_keys($N));

// define the sort order for the result with keys preserved
$Vt    = $Nu + $Nr;
$order = array();
foreach ($Vt as $key => $value) {
    $order[] = $orderMap[$key];
}

Then the sorting is done (here with preserving keys):

// sort an array by the defined order, preserve keys
$Vk = array_keys($Vt);
array_multisort($order, $Vt, $Vk);

The result then is:

array_combine($Vk, $Vt);

Again with the example values from above:

Array
(
    [0] => 3
    [1] => 4
    [2] => 2
    [3] => 1
    [4] => 7
    [5] => 0
    [6] => 5
    [7] => 8
    [8] => 6
    [9] => 9
)

This example output shows nicely that the keys are ordered from 0 to 9 as they were are well in the input array. Compared with the previous output you can for example see that the first added value 7 (keyed 4 ) is at the 5th position - same as the value keyed 4 in the original array. The order of the keys have been obtained as well.

If that is the result you strive for you can short-cut the path to this step as well by iterating the original arrays keys and in case each of those keys is not the first value of any duplicate value, you can pop from the new values array instead:

$result = array();
foreach ($N as $key => $value) {
    $result[$key] = array_key_exists($key, $Nu) ? $Nu[$key] : array_pop($Nr);
}

Again with the example array values the result (varies from previous because $Nr is shuffled:

Array
(
    [0] => 3
    [1] => 4
    [2] => 2
    [3] => 1
    [4] => 7
    [5] => 0
    [6] => 5
    [7] => 8
    [8] => 9
    [9] => 6
)

Which in the end might be the simplest way to answer your question. Hope this helps you answering the question. Keep the following in mind:

• divide your problem:
  1. you want to know if a value is unqiue or not - array_unique() helps you here.
  2. you want to create X new unique numbers/values. array_diff() helps you here.
• align the flow:
  1. obtain unique numbers first.
  2. obtain new numbers first.
  3. use both to process the original array.

Like in this example:

// original array
$array = array(3, 4, 2, 1, 4, 0, 5, 0, 3, 5);

// obtain unique values (1.)
$unique = array_unique($array);

// obtain new unique values (2.)
$new = range(0, count($array) - 1);
$new = array_diff($new, $unique);
shuffle($new);

// process original array (3.)
foreach ($array as $key => &$value) {
    if (array_key_exists($key, $unique)) {
        continue;
    }
    $value = array_pop($new);
}
unset($value, $new);

// result in $array:
print_r($array);

Which then (exemplary because of shuffle($new) ) outputs:

Array
(
    [0] => 3
    [1] => 4
    [2] => 2
    [3] => 1
    [4] => 9
    [5] => 0
    [6] => 5
    [7] => 8
    [8] => 7
    [9] => 6
)