PHP program to find missing elements from an array

We have to find element(s) in array that are missing from an array in range from array minimum to array maximum.

Examples:

Input : arr[] = (1, 2, 3, 4, 6, 7, 8)
Output : 5
The array minimum is 1 and maximum is
8. The missing element in range from
1 to 8 is 5.

Input : arr[] = (10, 11, 14, 15)
Output : 12, 13

This problem can be solved by iterating in an array by observing the contiguous difference between elements. But in PHP we can make use of some inbuilt functions to solve the problem. We will have to use the below two functions for this purpose:

1. range() function: This function is used to create an array of elements of any kind such as integer, alphabets within a given range(from low to high) i.e, list’s first element is considered as low and last one is considered as high.
2. array_diff() function: if we have an array of elements, we can find missing element(s) by comparing two arrays.

The idea to solve this problem using two inbuilt functions is to, first use range() function to create a new array from starting element and maximum element of original array using max() function. After this we apply array_diff() function to compare newly created array and original array thus getting all the missing elements of the original array.

<?php
    // PHP code to find missing elements

    function not_present($list)
    {
        // Create an array with range from array 
        // minimum to maximum.
        $new_array = range(min($list), max($list));

        // Find those elements that are present
        // in new_array but not in given list
        return array_diff($new_array, $list);
    }

    // Driver code
    print_r(not_present(array(1, 2, 3, 4, 7, 8)));
    print_r(not_present(array(10, 11, 12, 14, 15, 16)));    
?>

Output:

Array
(
    [4] => 5
    [5] => 6
)
Array
(
    [3] => 13
)

Using Array Keys

In the Array Keys approach, create an associative array using array_fill_keys() with the given array values as keys. Then, iterate through the expected range and use array_key_exists() to identify missing elements, appending them to the result array if they don't exist.

Example

<?php
function findMissingElements($array, $start, $end) {
    $arrayKeys = array_fill_keys($array, true);
    $missingElements = [];
    for ($i = $start; $i <= $end; $i++) {
        if (!array_key_exists($i, $arrayKeys)) {
            $missingElements[] = $i;
        }
    }
    return $missingElements;
}

// Example usage
$array = [1, 2, 4, 6, 7];
$start = 1;
$end = 7;
$missing = findMissingElements($array, $start, $end);
print_r($missing);
?>

Array
(
    [0] => 3
    [1] => 5
)

Using array_flip()

Use array_flip() to flip the original array, making the values as keys. Iterate through the expected range and check if each element exists in the flipped array. If not, add it to the missing elements array.

Example

<?php

function findMissingElementsWithFlip($arr) {
    $min = min($arr);
    $max = max($arr);
    $flippedArray = array_flip($arr);
    $missingElements = [];
    for ($i = $min; $i <= $max; $i++) {
        if (!isset($flippedArray[$i])) {
            $missingElements[] = $i;
        }
    }
    return $missingElements;
}

// Examples
$arr1 = [1, 2, 3, 4, 6, 7, 8];
print_r(findMissingElementsWithFlip($arr1));

$arr2 = [10, 11, 14, 15];
print_r(findMissingElementsWithFlip($arr2));

?>

Array
(
    [0] => 5
)
Array
(
    [0] => 12
    [1] => 13
)