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PHP Second most frequent element in an array

Last Updated : 20 Aug, 2024
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Given an array we have to find the second most frequent element present in it. Examples: 

Input : array(3, 3, 4, 5, 5, 5, 9, 8, 8, 8, 8, 8);
Output : Second most frequent element is: 5

Input : array("geeks", "for", "geeks");
Output : Second most frequent element is: for

Here are some common approaches

Table of Content

Using array_count_values and array_keys

At first, we make a new array containing the frequency of all the elements with values as key and count as values using the array_count_values.

Sorting the new array in reverse order using arsort, then take all the keys of the sorted array using array_keys. Second key will be the second most frequent element of the original array.

Example : In this example we counts the frequency of elements in an array, sorts them in descending order, and prints the second most frequent element. The second most frequent element is "2".

php 
<?php    
    
    $arr = array(2, 2, 3, 4, 4, 4, 8, 8, 6, 6, 9, 9, 9, 9);
    
    // new array containing frequency of values of $arr
    $arr_freq = array_count_values($arr);    
    
     // arranging the new $arr_freq in decreasing order 
     // of occurrences
     arsort($arr_freq);
     
     // $new_arr containing the keys of sorted array
     $new_arr = array_keys($arr_freq);
     
     // Second most frequent element
     echo "Second most frequent element is:"." ".$new_arr[1];
?>

Output
Second most frequent element is: 4

Example 2: In this example we counts the frequency of elements in an array, sorts them in descending order, and then prints the second most frequent element. The second most frequent string is "for".

php 
<?php
    
    
    $arr = array("Geeks", "for", "Geeks");
    
    // new array containing frequency of values of $arr
    $arr_freq = array_count_values($arr);
       
    // arranging the new $arr_freq in decreasing 
    // order of occurrences
    arsort($arr_freq);
     
    // $new_arr containing the keys of sorted array
    $new_arr = array_keys($arr_freq);
     
    // Second most frequent element
    echo "Second most frequent string is:"." ".$new_arr[1];
?>

Output
Second most frequent string is: for

Using forEach method

This manual method in PHP iterates through an array to count occurrences of each element, updating variables to track the second most frequent element without relying on built-in functions like array_count_values or arsort.

Example:

PHP 
<?php
function secondMostFrequent($array) {
    $mostFrequent = null;
    $secondMostFrequent = null;
    $mostFrequentCount = 0;
    $secondMostFrequentCount = 0;

    // Iterate through the array and count frequencies manually
    foreach ($array as $element) {
        $count = 0;
        foreach ($array as $compareElement) {
            if ($element === $compareElement) {
                $count++;
            }
        }

        // Update most frequent and second most frequent elements
        if ($count > $mostFrequentCount) {
            $secondMostFrequent = $mostFrequent;
            $secondMostFrequentCount = $mostFrequentCount;
            $mostFrequent = $element;
            $mostFrequentCount = $count;
        } elseif ($count > $secondMostFrequentCount && $element !== $mostFrequent) {
            $secondMostFrequent = $element;
            $secondMostFrequentCount = $count;
        }
    }

    return $secondMostFrequent;
}

// Example usage
$array = [1, 2, 3, 2, 2, 1, 4, 4, 4, 5];
$result = secondMostFrequent($array);
echo "Second most frequent element: $result\n";
?>

Output
Second most frequent element: 4

Using Min-Heap

A Min-Heap can be used to keep track of the two most frequent elements in the array. The idea is to iterate through the array to count the frequency of each element. Then, we maintain a Min-Heap of size 2, where the root of the heap always contains the second most frequent element.

Steps:

  1. Count the frequency of each element in the array.
  2. Use a Min-Heap to keep track of the two most frequent elements.
  3. The root of the heap will be the second most frequent element.

Example: Here's the implementation of above approach.

PHP 
<?php
class CustomMinHeap extends SplMinHeap {
    // Customize the comparison to sort by frequency (count)
    protected function compare($value1, $value2) {
        return $value1['count'] - $value2['count'];
    }
}

function secondMostFrequent($array) {
    // Step 1: Create a frequency map
    $frequency = array_count_values($array);

    // Step 2: Use a custom min-heap that sorts based on frequency
    $minHeap = new CustomMinHeap();

    foreach ($frequency as $element => $count) {
        $minHeap->insert(['element' => $element, 'count' => $count]);

        // If heap size exceeds 2, remove the smallest element
        if ($minHeap->count() > 2) {
            $minHeap->extract();
        }
    }

    // Extract the least frequent of the top 2 to get the second most frequent
    return $minHeap->extract()['element'];
}

// Example usage
$array = [3, 3, 4, 5, 5, 5, 9, 8, 8, 8, 8, 8];
echo "Second most frequent element is: " . secondMostFrequent($array) . "\n";
?>

Output
Second most frequent element is: 9

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Sort array of objects by object fields in PHP

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