PHP Program for Range Queries for Frequencies of Array Elements

Given an array of n non-negative integers. The task is to find the frequency of a particular element in the arbitrary range of array[]. The range is given as positions (not 0 based indexes) in array. There can be multiple queries of given type. 

Examples:  

Input  : arr[] = {2, 8, 6, 9, 8, 6, 8, 2, 11};
         left = 2, right = 8, element = 8
         left = 2, right = 5, element = 6      
Output : 3
                1
The element 8 appears 3 times in arr[left-1..right-1]
The element 6 appears 1 time in arr[left-1..right-1]

Naive Approach

This approach is to traverse from left to right and update the count variable whenever we find the element. 

Below is the code of Naive approach: 

<?php
// PHP program to find total count of
// an element in a range

// Returns count of element in
// arr[left-1..right-1]
function findFrequency(&$arr, $n, $left, $right, $element) {
    $count = 0;
    for ($i = $left - 1; $i <= $right; ++$i) {
        if ($arr[$i] == $element) {
            ++$count;
        }
    }
    return $count;
}

// Driver Code
$arr = [2, 8, 6, 9, 8, 6, 8, 2, 11];
$n = sizeof($arr);

// Print frequency of 2 from position 1 to 6
echo "Frequency of 2 from 1 to 6 = " .
    findFrequency($arr, $n, 1, 6, 2) . "";

// Print frequency of 8 from position 4 to 9
echo "\nFrequency of 8 from 4 to 9 = " . 
  	findFrequency($arr, $n, 4, 9, 8);

?>

Frequency of 2 from 1 to 6 = 1
Frequency of 8 from 4 to 9 = 2

Time complexity of this approach is O(right - left + 1) or O(n) 

Auxiliary space: O(1)

Please refer complete article on Range Queries for Frequencies of array elements for more details!

kartik

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Article Tags :

• PHP
• array-range-queries