PHP to list all indices of an array whose values meet a condition

PHP Code:


// Input array
$arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

// Condition to check
$condition = function ($value) {
    return $value % 2 == 0;

// Get all indices satisfying the condition
$indices = array_filter(array_keys($arr), $condition);

// Print the indices



  1. Define an input array $arr with sample values.

  2. Create an anonymous function $condition to check if a value is even (divisible by 2).

  3. Use array_filter to go through each element in $arr and filter out the keys (indices) that satisfy the condition specified in $condition.

  4. The result of array_filter is stored in the $indices variable.

  5. Finally, print the $indices array to display the indices of the even numbers in $arr.


To effectively implement this solution, consider the following:

  • Time Complexity: array_filter has a time complexity of O(n), where n is the number of elements in the array.

  • Memory Usage: The solution uses an additional array ($indices) to store the valid indices, so it requires O(n) extra memory.

  • Handling Empty Arrays: Check if the input array is empty before applying array_filter to avoid errors or incorrect results.

  • Custom Conditions: You can modify the $condition function to specify any desired condition for filtering the indices. For example, you could check for odd numbers, values within a certain range, or any other custom criteria.

  • Additional Features: You can enhance the solution by adding error handling, supporting multiple conditions, or returning the values instead of the indices.