PHP Exercises: Group the elements of an array based on the given function

Last update on May 27 2025 13:08:29 (UTC/GMT +8 hours)

83. Group Array Elements by Callback Result

Write a PHP program to group the elements of an array based on the given function.

Sample Solution:

PHP Code:

<?php
// License: https://bit.ly/2CFA5XY

// Function definition for 'groupBy' that takes an array of items and a grouping function as parameters
function groupBy($items, $func)
{
    // Initialize an empty associative array to store the grouped items
    $group = [];

    // Iterate through each item in the array
    foreach ($items as $item) {
        // Check if the grouping function is a callable function or a valid function name
        if ((!is_string($func) && is_callable($func)) || function_exists($func)) {
            // If callable, use 'call_user_func' to get the key and group the item
            $key = call_user_func($func, $item);
            $group[$key][] = $item;
        } 
        // Check if the item is an object
        elseif (is_object($item)) {
            // If an object, use the property specified by the grouping function as the key and group the item
            $group[$item->{$func}][] = $item;
        } 
        // Check if the item has a key specified by the grouping function
        elseif (isset($item[$func])) {
            // If set, use the specified key as the grouping key and group the item
            $group[$item[$func]][] = $item;
        }
    }

    // Return the associative array containing the grouped items
    return $group;
}

// Call 'groupBy' with an array of strings and a grouping function based on string length, then display the result using 'print_r'
print_r(groupBy(['one', 'two', 'three', 'four'], 'strlen'));

?>

Explanation:

Define groupBy Function:

The groupBy function takes two parameters:

$items: an array of items to group.
$func: a function or property name used to determine the grouping criteria.

Initialize Grouping Structure:

An empty associative array $group is created to store the grouped items.

Iterate Through Items:

A foreach loop iterates over each item in the $items array.

Check Grouping Function:

Callable Function:

If $func is a callable function, call_user_func($func, $item) is used to invoke it and get the key for grouping.
The item is added to $group under that key.

Object Property:

If the item is an object, it checks if $func corresponds to a property of the object and groups the item by that property.

Array Key:

If the item is an associative array and the key specified by $func exists, it uses that key to group the item.

Return Grouped Items:

After processing all items, the function returns the $group associative array containing the grouped results.

Example Usage:

The function is called with an array of strings and the strlen function to group the strings by their length.
The result is displayed using print_r, showing how the strings are grouped based on their lengths (e.g., all strings with length 3 will be in one group).

Purpose:

The groupBy function provides a flexible way to organize and categorize items in an array based on custom criteria, whether through a function, object property, or array key.

Output:

Array
(
    [3] => Array
        (
            [0] => one
            [1] => two
        )

    [5] => Array
        (
            [0] => three
        )

    [4] => Array
        (
            [0] => four
        )

)

Flowchart:

For more Practice: Solve these Related Problems:

Write a PHP script to group elements of an array into sub-arrays based on the result of a provided function.
Write a PHP script to classify elements into groups using a callback and return an associative array of these groups.
Write a PHP function to aggregate array values by computed key and output grouped results.
Write a PHP script to partition an array into groups where each group shares the same value as returned by a custom function.

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PHP Code Editor:

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