Transform array to key-value array without iteration

Question

What is the easiest way to map array of objects to key-value array of objects, where key is some property of an object?

For example:

class Cartoon
{
  public $title;
  public function __construct($title)
  {
    $this->title = $title;
  }
}

$cartoons = array(new Cartoon('Tom and Jerry'), new Cartoon('Cheburashka'));

$mappedCartoons = array();

foreach ($cartoons as $cartoon)
{
  $mappedCartoons[$cartoon->title] = $cartoon;
}

print_r ($mappedCartoons);

PS. I wonder if iteration and the extra variable $mappedCartoons can be disposed of?

Decent Dabbler · Accepted Answer · 2012-11-21 14:53:31Z

4

You could use array_reduce to achive your goal.

<?php

$cartoons = array_reduce($cartoons, function($result, $cartoon) {
  $result[$cartoon->title] = $cartoon;
  return $result;
}, array());

?>

the idea is not mine, found here: http://www.danielauener.com/howto-use-array_map-functionality-on-an-associative-array-changing-keys-and-values/

edited Nov 21, 2012 at 14:53

Decent Dabbler

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answered Nov 21, 2012 at 14:25

Agi Sferro

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2 Comments

Dziamid Over a year ago

This is brilliant! Now array_reduce is my second favourite friend after array_map.

Decent Dabbler Over a year ago

Which, behind the scenes, still involves iteration of course. But nice suggestion nonetheless. +1 And perhaps I've misunderstood OP in this respect, to begin with.

complex857 · Accepted Answer · 2012-11-21 14:11:08Z

0

If you must use basic arrays you can simply type out the keys when creating them:

$cartoons = array(
    'Tom and Jerry' => new Cartoon('Tom and Jerry'), 
    'Cheburashka' => new Cartoon('Cheburashka'),
);

Alternatively you can create container object that implements various array/iteration related interfaces and can be used as a regular array:

class CartoonArray extends ArrayObject {

    public function __construct($input, $flags = 0, $iterator_class = "ArrayIterator") {
        parent::__construct(array(), $flags, $iterator_class);
        // since the original ArrayObject implemented in C and they don't it won't pick up our overriden ossetSet we have to type it out here explicitly
        foreach ($input as $value) {
            $this->offsetSet(null, $value);
        }
    }

    public function offsetSet($index, $value) {
        parent::offsetSet($value->title, $value);
    }
}

$cartoons = new CartoonArray(array(new Cartoon('Tom and Jerry'), new Cartoon('Cheburashka')));
$cartoons[] = new Cartoon('Sesame street');

print $cartoons['Tom and Jerry']->title;
unset($cartoons['Tom and Jerry']);

foreach ($cartoons as $k => $v) {
    print $k."\n";
}

answered Nov 21, 2012 at 14:11

complex857

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1 Comment

Dziamid Over a year ago

Thanks, but I don't intend to use generic classes for collections of objects, native arrays only.

Decent Dabbler · Accepted Answer · 2012-11-21 14:17:51Z

I can't think of a way that doesn't involve iterating. I can think of a way of not needing an additional variable though:

$cartoons = array(new Cartoon('Tom and Jerry'), new Cartoon('Cheburashka'));
foreach( $cartoons as $key => $cartoon )
{
    $cartoons[ $cartoon->title ] = $cartoon;
    unset( $cartoons[ $key ] );
}

var_dump( $cartoons );

However, the documentation of foreach notes:

As foreach relies on the internal array pointer changing it within the loop may lead to unexpected behavior.

I'm not entirely sure that this applies to my example as well. Maybe someone else can chime in here.

To be safe, perhaps this alternative is more appropriate, as it initiates $len with the initial length of $cartoons, before iteration:

$cartoons = array(new Cartoon('Tom and Jerry'), new Cartoon('Cheburashka'));
for( $i = 0, $len = count( $cartoons ); $i < $len; $i++ )
{
    $cartoon = $cartoons[ $i ];
    $cartoons[ $cartoon->title ] = $cartoon;
    unset( $cartoons[ $i ] );
}

var_dump( $cartoons );

This example assumes the initial $cartoons has 'proper' sequentially numbered keys, starting at 0, though.

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