Convert array with 3-element rows into hierarchical array with 3 levels

Question

I have an array that looks like this:

[98] => Array
(
    [City] => Caracas
    [Country] => Venezuela
    [Continent] => Latin America
)

[99] => Array
(
    [City] => Cairo
    [Country] => Egypt
    [Continent] => Middle East
)

[105] => Array
(
    [City] => Abu Dhabi
    [Country] => United Arab Emirates
    [Continent] => Middle East
)

[106] => Array
(
    [City] => Dubai
    [Country] => United Arab Emirates
    [Continent] => Middle East
)

[107] => Array
(
    [City] => Montreal
    [Country] => Canada
    [Continent] => North America
)

I am trying to group this array into a new multi-dimensional array structure, so that it outputs something like:

Continent
    - Country Name    
      -- Cities under Every Country

Exact desired result:

array (
  'Latin America' => 
  array (
    'Venezuela' => 
    array (
      0 => 'Caracas',
    ),
  ),
  'Middle East' => 
  array (
    'Egypt' => 
    array (
      0 => 'Cairo',
    ),
    'United Arab Emirates' => 
    array (
      0 => 'Abu Dhabi',
      1 => 'Dubai',
    ),
  ),
  'North America' => 
  array (
    'Canada' => 
    array (
      0 => 'Montreal',
    ),
  ),
)

Scute

Out of curiosity, how is the array being generated? – Discotheque

I have a SQL query that returns the array. – Scute

Maybe it's much easier to organize data by SQL grouping. – Gavelkind

Answer 1

$array = array(
    98 => array(
        'City' => 'Caracas',
        'Country' => 'Venezuela',
        'Continent' => 'Latin America',
    ),
    99 => array(
        'City' => 'Cairo',
        'Country' => 'Egypt',
        'Continent' => 'Middle East',
    ),
    105 => array(
        'City' => 'Abu Dhabi',
        'Country' => 'United Arab Emirates',
        'Continent' => 'Middle East',
    ),
    106 => array(
        'City' => 'Dubai',
        'Country' => 'United Arab Emirates',
        'Continent' => 'Middle East',
    ),
    107 => array(
        'City' => 'Montreal',
        'Country' => 'Canada',
        'Continent' => 'North America',
    )
);

$newArray = array();
foreach ($array as $row)
{
   $newArray[$row['Continent']][$row['Country']][] = $row['City'];
}

print_r($newArray);

Rodmur

Answer 2

A couple more alternative approaches:

Use "array destructuring" in a body-less foreach() to populate the hierarchical structure (Demo) (Further about the technique)

$result = [];
foreach (
    $array as
    [
        'Continent' => $a,
        'Country' => $b,
        'City' => $result[$a][$b][]
    ]
);
var_export($result);

---

Use array_reduce() to avoid declaring a result variable in the global scope. (Demo)

var_export(
    array_reduce(
        $array,
        function($result, $row) {
            $result[$row['Continent']][$row['Country']][] = $row['City'];
            return $result;
        }
    )
);

Dragonroot

Answer 3

What about this solution:

function arrayToMultiDimensionalArray(array $elements, array $dimensions)
{
    $output = [];

    foreach ($elements as $element) {
        $outputElement =& $output;

        // Foreach to build up the dimensions
        foreach ($dimensions as $dimension) {
            $outputElement =& $outputElement[$element[$dimension]];
        }

        $outputElement[] = $element['City'];
    }

    return $output;
}

$dimensions = ['Continent', 'Country'];

$tree = arrayToMultiDimensionalArray($array, $dimensions);

Sarre

@Dragonroot Just change $outputElement[] = $element; to $outputElement[] = $element['City']; problem solved – Sarre

@Dragonroot A "thank you" would be nice. – Sarre

Stack Overflow doesn't do "thank you"s. Just the same, you can thank me for bringing to your attention that your answer wasn't (and still isn't) the best version of itself. I know what your script is doing, but there will be many PHP developers who won't and they might not use it because they don't understand how it works. – Dragonroot

Answer 4

I think you just need to loop in your array and create a new one using some values as keys and some other as value, for example

$new_array = array();
foreach($array as $val)
{ 
     $new_array[$val['Continent']][$val['Country']] = array('City' => $val['City']);
}  

Live Sample

Tenuous

This answer is incorrect because it only allows one city per subset. Proof: 3v4l.org/aDqFQ – Dragonroot