Array parts access

Question

I'm trying to understand arrays better. Pardon my elementary questions as I just opened my first php book three weeks ago.

I get that you can retrieve key/value pairs using a foreach (or for loop) as below.

 $stockprices= array("Google"=>"800", "Apple"=>"400", "Microsoft"=>"4",  "RIM"=>"15",  "Facebook"=>"30");

foreach ($stockprices as $key =>$price)

What I get confused about are multi dimensional arrays like this one:

$states=(array([0]=>array("capital"=> "Sacramento", "joined_union"=>1850, "population_rank"=> 1),
              [1]=>array("capital"=> "Austin", "joined_union"=>1845,"population_rank"=> 2),
              [2]=>array("capital"=> "Boston", "joined_union"=>1788,"population_rank"=> 14)
              ));

My first question is really basic: I know that "capital', "joined_union", "population_rank" are keys and "Sacramento", "1850", "1" are values (correct?). But what do you call [0][1][2]? Are they "main keys" and "capital" etc. sub-keys? I can't find any definition for those; neither in books nor online.

The main question is how do I retrieve Arrays [0][1][2]? Say I want to get the array that joined_union in 1845 (or even trickier during the 1800s), then remove that array.

Finally, can I name Arrays [0][1][2] as California, Texas and Massachusetts correspondingly?

$states=(array("California"=>array("capital"=> "Sacramento", "joined_union"=>1850, "population_rank"=> 1),
              "Texas"=>array("capital"=> "Austin", "joined_union"=>1845,"population_rank"=> 2),
              "Massachusetts"=>array("capital"=> "Boston", "joined_union"=>1788,"population_rank"=> 14)
              ));

Answer 1

1: Multidimensional arrays are basically 'arrays of arrays'.

So if we look here:

array("0"=>array("capital"=> "Sacramento", "joined_union"=>1850, "population_rank"=> 1)

0 is the key, and the array is the value.

Then, inside the value, you have capital as a key, and Sacramento as a value.

2: For removing arrays: Delete an element from an array

3: State names for keys

Yes, you can change that 0, 1, 2 into state names. They become key-values instead of a numbered array. Which makes it much easier to remove exactly the one you want to remove.

Answer 2

Unlike other languages, arrays in PHP can use numeric or string keys. You choose. (This is not a well loved feature of PHP and other languages sneer!)

$states = array(
    "California"    => array(
        "capital"         => "Sacramento",
        "joined_union"    => 1850,
        "population_rank" => 1
    ),
    "Texas"         => array(
        "capital"         => "Austin",
        "joined_union"    => 1845,
        "population_rank" => 2
    ),
    "Massachusetts" => array(
        "capital"         => "Boston",
        "joined_union"    => 1788,
        "population_rank" => 14
    )
);

As for querying the structure you have, there are two ways
1) Looping

$joined1850_loop = array();
foreach( $states as $stateName => $stateData ) {
    if( $stateData['joined_union'] == 1850 ) {
        $joined1850_loop[$stateName] = $stateData;
    }
}
print_r( $joined1850_loop );
/*
Array
(
    [California] => Array
        (
            [capital] => Sacramento
            [joined_union] => 1850
            [population_rank] => 1
        )

)
*/

2) Using the array_filter function:

$joined1850 = array_filter(
    $states,
    function( $state ) {
        return $state['joined_union'] == 1850;
    }
);
print_r( $joined1850 );
/*
Array
(
    [California] => Array
        (
            [capital] => Sacramento
            [joined_union] => 1850
            [population_rank] => 1
        )

)
*/

-

$joined1800s = array_filter(
    $states,
    function ( $state ){
        return $state['joined_union'] >= 1800 && $state['joined_union'] < 1900;
    }
);
print_r( $joined1800s );
/*
Array
(
    [California] => Array
        (
            [capital] => Sacramento
            [joined_union] => 1850
            [population_rank] => 1
        )

    [Texas] => Array
        (
            [capital] => Austin
            [joined_union] => 1845
            [population_rank] => 2
        )

)
*/

Answer 3

Multidimensional arrays are simply arrays which have arrays as values. Simple or "scalar" types are types likes int, string and bool, they hold one value and one value only. Arrays are a compound type, meaning it's a thing that combines several other things together. An array can contain values of other types, including arrays.

The simplest visualization is probably this longhand:

$array = array('foo' => array('bar' => 'baz'));

$foo = $array['foo']; // $foo is now array('bar' => 'baz')

echo $foo['bar']; // outputs 'baz'

This is just shorthand for the same thing as above:

echo $array['foo']['bar'];

$array['foo'] gives you access to the value array('bar' => 'baz'), ['bar'] on that array gives you the value 'baz'. It doesn't matter whether you assign the intermediate value into a variable or continue directly.

The other way around, here's a demonstration of the concept that multidimensional arrays are just arrays in arrays:

$baz   = 'baz';
$bar   = array('bar' => $baz);
$array = array('foo' => $bar);

That's all there is to it.

You can call nested arrays in array "sub arrays", though there's no real definition and it's not really necessary to define this, since nested arrays aren't a special case of anything.

You may stumble upon explanations using the terms "two dimensional", "three dimensional" etc. arrays. Do not imagine this as tables and cubes, because it's not accurate and will make your head explode when it goes beyond three dimensions. This "dimensionality" simple refers to the depth of the array, i.e. how many arrays are nested within each other.