从PHP数组访问数据

Question

I have an array as follows

$user_followers = [
    [
        'user_id'   => '1',
        'followers' => ['3', '4', '5']
    ],
    [
        'user_id'   => '2',
        'followers' => ['1', '5']
    ],
    [
        'user_id'   => '3',
        'followers' => ['1', '5', '4']
    ],
    [
        'user_id'   => '4',
        'followers' => ['3', '1', '5']
    ],
    [
        'user_id'   => '5',
        'followers' => ['1', '2', '4']
    ],
];

What I need to do is obtain the followers data array based on the user_id value therefore if I have a user_id of 2 it will return an array containing the followers array data. I am not sure whether I need to re organise my array structure to do this.

Answer 1

You can easily key the followers to all user_id s with array_column() , then you can access the followers by their user id:

$followers = array_column($user_followers, 'followers', 'user_id');

print_r($followers[2]);

Which gives ( Demo ):

Array
(
    [0] => 1
    [1] => 5
)

This works as long as the key is unique (which is the third parameter of array_column and seems the case here as user_id sounds like a unique id).

Answer 2

If you version php < 5.5 you can't use array_column

Fatal error: Call to undefined function array_column()

So you can create your own function:

if ( !function_exists('array_column') ) {
  function array_column( $collection, $field, $keyfield = null, $desired_id = null ) {
    $items = array();
    foreach ( $collection as $k => $item ) {
      $key = $keyfield ? $item[$keyfield] : $k;
        if( $desired_id == $key){     
            $items[] = $item[$field];
        }
    }
    return $items;
  }
}

var_dump(array_column($user_followers, 'followers', 'user_id', 2));

Result:

array (size=1)
  0 => 
    array (size=2)
      0 => string '1' (length=1)
      1 => string '5' (length=1)

Example - Demo

Answer 3

Well, first of all, your array should look like this:

$users = array(
        array(
            'user_id'   => '1',
            'followers' => array('3', '4', '5')
        ),
        array (
            'user_id'   => '2',
            'followers' => array('1', '5')
        ),
        array(
            'user_id'   => '3',
            'followers' => array('1', '5', '4')
        ),
        array(
            'user_id'   => '4',
            'followers' => array('3', '1', '5')
        ),
        array(
            'user_id'   => '5',
            'followers' => array('1', '2', '4')
        ),
    );

You can create a function like this:

// Returns the array of user data
function getUserFollowers($users, $desired_id) {
    foreach($users as $user) {
        if ( $user['user_id'] == $desired_id ) {
            return $user['followers'];
        }
    }
}

Where the parameter $users is the array of all your users and $id is the id of the user you are looking for. The foreach loop iterates through all of your users and checks the id of all the users. If it matches, it returns the array of followers.

Note, this process can use up a lot of memory if the array is huge. Where are you loading this data from?