How to slice an array by key, not offset?

Question

PHP function array_slice() returns sequence of elements by offset like this:

// sample data
$a = array('a','b','c',100=>'aa',101=>'bb',102=>'cc'); 

// outputs empty array because offset 100 not defined
print_r(array_slice($a,100));

Current function arguments:

array_slice ( $array, $offset, $length, $preserve_keys)

I need something like this:

array_slice ( $array, **$key**, $length, $preserve_keys)

That outputs this according to above print_r:

array (
   100 => aa,
   101 => bb,
   102 => cc
)

salathe · Accepted Answer · 2013-05-22 19:27:03Z

19

To find the offset of the key, use array_search() to search through the keys, which can be retrieved with array_keys(). array_search() will return FALSE when the specified key (100) is not present in the array ($a).

$key = array_search(100, array_keys($a), true);
if ($key !== false) {
    $slice = array_slice($a, $key, null, true);
    var_export($slice);
}

Prints:

array (
  100 => 'aa',
  101 => 'bb',
  102 => 'cc',
)

edited May 22, 2013 at 19:27

answered May 22, 2013 at 19:21

salathe

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Comments

CragMonkey · Accepted Answer · 2019-08-07 13:17:03Z

11

Returns parts of $array whose keys exist in the array $keys:

array_intersect_key($array,array_flip($keys));

answered Aug 7, 2019 at 13:17

CragMonkey

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2 Comments

brett Over a year ago

Not sure if your answer answers the OP's question, but this is exactly what I've been googling for :)

Wael Mahmoud Over a year ago

@brett me too :)

Philip · Accepted Answer · 2018-03-26 22:32:49Z

3

Alternative solution

If you want to slice the array by using keys for the items you want to get, you could use the following custom function to do so:

function array_slice_keys($array, $keys = null) {
    if ( empty($keys) ) {
        $keys = array_keys($array);
    }
    if ( !is_array($keys) ) {
        $keys = array($keys);
    }
    if ( !is_array($array) ) {
        return array();
    } else {
        return array_intersect_key($array, array_fill_keys($keys, '1'));
    }
}

Usage:

$ar = [
    'key1' => 'a',
    'key2' => 'b',
    'key3' => 'c',
    'key4' => 'd',
    'key5' => 'e',
    'key6' => 'f',
];

// Get the values from the array with these keys:
$items = array_slice_keys( $ar, array('key2', 'key3', 'key5') );

Result:

Array
(
    [key2] => b
    [key3] => c
    [key5] => e
)

answered Mar 26, 2018 at 22:32

Philip

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2 Comments

GuyPaddock Over a year ago

Why is $keys an optional argument? It seems odd that this function handles so many defensive cases of getting no keys, getting an array without keys, getting... an empty array?

YTZ Over a year ago

+1 I prefer this over array_search answer because it's easier to add more keys this way. But you could've just answered array_intersect_key($array, array_fill_keys($keys, '1')) without the (unnecessary) function.

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