Remove all elements from array that have a key that does not start with a certain string

Question

I have an array that looks like this:

array(
  'abc' => 0,
  'foo-bcd' => 1,
  'foo-def' => 1,
  'foo-xyz' => 0,
  // ...
)

How can I retain only the elements that start with foo-?

Having an array with prefixes like this is a code smell. Extract those values into an object holding these values or at least make the prefix point to an array and add the values to it. — Gordon
– Gordon, Commented Feb 12, 2011 at 17:29

Black · Accepted Answer · 2021-02-01 16:02:47Z

153

Functional approach:

$array = array_filter($array, function($key) {
    return strpos($key, 'foo-') === 0;
}, ARRAY_FILTER_USE_KEY);

Procedural approach:

$only_foo = array();
foreach ($array as $key => $value) {
    if (strpos($key, 'foo-') === 0) {
        $only_foo[$key] = $value;
    }
}

Procedural approach using objects:

$i = new ArrayIterator($array);
$only_foo = array();
while ($i->valid()) {
    if (strpos($i->key(), 'foo-') === 0) {
        $only_foo[$i->key()] = $i->current();
    }
    $i->next();
}

edited Feb 1, 2021 at 16:02

Black

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answered Feb 12, 2011 at 16:58

erisco

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4 Comments

Jason Over a year ago

Note PHP 5.6 adds the ARRAY_FILTER_USE_KEY flag to array_filter(), which you can use instead of supplying a custom array_filter_key() function.

Chris Sprague Over a year ago

Can you please just add the array_filter_key function to your example? I kept getting an error and had to re-read your post multiple times before recognizing my mistake. It's not hard to just post the code, save a click, and save somebody time troubleshooting. Thanks.

mickmackusa Over a year ago

This answer will be fooling a lot of researchers and wasting their time until they realize that array_filter_key() is not a native function. Researchers shouldn't have to link-chase to receive the whole solution.

Tofandel Over a year ago

With php8 you can also use str_starts_with($key, 'foo-')

Matías Cánepa · Accepted Answer · 2016-02-29 13:42:06Z

38

This is how I would do it, though I can't give you a more efficient advice before understanding what you want to do with the values you get.

$search = "foo-";
$search_length = strlen($search);
foreach ($array as $key => $value) {
    if (substr($key, 0, $search_length) == $search) {
        ...use the $value...
    }
}

edited Feb 29, 2016 at 13:42

Matías Cánepa

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answered Feb 12, 2011 at 16:56

user613857

2 Comments

Iago Over a year ago

You also can use: foreach ($array as $key => $value) { if (0 === strpos($key, 'foo-')) { ...use the $value... } }

Jason Over a year ago

The "4" here needs to be adjusted to whatever length "foo-" is.

Suresh Velusamy · Accepted Answer · 2017-09-25 10:56:40Z

26

Simply I used array_filter function to achieve the solution as like follows

<?php

$input = array(
    'abc' => 0,
    'foo-bcd' => 1,
    'foo-def' => 1,
    'foo-xyz' => 0,
);

$filtered = array_filter($input, function ($key) {
    return strpos($key, 'foo-') === 0;
}, ARRAY_FILTER_USE_KEY);

print_r($filtered);

Output

Array
(
    [foo-bcd] => 1
    [foo-def] => 1
    [foo-xyz] => 0
)

For live check https://3v4l.org/lJCse

answered Sep 25, 2017 at 10:56

Suresh Velusamy

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1 Comment

Netsurfer Over a year ago

Pay attention that this needs at least PHP 5.6.0 (due to the use of the constant ARRAY_FILTER_USE_KEY). In prior versions you can use: [code]

heena Jethva · Accepted Answer · 2017-02-03 12:47:18Z

20

From PHP 5.3 you can use the preg_filter function: here

$unprefixed_keys = preg_filter('/^foo-(.*)/', '$1', array_keys( $arr ));

// Result:
// $unprefixed_keys === array('bcd','def','xyz')

edited Feb 3, 2017 at 12:47

heena Jethva

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answered Oct 30, 2013 at 20:58

biziclop

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Comments

Developer-Sid · Accepted Answer · 2012-12-05 17:39:32Z

19

$arr_main_array = array('foo-test' => 123, 'other-test' => 456, 'foo-result' => 789);

foreach($arr_main_array as $key => $value){
    $exp_key = explode('-', $key);
    if($exp_key[0] == 'foo'){
         $arr_result[] = $value;
    }
}

if(isset($arr_result)){
    print_r($arr_result);
}

answered Dec 5, 2012 at 17:39

Developer-Sid

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2 Comments

Martijn Over a year ago

Before you take this solution, the one below is better. Never use array functions where simple string function will suffice. String functions are WAY faster. This will 'lag' with large arrays.

Jason Over a year ago

I don't see the point of exploding the array keys into arrays. They are strings and the original question is just asking about a string prefix. Also OP is asking for "the elements" rather than just the values, so the numerically-indexed result is going to be useless - the keys are clearly important here, with the values being (presumably) anonymous 1/0 flags.

user142162 · Accepted Answer · 2011-02-12 16:55:15Z

14

foreach($arr as $key => $value)
{
   if(preg_match('/^foo-/', $key))
   {
        // You can access $value or create a new array based off these values
   }
}

answered Feb 12, 2011 at 16:55

user142162

3 Comments

jfoucher Over a year ago

A regex is not really necessary in this case. The substr method is slightly more efficient

user142162 Over a year ago

@jfoucher: That's true, however, personally, it's easier to read.

Jason Over a year ago

It may be more flexible, but comes at the cost of needing to escape special preg characters if you don't want to do a regular expression match. So use with caution. I would suggest you use REs only if you need REs.

Keyur · Accepted Answer · 2017-09-15 10:37:53Z

8

Modification to erisco's Functional approach,

array_filter($signatureData[0]["foo-"], function($k) {
    return strpos($k, 'foo-abc') === 0;
}, ARRAY_FILTER_USE_KEY);

this worked for me.

edited Sep 15, 2017 at 10:37

answered Nov 30, 2016 at 7:15

Keyur

1,1213 gold badges23 silver badges42 bronze badges

3 Comments

Maciek Over a year ago

Any difference in performance or usefulness compared to array_filter_key version provided by @erisco?

Keyur Over a year ago

Nope. There should not any difference in performance I guess. @MaciekSemik

Jeremy John Over a year ago

This is a core PHP function so it should be preferred.

Netsurfer · Accepted Answer · 2017-12-01 09:02:39Z

In addition to @Suresh Velusamy's answer above (which needs at least PHP 5.6.0) you can use the following if you are on a prior version of PHP:

<?php

$input = array(
    'abc' => 0,
    'foo-bcd' => 1,
    'foo-def' => 1,
    'foo-xyz' => 0,
);

$filtered = array_filter(array_keys($input), function($key) {
    return strpos($key, 'foo-') === 0;
});

print_r($filtered);

/* Output:
Array
(
    [1] => foo-bcd
    [2] => foo-def
    [3] => foo-xyz
)
// the numerical array keys are the position in the original array!
*/

// if you want your array newly numbered just add:
$filtered = array_values($filtered);

print_r($filtered);

/* Output:
Array
(
    [0] => foo-bcd
    [1] => foo-def
    [2] => foo-xyz
)
*/

Please note (or at least for me) that this only returns the array key's that match. The answer below by @suresh-velusamy extracts the actual array with the original $key => $value pair. It is stated in the output of this answer, but was not yet mentioned explicitly

mickmackusa · Accepted Answer · 2020-11-27 12:50:17Z

From PHP5.6, the array keys can be the sole subject of the filtration by using the ARRAY_FILTER_USE_KEY constant/flag.

From PHP7.4, arrow functions make custom functions more concise and allow values to be passed into the custom function's scope without use().

From PHP8, str_starts_with() can take the place of strpos(...) === 0

Code: (Demo)

$array = [
  'abc' => 0,
  'foo-bcd' => 1,
  'foo-def' => 1,
  'foo-xyz' => 0,
];

$prefix = 'foo';

var_export(
    array_filter(
        $array,
        fn($key) => str_starts_with($key, $prefix),
        ARRAY_FILTER_USE_KEY
    )
);

Output:

array (
  'foo-bcd' => 1,
  'foo-def' => 1,
  'foo-xyz' => 0,
)

mondayrris · Accepted Answer · 2022-04-02 17:35:47Z

You might pass a key-value single dimensional array and a specific prefix string 'foo-' to the function just made: filter_array_key_with_prefix

<?php

function filter_array_key_with_prefix($arr, $prefix) {
  return array_filter($arr, function($k) use ($prefix) {
    return strpos($k, $prefix) === 0;
  }, ARRAY_FILTER_USE_KEY);
}

$arr = array(
  'abc' => 0,
  'foo-bcd' => 1,
  'foo-def' => 1,
  'foo-xyz' => 0,
  // ...
);

$filtered = filter_array_key_with_prefix($arr, 'foo-');

var_dump($filtered);

Output:

array(3) {
  ["foo-bcd"]=>
  int(1)
  ["foo-def"]=>
  int(1)
  ["foo-xyz"]=>
  int(0)
}

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