PHP foreach that returns keys only

Question

Theoretical question that perhaps does not make any sense but still, maybe there is a clever answer.

I want to iterate through array and get its keys and to something with them. A quick example of what I do:

foreach($array as $key => $value) {
    $other_array[$key] = 'something';
}

Now, PHP Mess Detector screams that $value is unused in this scope. Therefore I was thinking that perhaps this is not the best way to access keys of my array.

Any idea how to do it without unnecessarily taking out values out of my array? Does it have any significant performance impact ... or perhaps I am just being paranoid and should carry on without wasting anyone's time with stupid questions :).

It's normal. But U may use this: $keys = array_key ($array); foreach($keys as $key) { $other_array[$key] = 'something'; } — Sergey
– Sergey, Commented Sep 10, 2012 at 7:15
You can suppress mess detector's warnings for instances like these: phpmd.org/documentation/suppress-warnings.html — wkm
– wkm, Commented Mar 11, 2015 at 11:55

inquam · Accepted Answer · 2012-09-10 07:17:26Z

90

You could do something like this

foreach(array_keys($array) as $key) {
 // do your stuff
}

That would make the foreach iterate over an array consisting of the keys from your array instead of the actual array. Note that it's probably not better from a performance standpoint though.

answered Sep 10, 2012 at 7:17

inquam

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9 Comments

Zathrus Writer Over a year ago

I believe array_keys() would be called for each iteration here, so it's probably not the smartest move?

inquam Over a year ago

No from what I remember the array that goes into the foreach if evaluated once. Just like doing foreach(array("a", "b", "c") as $letters) does not create a new array on every iteration. If that's incorrect then sure, create a $keys variable before the foreach... Do you have any reference pointing to the fact that foreach evaluates the array on each iteration?

inquam Over a year ago

From what I can see foreach() is implemented using iterators and thus only calls the function that returns an array once. Then it uses the iterator which points to the existing result set to continue with each item. So it's perfectly safe to to foreach(array_keys() ...

Zathrus Writer Over a year ago

@inquam thank you for your insight! This came from my JavaScript experience, where doing for (var i = 0; i < abc.indexOf('aa'); i++) would call indexOf in each iteration. I guess my idea was that all languages would do it the same way - and I'm glad they don't!

AbcAeffchen Over a year ago

I did some testing and foreach(array_keys($arr) as $key) is about 50% to 60% slower than foreach($arr as $key => $val). But it is right, that there is no difference where you put the array_keys($arr). (tested with an array of 255 elements and 100000 round with php 5.6.0)

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AbcAeffchen · Accepted Answer · 2019-02-15 13:44:44Z

Just ignore this message.

In PHP the way you used foreach is the fastest. It is right, that you should avoid unused variables, but in this case you cannot avoid it, without losing some performance.

E.g. foreach(array_keys($arr) as $key) is about 50% to 60% slower
than foreach($arr as $key => $notUsed).

This issue of phpmd is already reported here and there is also already a pull request here.

Until phpmd is updated you can also use this little hack

In the file /src/main/php/PHPMD/Rule/UnusedLocalVariable.php in the method collectVariables(..) (line 123 in my case) replace

if ($this->isLocal($variable))

by

if ($this->isLocal($variable) && !($this->isChildOf($variable, 'ForeachStatement') && $variable->getName() === '$notUsed'))

This will stop phpmd from reporting $notUsed anywhere inside a foreach loops.

UPDATE: The recommendation above assumes PHP 5.6 (the relevant version at the time of writing this answer). But time went by and now using PHP 7.2 it seems to be the other way around. As always it depends on the exact use case, but for associative arrays with less than 100.000 keys it is faster to store array_keys($arr) in a variable and use this in a foreach loop.

botris · Accepted Answer · 2018-09-12 08:03:08Z

5

It's valid PHP code, don't fix the code, but fix PHP MD. There is a config in PHP MD 2.2 And needs this rule:

<properties>
    <property
        name="allow-unused-foreach-variables"
        description="Allow unused variables in foreach language constructs."
        value="false" />
</properties>

answered Sep 12, 2018 at 8:03

botris

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3 Comments

Olle Härstedt Over a year ago

Uh, where to put those lines??

botris Over a year ago

@OlleHärstedt In the config file you are using, like ruleset.xml and then phpmd codebase/ some_output_format rulesets.xml

Olle Härstedt Over a year ago

There's an answer below that provides context. If anyone else wonders.

18C · Accepted Answer · 2019-11-21 21:39:03Z

2

Hack by 18C :-)

In construction as $key=>$value, the $value is first assigned in PHP before assigning $key.

$array=[1,3,6,10];

foreach($array as $k => $k) 
   print($k.'<br/>');

For your code:

foreach($array as $key => $key) 
{
    $other_array[$key] = 'something';
}

Btw. do not use: foreach($array as $k => &$k). It can disrupt your array.

edited Nov 21, 2019 at 21:39

answered Nov 21, 2019 at 21:31

18C

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3 Comments

finder2 Over a year ago

You depend on an internal behavior. This could end bad.

Robo Robok Over a year ago

@finder2 I agree. They can change this behavior in future versions and the bug will be hard to track. I'd consider foreach ($array as $key => $_)

18C Over a year ago

@RoboRobok Later versions should be compatible with previous versions. If not, it's time to replace PHP with a better model. The same can be said about CSS hacks.

atwixtor · Accepted Answer · 2020-07-30 05:56:54Z

2

If you are using an XML ruleset:

    <rule ref="rulesets/unusedcode.xml/UnusedLocalVariable">
        <properties>
            <property name="allow-unused-foreach-variables" value="true" />
        </properties>
    </rule>

answered Jul 30, 2020 at 5:56

atwixtor

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Comments

core1024 · Accepted Answer · 2012-09-10 07:26:18Z

1

If you want to set all keys to certain value you can just do it this way:

$array = array(
        'foo'=> 'oldval1',
        'bar'=> 'oldval2',
        'baz'=> 'oldval3'
);

$other_array = array_fill_keys(array_keys($array), 'something');
print_r($other_array);

This will produce:

Array
(
    [foo] => something
    [bar] => something
    [baz] => something
)

answered Sep 10, 2012 at 7:26

core1024

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1 Comment

RandomWhiteTrash Over a year ago

+1 for the idea although that was not what I had in mind - example was oversimplified :). Thanks anyway - learning something new :)

Darksynx · Accepted Answer · 2017-10-13 22:58:29Z

1

loop to avoid a foreach in a while.

$a = ['1','A','B','12','ui'];

 while(true) { sleep(1);
    $b = next($a) ? current($a): reset($a);
    echo key($a) , ':' , $b , PHP_EOL;
 }

answered Oct 13, 2017 at 22:58

Darksynx

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1 Comment

18C Over a year ago

while(3) is shorter. :-) Additionally you should avoid potentially infinite loops.

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