How to "skip" iteration when building a new array using recursion

Question

I am stuck on something that might be very simple.

I am creating a new array by looping through an existing array using a recursion function yet I can not seem to get the values to stick to the new array. The function, in the end, will be a bit more complex, but for now I need some help.

I have tried soooo many ways to get this to work but I am at a loss right now.

Here is my php function

function recursive($array) {

    $newArray = array();

    foreach($array as $key => $value) {

        if(is_array($value)){
            recursive($value);                              
        }else{
            $newArray[] = $value;
        }   

    }

    return $newArray;

}

As is, the new array never gets filled...BUT, if I change this line...

recursive($value); // Why can't I just call the recursive function here?

...to...

$newArray[] = recursive($value); // Instead of having to set a new value to the new array?

everything works properly...except that my goal was to create a flat array with only the values.

So my question is, why is it necessary to set a new array value in order to call the recursive function again? Ideally, I want to skip setting a new array value if the value is an array and just continue the loop through the original array.

I guess I should've made that an answer instead of a comment — JohannesB
– JohannesB, Commented Aug 24, 2016 at 17:26

ofca · Accepted Answer · 2016-08-24 17:31:11Z

2

Use array_merge:

function recursive($array) {

    $newArray = array();

    foreach($array as $key => $value) {

        if(is_array($value)){
            $newArray = array_merge($newArray, recursive($value));                              
        }else{
            $newArray[] = $value;
        }   

    }

    return $newArray;

}

...or you could use special operator:

function recursive($array) {

    $newArray = array();

    foreach($array as $key => $value) {

        if(is_array($value)){
            $newArray += recursive($value);                              
        }else{
            $newArray[] = $value;
        }   

    }

    return $newArray;

}

...or pass a variable by reference like this:

function recursive($array, &$newArray = null) {

    if (!$newArray) {
        $newArray = array();
    }

    foreach($array as $key => $value) {

        if(is_array($value)){
            recursive($value, $newArray);                              
        }else{
            $newArray[] = $value;
        }   

    }

    return $newArray;   
}

edited Aug 24, 2016 at 17:31

answered Aug 24, 2016 at 17:24

ofca

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2 Comments

VIDesignz Over a year ago

Can I ask you a related question? When the $newArray gets returned each cycle, is it always the sum of all the other cycles? I am just confused a bit on what is actually happening when the return is made.

ofca Over a year ago

Yes it is always the sum of all other cycles. Did you read about array_merge php.net/array_merge ?

Joseph Young · Accepted Answer · 2016-08-24 17:22:42Z

1

use array_merge() to merge the array returned from recursive($value); and $newArray

$newArray = array_merge($newArray,recursive($value));

You can guarantee that $newArray will be flat after this, as the previous value of $newArray was flat, and recursive always returns a flat array, so the combination of both should be a flat array.

answered Aug 24, 2016 at 17:22

Joseph Young

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Comments

nick.graziano · Accepted Answer · 2016-08-24 17:25:19Z

1

You aren't doing anything with the return from your recursive function. Try this:

function recursive($array) {

    $newArray = array();

    foreach($array as $key => $value) {

        if(is_array($value)){
            // This is what was modified
            $newArray = array_merge($newArray, recursive($value));                              
        }else{
            $newArray[] = $value;
        }   

    }

    return $newArray;

}

answered Aug 24, 2016 at 17:25

nick.graziano

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