Using nested foreach loop on same array

Question

Is it ok to loop array again in nested loop and also change the array?

I've an URL's array with entries(as array key) of either an URL or domain:example.com

In case of this entry : domain:example.com I want to remove all URLS containing example.com as domain:

foreach (array_keys($urls1) as $line) {
        if (preg_match('/domain:(.*)/i', $line, $matches)) {
            $domain = $matches[1];
            foreach (array_keys($urls1) as $line2) {
                if ($url_domains[$line2] == $domain) {
                    unset($urls1[$line2]);
                }
            }
        }
    }

It's certainly ok to loop over it. But I'm not sure wheter it's ok to modify it and how. — Pavel Šimerda, Apr 06 '14 at 06:01
It might be better to do an [`array_walk`](http://php.net/array_walk) or [`array_map`](http://php.net/array_map). If it works fine with this code, I don't see any reason to change it. — Dave Chen, Apr 06 '14 at 06:09
http://stackoverflow.com/questions/10057671/how-foreach-actually-works?rq=1 http://stackoverflow.com/questions/1685689/how-is-an-array-in-a-php-foreach-loop-read?rq=1 http://stackoverflow.com/questions/2008866/unsetting-array-values-in-a-foreach-loop?rq=1 — Andy Librian, Apr 06 '14 at 06:16

Toby Allen · Answer 1 · 2016-10-24T06:51:49.157

There is no problem looping over it a second time, however you will get yourself and your code into a big knot if you start removing items. My suggestion would be to save a copy and modify that.

This is not ideal, but I'm not sure what you wish to do.

//Make a copy of your array
$URLCopy  = $urls1;

foreach (array_keys($urls1) as $line) {
    if (preg_match('/domain:(.*)/i', $line, $matches)) {
        $domain = $matches[1];
        foreach (array_keys($urls1) as $line2) {
            if ($url_domains[$line2] == $domain) {
                unset($URLCopy[$line2]);
            }
        }
    }
 }

score 0 · Answer 2 · answered Feb 09 '17 at 04:16

I ran into a similar problem and making a copy of the array was the answer. This was my problem:

If a particular text string existed towards the beginning of the file and (an array of approximately 80 members) matched a string towards the end of the file, I had to remove three lines towards the end. The problem that happened when I didn't use a copy is that the index would reset from 30, back to 9, and this caused me some issues.

This is what worked for me.

$rowCopy = $row

foreach($row as $index => &$line) {

    ////previous code

    if ($line[0] === "NM1" && $line[1] === "77") {
        //read through the $row array and find the NM*85 string
        foreach ($rowCopy as $index2 => $lineT) {

            if ($s = strpos($lineT, "NM1*85") !== false) {
                $npiTest = explode("*", $lineT);
                if (strcmp(preg_replace("/[^0-9,.]/", "", $npiTest[9]), $line[9]) === 0) {
                    // $line = false;
                    $index--;
                    unset($row[$index + 1]);
                    $index++;
                    unset($row[$index + 1]);
                    $index++;
                    unset($row[$index + 1]);
                    $erased = $erased + 3;
                    $index++
                }
            }
        }

    }

}

Using nested foreach loop on same array

2 Answers2