Using foreach effectively in PHP

Question

So I don't think I'm making full use of the foreach loop. Here is how I understand foreach.

It goes like foreach(arrayyouwanttoloopthrough as onevalueofthatarray)
No counter or incrementing required, it automatically pulls an array, value by value each loop, and for that loop it calls the value whatever is after the "as".
Stops once it's done with the array.
Should basically replace "for", as long as dealing with an array.

So something I try to do a lot with foreach is modify the array values in the looping array. But in the end I keep finding I have to use a for loop for that type of thing.

So lets say that I have an array (thing1, thing2, thing3, thing4) and I wanted to change it....lets say to all "BLAH", with a number at the end, I'd do

$counter = 0;
foreach($array as $changeval){
$counter++;
$changeval = "BLAH".$counter;
}

I would think that would change it because $changeval should be whatever value it's at for the array, right? But it doesn't. The only way I could find to do that in a] foreach is to set a counter (like above), and use the array with the index of counter. But to do that I'd have to set the counter outside the loop, and it's not even always reliable. For that I'd think it would be better to use a for loop instead of foreach.

So why would you use foreach over for? I think I'm missing something here because foreach has GOT to be able to change values...

Thanks

OH HEY One more thing. Are variables set in loops (like i, or key) accessible outside the loop? If I have 2 foreach loops like

foreach(thing as value)

would I have to make the second one

foreach(thing2 as value2) ]

or else it would have some problems?

Will Vousden · Accepted Answer · 2012-02-01 07:14:54Z

6

You can use a reference variable instead:

foreach ($array as &$value)
{
    $value = "foo";
}

Now the array is full of foo (note the & before $value).

Normally, the loop variable simply contains a copy of the corresponding array element, but the & (ampersand) tells PHP to make it a reference to the actual array element, rather than just a copy; hence you can modify it.

However, as @Tadeck says below, you should be careful in this case to destroy the reference after the loop has finished, since $value will still point to the final element in the array (so it's possible to accidentally modify it). Do this with unset:

unset($value);

The other option would be to use the $key => $value syntax:

foreach ($array as $key => $value)
{
    $array[$key] = "foo";
}

To answer your second question: yes, they are subsequently accessible outside the loop, which is why it's good practice to use unset when using reference loop variables as in my first example. However, there's no need to use new variable names in subsequent loops, since the old ones will just be overwritten (with no unwanted consequences).

edited Feb 1, 2012 at 7:14

answered Feb 1, 2012 at 6:20

Will Vousden

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2 Comments

Tadeck Over a year ago

+1, but please add a note about destroing reference to the last element.

user1159454 Over a year ago

Thank you so much...WOW, I was doing custom named key checks through multiple blocks....looks like I just needed full foreach. Wow. Also can you answer my other question please? Do I need to use new variables each time for foreach loops or will they not carry out of the loop? I edited my post with that, at the end. Thanks!

Ryan Pendleton · Accepted Answer · 2015-04-28 05:39:07Z

2

You want to pass by reference:

$arr = array(1, 2, 3, 4);

foreach ($arr as &$value)
{
    $value = $value * 2;
}

// $arr is now array(2, 4, 6, 8)

edited Apr 28, 2015 at 5:39

answered Feb 1, 2012 at 6:21

Ryan Pendleton

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Comments

aorcsik · Accepted Answer · 2012-02-01 06:22:06Z

0

The extended foreach syntax is like this:

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

Using the $key, you can index the original array:

foreach ($array as $key => $value) {
    $array[$key] = "BLAH";
}

answered Feb 1, 2012 at 6:22

aorcsik

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Comments

djot · Accepted Answer · 2012-02-01 06:22:52Z

0

Incrementing from 0 to ...

foreach($array as $changeval){
  if (!isset($counter)) { $counter = 0; }
  $counter++;
  $changeval = "BLAH".$counter;
}

Using the index/key of the ARRAY

foreach($array as $key => $changeval){
  $changeval = "BLAH".$key;
}

answered Feb 1, 2012 at 6:22

djot

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1 Comment

djot Over a year ago

AND, you should use it as reference in this case, otherwise you have to write $array[$key] = "BLAH".$key;

Community · Accepted Answer · 2017-05-23 10:24:06Z

0

You can use the key when looping with foreach:

foreach ($array as $key => $value)
{
    $array[$key] = "foo";
}

But note that using a reference like Will suggested will be faster for such a case - but anyway, the $key => $value-syntax is quite useful sometimes.

edited May 23, 2017 at 10:24

CommunityBot

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answered Feb 1, 2012 at 6:24

oezi

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