Undefined offset when creating/adding Multi-Dimensional array values

Question

I am adding to an array on the fly, using code similar to this;

 $arrayF[$f+1][$y][$x+1] = $value+1;

But I receive this in the error report:

Undefined offset: 1

QUESTION: Why do I get an undefined offset when I am trying to CREATE an array value?

What can I do about it?

Further info, if relevant: It occurs in a loop as so, where I am 'sprawling' through a masterArray

if (is_array($arrayF[$f])){ 
    foreach ($arrayF[$f] as  $key2 => $arrayF2) {
        $y = $key2;
        foreach ($arrayF2 as $key3 =>$value) {
            $x = $key3;
            if (($y<=100)&& ($y>=1)&&($x<=100)&&($x>=1)){
                if ($value < $arrayMaster[$y][$x])  {
                    $arrayMaster[$y][$x] = $value;//resets value in a master array
                    $arrayF[$f+1][$y][$x+1] = $value+1;//creates a new array for F to 'sprawl' with
                    $max = $f+1;
                }
            }
        }
    }
}

have you tried specify that is an array with $arrayF = new array()? it can help maybe — Marco Mura
– Marco Mura, Commented Dec 18, 2014 at 10:53
@MarcoMura: arrays don't require the new keyword. They never have, and never will and for this code to even get in the loop $arrayF has to be an array, or the outer if would evaluate to false, so initializing $arrayF won't make any difference — Elias Van Ootegem
– Elias Van Ootegem, Commented Dec 18, 2014 at 11:10

Elias Van Ootegem · Accepted Answer · 2014-12-19 12:23:32Z

2

Simple, because when you do this: $arrayF[$f+1][$y][$x+1] = $value+1;, you can't be sure that $arrayF[$f+1] is a valid offset/index/key. All you know for sure is that is_array($arrayF[$f]) is true.

The fix is rather simple:

if (!isset($arrayF[$f+1]) || !is_array($arrayF[$f+1])) {
    $arrayF[$f+1] = array(
        $y => array()
    );
} else if (!is_array($arrayF[$f+1][$y])) {
        $arrayF[$f+1][$y] = array();
}
$arrayF[$f+1][$y][$x+1] = $value+1;

Now why are you getting the notice? That's because, if $arrayF[$f+1] doesn't exist, PHP will happily create it for you when you do a simple assignment like $arrayF[$f+1] = 'foobar';. However, you're accessing a (possibly) non-existent offset (which would evaluate to null), and try to use it as an array: $arrayF[$f+1][$y], if $arrayF[$f+1] doesn't exist, there can't be a $y index in there, hence the notice: PHP is warning you about a possible bug in your code

Update:
As discussed in the comments below: is_array can produce an undefined offset notice, seeing as it assumes that the argument you pass to it actually exists. To avoid such notices from being produced, an isset check is required, so I've updated the code above accordingly.

edited Dec 19, 2014 at 12:23

answered Dec 18, 2014 at 10:57

Elias Van Ootegem

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7 Comments

EnglishAdam Over a year ago

Thanks, So I would have to prepopulate ALL values of a multi-dimensional arrays until level dimensions-1?

Elias Van Ootegem Over a year ago

@Gamemorize: Then leave out the if, and just write $arrayF[$f+1] = array($y => array($x+1 => $value+1));, but be weary: in that case, you're bound to re-assign $arrayF[$f+1] endlessly, effectively making your loops redundant

EnglishAdam Over a year ago

Thx Elias, I'll have a go at your first suggestions, which effectively dynamically declares then assigns. Should be back 'shortly' to accept answer. Thx again!

Elias Van Ootegem Over a year ago

think of $array[$i] as saying "give me value $i in variable $array, and do something with it", if you want $array[$i][$j], then think of it as ($array[$i])[$j] (take the value of $array[$i], then give me index $j of that value). So in order to add a value 3 levels deep, you have to make sure that all the arrays exist: the main variable must be an array ($arrayF), the offset you use must resolve to an array ($f+1) and the offset in there ($y) must be an array, too. Only then can you assign safely (create $x+1, and assign a value). Hence the checks are required

EnglishAdam Over a year ago

It seems to be that is_array($array[$x]) will throw undefined offset but isset($array[$x]) does not. Is this the case as expected?

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