Array inside array by index not working

Question

I seem to be unable to access an array inside an array with index.

this

var_dump($graphed[0]);

give me this

array (size=2)
  'date' => string '02-03-15' (length=8)
  'weight' => string '82.327015155' (length=12)

this

var_dump($graphed[0]['weight']);

gives me this

 string '82.327015155' (length=12)

BUT this

var_dump($graphed[0][1]);

gives me THIS

A PHP Error was encountered

Severity: Notice

Message: Undefined offset: 1

Filename: progress/compare.php

Line Number: 17

null

I have no idea where to go from here. Everything I know about PHP tells me this shouldn't be happening.

By the way, this

echo phpversion();

gives me

 5.5.12

Am I crazy? What's going on?

Why shouldn't it be happening? The inside array is an associative array. It has elements indexed by the keys `date` and `weight`. It doesn't have anything at index `1`. — Barmar, Feb 20 '15 at 01:26
I expect $graphed[0][1] to return the same as $graphed[0]['weight'] because 1 is the index key to 'weight'. date is index 0, weight is index 1. This is something I've learned and used for nearly a year now, and it's always worked. If I'm overlooking something simple or my understanding of index keys is wrong, please correct me. — Goose, Feb 20 '15 at 01:29
Have you been doing it with arrays returned by functions like `mysql_fetch_array()`? This function returns the row with BOTH numeric and named keys. But that's not a general feature of PHP arrays, it's specific to that function. — Barmar, Feb 20 '15 at 01:33

score 2 · Accepted Answer · edited May 23 '17 at 11:50

PHP makes a clear destinction between associative elements (with a key) and indexed elements (with an index). You can't access an element with a key in such way. In other words: they are not interleaved. PHP sees an array as a composition of an Array(List) (like you known them in Java) and a HashMap. The difference with Java is that the keys are furthermore guaranteed to be ordered. But that doesn't mean the keys themselve correspond to an index. In a HashMap<T> you neither can get the i-th value.

Example:

php > var_dump(array(1,2,3,'foo'=>'bar',7));
array(5) {
  [0]=>
  int(1)
  [1]=>
  int(2)
  [2]=>
  int(3)
  ["foo"]=>
  string(3) "bar"
  [3]=>
  int(7)
}

In other words, "foo" doesn't map on an index, the index numbering only considers indexed elements. For the indexer, it's as if "foo" => "bar" doesn't exist.

You can however obtain the list of keys - which is an indexed array - and then pick that key as is written here:

$keys = array_keys($graphed[0]);
echo $graphed[0][$keys[1]];

PHP guarantees key ordering so it's safe to do so (given you know of course the order in advance, or know what you are doing).

Note that the keys include the indices:

php > var_dump(array_keys(array(1,2,3,'foo'=>'bar',7)));
array(5) {
  [0]=>
  int(0)
  [1]=>
  int(1)
  [2]=>
  int(2)
  [3]=>
  string(3) "foo"
  [4]=>
  int(3)
}

Excellent explanation and excellent answer. I always appreciate getting both. Will checkmark if I have success with it. — Goose, Feb 20 '15 at 01:37
A note of advice, the comments on java didn't apply to me, perhaps my assumption made it seem so. — Goose, Feb 20 '15 at 01:57
@Goose: well it is only meant in case you don't know how `HashMap`s work and want to look them up, you can find the mechanism in the java documentation. But it is probably not the essential part of the answer ;). — Willem Van Onsem, Feb 20 '15 at 01:58

Array inside array by index not working

1 Answers1