This is such a basic question, but I'm unable to find a clear answer anywhere. As I have understood it, storing an object in an array should store a reference, not a copy... so any changes you make to the object subsequently should be visible when you access the object via the array.
When searching this topic, I've run across many questions asking how to store copies of objects in an array, so that this doesn't happen, and the answer is always that you need to use clone. To me, this would SEEM to indicate that by default a reference would be stored.
So I was really confused when I encountered the following behavior...
$inner = ['key1'=>"value1"];
$outer = [];
$outer['inner'] = $inner;
$inner['key2'] = "value2";
print_r($inner);
echo "<br>";
print_r($outer['inner']);
OUTPUT:
Array
(
[key1] => value1
[key2] => value2
)
Array
(
[key1] => value1
)
I've been doing pretty serious PHP coding for 2 years now, and this seems to go against everything I thought I knew about arrays, so it's really tripping me up.
Similar questions on Stack Exchange tend to get answers saying "you should refer to the documentation". But nothing I can find in the docs address this clearly.
You're just assigning the value of $inner at that time to $outer['inner']. After that, $outer['inner'] is its own array, it doesn't maintain a reference to $inner. This is expected behaviour with arrays.
From the PHP documentation on arrays:
Array assignment always involves value copying. Use the reference operator to copy an array by reference.
So if you want to maintain a reference, you need to use the reference operator:
// Set the original values
$inner = ['key1' => 'value1'];
// Assign by reference
$outer = [];
$outer['inner'] = &$inner;
// Modify the original array afterwards
$inner['key2'] = 'value2';
You will then find that $outer['inner'] maintains the reference to $inner, even though it was changed after the assignment.
print_r($inner);
print_r($outer['inner']);
They will be the same:
Array ( [key1] => value1 [key2] => value2 )
Array ( [key1] => value1 [key2] => value2 )
Note: Using references is generally not a good idea and I can't remember ever really needing them. There is a good discussion on Stack Overflow about this already.
Related
I have an array such as:
$var = array('hi','ho',rand(2,5));
What I would like to echo is the entire array, exactly as written.
Normally when you try a print_r, it shows as:
Array (
[0] => hi
[1] => ho
[2] => 3
)
But I want:
Array (
[0] => hi
[1] => ho
[2] => rand(2,5)
)
You can get this with file_get_contents, but is there any way to do so within the actual PHP file?
I don't think it's possible because when array is created, random value is assigned to element with index 2 and you cannot check how this value was created.
I don't think it's possible, since the rand is already evaluated as soon as you set the array to some variable.
A workaround would be the hold the expression as a string and then eval it when you need it. Like this:
$varStr = "array('hi','ho',rand(2,5))";
echo $varStr;
// when you actually need it
$var = eval($varStr);
However, this is almost never a good idea. Providing a use-case where you need this might help come up with a better solution.
I want to check, if array A contains all the items from array B (may contain others, but must contain all), when both arrays are multidimensional, i.e. can contains different variable types.
I've seen a lot (particularly this, this, this, this, this and this, also this, this and this as well). I've read PHP doc. Everything, that I checked, fails with "Array to string conversion" notice. Especially wen using array_intersect() or array_diff().
I'm using strict error checking, so notices actually holds further execution of entire script and are something, I don't generally like and want to avoid. Is it possible in this case?
My array A is:
Array
(
[0] => content/manage/index
[Content] => Array
(
[title] =>
[type] => 5
[category] =>
[recommended] =>
[featured] =>
[status] =>
[views] =>
[last_access_date] =>
[creation_date] =>
[modification_date] =>
[availability_date] =>
[author_id] =>
)
)
My array B is:
Array
(
[0] => /content/manage/index
[Content] => Array
(
[type] => 1
)
)
So, is there any way I can if I can use array_intersect on multidimensional arrays containing different variable types without getting notice?
My problem (and question) came out of misunderstanding, what "Array to string conversion" notice really means. In my case, it was trying to tell me, that I'm trying to walk multidimensional array with functions designed to be used on single dimension array.
Understanding that led me to a solution within few seconds. There are many of them here, on SO, but the one given by deceze here looked the best for me. So I adopted it into the form of such function:
function recursiveArrayIntersect($array1, $array2)
{
$array1 = array_intersect_key($array1, $array2);
foreach($array1 as $key=>&$value)
{
if(is_array($value)) $value = recursiveArrayIntersect($value, $array2[$key]);
}
return $array1;
}
I adopted it to my project and my way of coding, but all the credits still goes to deceze (his answer here)!
Now I can find an intersection of virtually any array, no matter what kind of variable types it contain and no matter of, how deep it is (how many subarrays it contains).
I have a very strange array sorting related problem in PHP that is driving me completely crazy. I have googled for hours, and still NOTHING indicates that other people have this problem, or that this should happen to begin with, so a solution to this mystery would be GREATLY appreciated!
To describe the problem/question in as few words as possible: When sorting an array based on values inside a multiple levels deeply nested array, using a foreach loop, the resulting array sort order reverts as soon as execution leaves the loop, even though it works fine inside the loop. Why is this, and how do I work around it?
Here is sample code for my problem, which should hopefully be a little more clear than the sentence above:
$top_level_array = array('key_1' => array('sub_array' => array('sub_sub_array_1' => array(1),
'sub_sub_array_2' => array(3),
'sub_sub_array_3' => array(2)
)
)
);
function mycmp($arr_1, $arr_2)
{
if ($arr_1[0] == $arr_2[0])
{
return 0;
}
return ($arr_1[0] < $arr_2[0]) ? -1 : 1;
}
foreach($top_level_array as $current_top_level_member)
{
//This loop will only have one iteration, but never mind that...
print("Inside loop before sort operation:\n\n");
print_r($current_top_level_member['sub_array']);
uasort($current_top_level_member['sub_array'], 'mycmp');
print("\nInside loop after sort operation:\n\n");
print_r($current_top_level_member['sub_array']);
}
print("\nOutside of loop (i.e. after all sort operations finished):\n\n");
print_r($top_level_array);
The output of this is as follows:
Inside loop before sort operation:
Array
(
[sub_sub_array_1] => Array
(
[0] => 1
)
[sub_sub_array_2] => Array
(
[0] => 3
)
[sub_sub_array_3] => Array
(
[0] => 2
)
)
Inside loop after sort operation:
Array
(
[sub_sub_array_1] => Array
(
[0] => 1
)
[sub_sub_array_3] => Array
(
[0] => 2
)
[sub_sub_array_2] => Array
(
[0] => 3
)
)
Outside of loop (i.e. after all sort operations finished):
Array
(
[key_1] => Array
(
[sub_array] => Array
(
[sub_sub_array_1] => Array
(
[0] => 1
)
[sub_sub_array_2] => Array
(
[0] => 3
)
[sub_sub_array_3] => Array
(
[0] => 2
)
)
)
)
As you can see, the sort order is "wrong" (i.e. not ordered by the desired value in the innermost array) before the sort operation inside the loop (as expected), then is becomes "correct" after the sort operation inside the loop (as expected).
So far so good.
But THEN, once we're outside the loop again, all of a sudden the order has reverted to its original state, as if the sort loop didn't execute at all?!?
How come this happens, and how will I ever be able to sort this array in the desired way then?
I was under the impression that neither foreach loops nor the uasort() function operated on separate instances of the items in question (but rather on references, i.e. in place), but the result above seems to indicate otherwise? And if so, how will I ever be able to perform the desired sort operation?
(and WHY doesn't anyone else than me on the entire internet seem to have this problem?)
PS.
Never mind the reason behind the design of the strange array to be sorted in this example, it is of course only a simplified PoC of a real problem in much more complex code.
Your problem is a misunderstanding of how PHP provides your "value" in the foreach construct.
foreach($top_level_array as $current_top_level_member)
The variable $current_top_level_member is a copy of the value in the array, not a reference to inside the $top_level_array. Therefore all your work happens on the copy and is discarded after the loop completes. (Actually it is in the $current_top_level_member variable, but $top_level_array never sees the changes.)
You want a reference instead:
foreach($top_level_array as $key => $value)
{
$current_top_level_member =& $top_level_array[$key];
EDIT:
You can also use the foreach by reference notation (hat tip to air4x) to avoid the extra assignment. Note that if you are working with an array of Objects, they are already passed by reference.
foreach($top_level_array as &$current_top_level_member)
To answer you question as to why PHP defaults to a copy instead of a reference, it's simply because of the rules of the language. Scalar values and arrays are assigned by value, unless the & prefix is used, and objects are always assigned by reference (as of PHP 5). And that is likely due to a general consensus that it's generally better to work with copies of everything expect objects. BUT--it is not slow like you might expect. PHP uses a lazy copy called copy on write, where it is really a read-only reference. On the first write, the copy is made.
PHP uses a lazy-copy mechanism (also called copy-on-write) that does
not actually create a copy of a variable until it is modified.
Source: http://www.thedeveloperday.com/php-lazy-copy/
You can add & before $current_top_level_member and use it as reference to the variable in the original array. Then you would be making changes to the original array.
foreach ($top_level_array as &$current_top_level_member) {
I have an $array with some values stored on it. Now, if I do :
$array=array();
all values/index are deleted? Or I need to use unset() before it?
A new array is being created with array() and this new array object is assigned to the variable $array.
The variable ($array) no longer points to the original array object -- and because PHP is a garbage collected language -- the original array object will be eligible for reclamation if (and only if) it is no longer strongly reachable from a root object. (The actual time the previous array object and objects it contained are actually deleted depends on other factors.)
Happy coding.
See PHP Garbage Collection Manual for more details -- PHP uses a hybrid GC (ref-count and cycle-breaking).
Yes the reassignment just wipes out all the data from the array. But to get clear understanding of the garbage collection please check the PHP Reference Counting Basics.
$array = array('apples', 'oranges', 'bananas');
print_r($array);
//Array ( [0] => apples [1] => oranges [2] => bananas )
$array = array();
print_r($array);
//Array ( )
Your intent would be clearer if you used something like
$array = null;
(and even clearer if you used a better name than $array!)
I'm using cakephp and am getting back a "double array" where it is giving me 2 arrays where it should be 1, I have looked into the issue as far as cakephp and can't figure it out and just want to move past this for now so I am wondering if anyone knows how to unset a second array if a variable has 2 arrays.. below is the print_r of the array, its just one variable that has this, which I find odd.. so I want to make it so there is not a 2nd set of duplicate values, if I do an array_push it pushes both values for that index into the resulting new array index so that won't work
one variable is equal to the following:
Array ( [0] => 42 [1] => 62 ) Array ( [0] => 42 [1] => 62 )
EDIT:
This is not an issue of my printing out the array twice accidentally, as I said above, with a foreach array_push of the variable, i end up with this, which is odd:
Array ( [0] => 4242 [1] => 6262 )
EDIT:
This is the cakephp database call that I am using, I know I didn't ask this in regards to cakephp but since some people think this is impossible i am posting this just so you can see what it does if you want
$specificfields_array = $this->Mymodel->find('list', array('fields' =>'Mymodel.id'),
'conditions' => array('emailgroup' => $categorynumber, 'sent' => '0');));
EDIT:
This is what a "foreach" array_push is:
$mynewarray = array();
foreach ($specificfields as $specificfields_current) {
array_push ($mynewarray, $specificfields_current);
}
A variable cannot "have two arrays". It can be one array that has two arrays nested. The scenario you describe is impossible (probably there are two print_r there or there is a < character hiding stuff – check the HTML source).
Can you post the controller, the model and the view file with your print_r calls to the http://bin.cakephp.org/ site and post the links back here so we can see all of your code?