php array_merge associative arrays

Question

I'm trying to prepend an item to the beginning of an associative array. I figured the best way to do this is to use array_merge, but I'm having some odd consequences. I get the id and Name of products from a mysql database, and it gets returned as an associative array, like this (not the actual data coming back, but sample data for this question that represents what the data looks like approximately):

$products = array (1 => 'Product 1', 42 => 'Product 42', 100 => 'Product 100');

this is getting sent to an html helper to create a dropdown that associates the key with the value, and the value of the array item gets set as the text in the drop down select control. I need the first item to be something like "Please Select" with a key of 0, so I did this:

$products = array_merge(array(0 => "Select a product" ), $products);

The resulting array looks like this:

array(
  0 => 'Select a product', 
  1 => 'Product 1', 
  2 => 'Product 42', 
  3 => 'Product 100' 
);

when What I really wanted was not to lose the keys of the associative array. I was told that you can properly use array_merge with associative arrays in the manner I tried, however, I believe because my keys are ints that it is not treating the array as a true associative array, and compressing them as illustrated above.

The question is: Why is the array_merge function changing the keys of the items? can I keep it from doing this? OR is there another way for me to accomplish what I'm trying to do, to add the new item at the beginning of the array?

Answer 1

From the docs:

If you want to append array elements from the second array to the first array while not overwriting the elements from the first array and not re-indexing, use the + array union operator

The keys from the first array argument are preserved when using the + union operator, so reversing the order of your arguments and using the union operator should do what you need:

$products = $products + array(0 => "Select a product");

Answer 2

Just for the fun of it

$newArray = array_combine(array_merge(array_keys($array1),
                                      array_keys($array2)
                                     ),
                          array_merge(array_values($array1),
                                      array_values($array2)
                                     )
                         );

Answer 3

array_merge will recalculate numeric indexes. Because your associative array iuses numeric indexes they will get renumbered. You either insert a non-numeric charadter in front of the indices like:

$products = array ('_1' => 'Product 1', '_42' => 'Product 42', '_100' => 'Product 100');

Or you can create the resulting array manually:

$newproducts = array (0 => "Select a product");
foreach ($products as $key => $value)
    $newproducts[$key] = $value;

Answer 4

You could use array operator: +

$products = array(0 => "Select a product" ) + $products;

it will do a union and only works when the keys don't overlap.

Answer 5

From the docs:

Values in the input array with numeric keys will be renumbered with incrementing keys starting from zero in the result array.

Answer 6

You could try something like

$products[0]='Select a Product'
ksort($products);

That should put the 0 at the start of the array but it will also sort the other products in numeric order which you may not want.

Answer 7

You man want to look at array_replace function.

In this example they are function the same:

$products1 = array (1 => 'Product 1', 42 => 'Product 42', 100 => 'Product 100');
$products2 = array (0 => 'Select a product');

$result1 = array_replace($products1, $products2);
$result2 = $products1 + $products2;

Result for both result1 and result2: Keys are preserved:
array(4) {
  [1] => string(9) "Product 1"
  [42] => string(10) "Product 42"
  [100] => string(11) "Product 100"
  [0] => string(16) "Select a product"
}

However they differ if the same key is present in both arrays: + operator does not overwrite the value, array_replace does.