PHP Array_Fill_Keys & Array_Sum on Sequential and Associative Array

Question

So I have two arrays as follows:

$arr1 = array("apple","banana","lemon");
$arr2 = array("apple"=>4,"banana"=>1,"lemon"=>1);

And I use array_fill_keys to fill the above arrays:

$arr3 = array_fill_keys($arr1,1);
$arr4 = array_fill_keys($arr2,1);

When I do an array_sum for those 4 arrays above like this:

echo "Sum of Array1 is " . array_sum($arr1) . "\n";
echo "Sum of Array2 is " . array_sum($arr2) . "\n";
echo "Sum of Array3 is " . array_sum($arr3) . "\n";
echo "Sum of Array4 is " . array_sum($arr4) . "\n";

The result will be:

Sum of Array1 is 0
Sum of Array2 is 6
Sum of Array3 is 3
Sum of Array4 is 2

Why does Array4 return 2 instead of 3?

ArtisticPhoenix · Accepted Answer · 2015-06-30 05:35:21Z

1

Lets simplify this here are your arrays

$arr1 = array("apple","banana","lemon");
$arr2 = array("apple"=>4,"banana"=>1,"lemon"=>1);

//$arr3 = array_fill_keys($arr1,1);
$arr3 = array("apple"=>1,"banana"=>1,"lemon"=>1);
//$arr4 = array_fill_keys($arr2,1);
$arr4 = array(1=>1, 4=>1);

As you can see array #4 ( $arr4 ) has these keys added using array_fill_keys

array( 4, 1, 1 );

These are the values of $arr2, And your input value was

which produces this array

array(
    4=>1,
    1=>1,
    1=>1
);

Now as you may know php array keys are unique, so the first one(1) key is overwritten by the second one(1) key, resulting in this

$arr4 = array_sum(array(1=>1, 4=>1));

http://php.net/manual/en/function.array-fill-keys.php

And hence the sum of 2

edited Jun 30, 2015 at 5:35

answered Jun 30, 2015 at 5:24

ArtisticPhoenix

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Comments

Dan Belden · Accepted Answer · 2015-06-30 05:15:57Z

0

array_fill_keys() is given an array of keys and produces a new array using those keys and a defined value. $arr2 contains two values which are the same "1" which means when you use it to produce $arr4, only two values are created.

If you need to use the keys from the first arrays you should consider using array_keys to use the keys, rather than the values:

$keys1 = array_keys($arr1);
$keys2 = array_keys($arr2);
$arr3 = array_fill_keys($keys1, 1);
$arr4 = array_fill_keys($keys2, 1);

This way the keys of (arr1 and arr3) and (arr2 and arr4) will match up.

Enjoy.

answered Jun 30, 2015 at 5:15

Dan Belden

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Comments

Dave Chen · Accepted Answer · 2015-06-30 05:54:36Z

0

The reason is because array_fill_keys will use the the values 4, 1, and 1 and the keys and set their values to 1.

$arr2 = array("apple" => 4, "banana" => 1, "lemon"=> 1);

will become

$arr2 = array(4 => "apple", 1 => "banana", 1 => "lemon");

The two keys as 1 will cancel out, so you're left with only two values in the array. Then PHP will replace all the values with the value provided, which is one.

Your final array will look like this,

$arr2 = array(4 => 1, 1 => 1);

And array_sum, adds the values in the array, thus 1 + 1 = 2.

edited Jun 30, 2015 at 5:54

answered Jun 30, 2015 at 5:14

Dave Chen

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6 Comments

ArtisticPhoenix Over a year ago

array fill keys does not flip the values it uses the first parameter as the keys and the second as the value - php.net/manual/en/function.array-fill-keys.php, so $arr3 is array("apple"=>1,"banana"=>1,"lemon"=>1);

Dan Belden Over a year ago

Array fill keys uses an array's values as the keys for a new array, and fills that array with a given value. The problem is two values in $arr2 are the same so only one key is passed across.

Dave Chen Over a year ago

@ArtisiticPhoenix using the values of the keys array as keys, So it's taking 4 (the value of the key), and swapping it with apple (the key). It is flipping. It changes the values and keys. If it's following your logic, then array_sum should yield three, but it clearly does not.

ArtisticPhoenix Over a year ago

@DaveChen - clearly it does not flip the array according to the php documentation, that function is array_flip. such as this would give the same result array( 'a'=>1, 'b'=>1, 'a'=>1) why is that not 3? because keys are not duplicate has nothing to do with flipping them, the OP chose to use numbers thats not the fault of array_fill_keys .

Dave Chen Over a year ago

@ArtisiticPhoenix Can you check my explanation here?

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