Shift one value of array values to the beginning

Question

Given an integer $value in the range of 1, 2, 3, I want to get an array that starts with $value and proceeds with the remaining two integers from above in arbitrary order. The best I could think of was doing something like this:

switch ($value) {
      case '1':
        $array = [1,2,3];
        break;

      case '2':
        $array = [2,1,3];
        break;

      case '3':
        $array = [3,1,2];
        break;
    }

Is there a shorter and more beautiful way of doing that? Something like that:

$array = push_to_top_of_array($value,[1,2,3]);

Answer 1

Is there a shorter and more beautiful way of doing that?

• shorter: yes
• more beautiful: well, that sounds subjective... you can be the judge of the approaches below.

One approach would be to take off the value using $index with array_splice() and then put it (the first element from that spliced array) at the beginning using array_unshift():

$array = [1, 2, 3];
array_unshift($array, array_splice($array, $value - 1, 1)[0]);

See it demonstrated in this playground example.

Another approach would be to merge the spliced array and the original array using array_merge():

$array = [1, 2, 3];
$array = array_merge(array_splice($array, $value - 1, 1), $array);

See it demonstrated in this playground example.

Answer 2

Yes, for three array elements you can write:

$arr = [
    ($value - 1) % 3 + 1,
    ($value + 0) % 3 + 1,
    ($value + 1) % 3 + 1
];

The main ingredient here is the $value modulo 3 expression. The above code contains some redundancies, which I have kept to clearly show the construction of the code. If you absolutely need faster code instead of readable code, the above is equivalent to:

$arr = [
    $value,
    $value % 3 + 1,
    5 - $value - $value % 3
];

Or, the brute force variant:

// To be executed only once in the program.
$arrs = [[], [1, 2, 3], [2, 3, 1], [3, 1, 2]];

// And then, whenever you need it:
$arr = $arrs[$value];

Answer 3

I hate hardcoded solutions and prefer generalized ones. You never know when your array will get a 4th element and break all the application.

So a generalized solution would be like

• get a key for the desired value
• remove this element from array
• add its value to the beginning of an array

in PHP it would be like

function push_to_top_of_array($value, $array) {
   $key = array_search($value, $array);
   if ($key === false) {
       throw new \OutOfRangeException("Value not found"); 
   }
   unset($array[$key]);
   array_unshift($array, $value);
   return $array;
}

$value = 2;
$array = [3, 5, 2, 4];
$array = push_to_top_of_array($value, $array);

Answer 4

I'm not sure if "more beautiful" means you want a one-liner, but here are two more concise ways to perform the task:

*note, this doesn't validate the $value as being one of the element values -- if this is a necessary component of your project, then please clarify in your question.

Code: (Demo)

$value = 2;
$array = [1, 2, 3];
array_unshift($array,$value);  // prepend a duplicate
var_export(array_unique($array));  // kill the original

echo "\n---\n";

$value = 2;
$array = [1, 2, 3];
var_export(array_unique(array_merge([$value],$array)));  // prepend a duplicate, kill the original

Output:

array (
  0 => 2,
  1 => 1,
  3 => 3,
)
---
array (
  0 => 2,
  1 => 1,
  3 => 3,
)

On huge arrays, statistics have shown array_flip(array_flip()) out performs array_unique() but that would certainly be less beautiful and if you were dealing with big array I'm sure you would have mentioned that.