Advancing index keys in an array - php

Question

I have an array with numerical incrementing keys starting at 0, like 0 1 2 3 4 5 ....etc. I need to assign new keys to the array in the following manner
The first three keys keep their index number
Every 4th key gets incremented by 4
The two keys after the fourth gets incremented by 1again
I know I need to use a foreach loop ( seems the simplest way anyway ) to build a new array with newly assigned keys. My problem is the calculation of the keys
HERE IS WHAT I HAVE SEEN IN RELATION
Here is my current array keys
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
and this is what I need them to be
0 1 2 6 7 8 12 13 14 18 19 20 24 25 26 30
Lets break this down into 3 key groups with the old array key on the top and the new array key at the bottom
1st group
0 1 2
0 1 2
2nd group
3 4 5
6 7 8
3rd group
6 7 8
12 13 14
4th group
9 10 11
18 19 20
etc etc......
When you look at the relationship between the two array keys, you'll see the following:
The old array keys 0, 1 and 2 keep their key value
The old array keys 3, 6, 9 etc, if multiplied by 2, gives you the new array key value
The old array keys 4, 7, 10 etc, if multiplied by 2 and you subtract 1 from that total, you get the new array key for that specific key
The old array keys 5, 8, 11 etc, if multiplied by 2 and you subtract 2 from the total, you get the new array key for that specific key
Alternatively, if you subtract the old key from the new key, you get the following answer
0 for the 1st group
3 for the 2nd group
6 for the 3rd group
9 for the 4th group
etc etc.....
POSSIBLE SOLUTION
The only solution I can think of is to use the modulus operator (inside my foreach loop), check the current key against the modulus result and then calculate my new key
Example:
if ( $key%3 == 0 && $key != 0 ) {
$new_key = $key * 2;
} elseif ( ( $key - 1 ) %3 == 0 && $key != 1 ) {
$new_key = ( $key * 2 ) - 1;
} elseif ( ( $key - 2 ) %3 == 0 && $key != 2 ) {
$new_key = ( $key * 2 ) - 2;
} else {
$new_key = $key;
}
$new_array[$new_key] = $value;
MY QUESTION
Isn't there a smarter more mathematical way of doing this?

Answer 1

Try the following:
$new_key = floor($old_key / 3) * 3 + $old_key

Answer 2

If you use modulous you can generate a sequence with items missing. The below code demonstrates how to do that.
for ($x = 0; $x < 40; $x++)
{
echo ("$x, " . ($x % 6) . ", " . ($x % 6 < 3? "true": "false") . "\n");
}
The output is:
0, 0, true
1, 1, true
2, 2, true
3, 3, false
4, 4, false
5, 5, false
6, 0, true
7, 1, true
8, 2, true
9, 3, false
10, 4, false
11, 5, false
12, 0, true
13, 1, true
14, 2, true
15, 3, false
16, 4, false
17, 5, false
18, 0, true
19, 1, true
20, 2, true
21, 3, false
22, 4, false
23, 5, false
24, 0, true
25, 1, true
26, 2, true
27, 3, false
28, 4, false
29, 5, false
30, 0, true
31, 1, true
32, 2, true
33, 3, false
34, 4, false
35, 5, false
36, 0, true
37, 1, true
38, 2, true
39, 3, false
If you use $x as a key only when $x % 6 < 3 you can get the sequence you want.

Answer 3

$array = range(1,15);
For ($i=0; $i<count($array); $i++) echo $keys[] = $i + floor($i/3)*3;
$new_array = array_combine($keys, $array);
var_dump($new_array);