php - Alternating Algorithm Starting from Center -

i'm trying figure out algorithm come arrayed result equidistant alternating points center (can percent based). so, end result like: x = 20 (separation distance based on # of items) - 100 / 5 (5 items spread out on 100%)

• a = 50 (center point)
• b = 70 (a + 20)
• c = 30 (a - 20)
• d = 90 (b + 20)
• e = 10 (c - 20)

another result if have 10 items (x = 100 / 10):

• a = 50 (center point)
• b = 60 (a + 10)
• c = 40 (a - 10)
• d = 70 (b + 10)
• e = 30 (c - 10)
• f = 80 (d + 10)
• g = 20 (e - 10)
• h = 90 (f + 10)
• i = 10 (g - 10)
• j = 100 (h + 10)

if it's important, i'm trying arrive @ algorithm using php. i'm not of math wiz i'm not sure if there's name type of calculation. thanks!

you write function like:

function getarrayequidistant($startvalue, $step, $nbentries) {     $i = 0;     $count = 1;     $final = array();     $final[] = $startvalue;     while($i < $nbentries)     {         if ($i % 2 == 0)             $final[] = $startvalue + ($count * $step);         else         {             $final[] = $startvalue + ($count * $step);             $count++;         }          $i++;     }      retun $final; } 

where $startvalue initial value (at index 0), $step value added or sub @ each iteration, , $nbentries number of entries after initial value.

as example:

print_r(getarrayequidistant(50, 20, 10)); 

will give you:

array (     [0] => 50     [1] => 70     [2] => 30     [3] => 90     [4] => 10     [5] => 110     [6] => -10     [7] => 130     [8] => -30     [9] => 150     [10] => -50 )