Alternating Algorithm Starting from Center - php

Question

I'm trying to figure out an algorithm to come up with an arrayed result that are equidistant alternating points from the center (can be percent based). So, my end result would be something like:
X = 20 (separation distance based on # of items) - 100 / 5 (5 items spread out over 100%)
A = 50 (center point)
B = 70 (A + 20)
C = 30 (A - 20)
D = 90 (B + 20)
E = 10 (C - 20)
Another result would be if we 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 to arrive at this algorithm using PHP. I'm not much of a Math wiz so I'm not sure if there's a name for this type of calculation. Thanks!

Answer 1

You could write a 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 is the initial value (at index 0), $step is the value added or sub at each iteration, and $nbEntries the number of entries after the 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
)

Answer 2

If I understood you correctly, this should work:
function pointsArray($center, $numPoints) {
$arr = array();
$arr[0] = $center;
for($i = 1;$i < $numPoints; $i++)
$arr[$i] = $i%2 == 0 ? $center - ($i-1)*20 : $center + $i*20;
return $arr;
}
Than, you can use the function like this:
$PointsArray = pointsArray(50, 5); // {50,70,30,90,10}