PHP-Relooping array associative backwards - php

Question

I have a created a grid like this (some gaps has an obstacle, but it is not important for the question):
private function newGrid()
{
$grid = array();
for ($i = 0; $i < 10; $i++) {
for ($j = 0 ; $j < 10; $j++) {
$grid[$i][$j] = ['obstacle' => rand(0,1)];
}
}
return $grid;
}
So, the initial location is an array:
$loc = array('x' => 1, 'y' => 3);
For example, if i'm moving to x direction, I want wrapping from one edge of the grid to another(like an sphere). (Forward and backward)
When i'm going forward I'm using the modulus like this:
$loc['x'] = ($loc['x'] + 1) % 10 ;
But if I want to do the same, but going backwards, which is the better way to do that? when x gets to 0, go to position x = 9
Any suggestion?

Answer 1

For going backwards you can do:
$loc['x'] = ($loc['x'] + 9) % 10 ;
If you have dynamic "direction" variable which can take values 1 (forward) and -1 (backwards), then:
$loc['x'] = ($loc['x'] + 10 + $direction) % 10 ;
Maybe this would be a practical function:
function move($loc, $deltaX, $deltaY) {
$loc['x'] = ($loc['x'] + 10 + $deltaX) % 10;
$loc['y'] = ($loc['y'] + 10 + $deltaY) % 10;
return $loc;
}