Do something with every X item in while loop - php

Question

$i = 0;
while ($have_something) {
if($i == 0 || $i == 3 || $i == 6 || $i == 9) {
$color = 'black';
}
else {
$color = 'white';
}
$i++;
}
Problem occurs with this code when count of items in while loop is dynamic and changes regulary, also when it could have hundreds of items which means that if statement would be very long.
How to overcome this problem and "automate" it so that something happens with every X item and something else for the rest without hardcoding $i count?

Answer 1

Use the modulus operator %:
$i = 0;
while ($have_something) {
if($i % 3 == 0 ) {
$color = 'black';
}
else {
$color = 'white';
}
$i++;
}

Answer 2

An alternative to using php to generate what I assume will be a class or inline color definition would be purely in css by using the nth-of-type(3n) style selector.
<style>
#rep{ width:80%; float:none; margin:5rem auto; box-sizing:border-box; padding:1rem; }
#rep div{ width:100%; float:none; margin:0.1rem auto; box-sixing:border-box; border:1px solid black; }
#rep div:nth-of-type(2n){ background:whitesmoke; }
#rep div:nth-of-type(3n){ background:gray; }
#rep div:nth-of-type(4n){ background: #f0ffff; }
#rep div:nth-of-type(5n){ background: #f0fff0; }
#rep div:nth-of-type(6n){ background: #e0ffff; }
#rep div:nth-of-type(7n){ background: #f0f8ff; }
#rep div:nth-of-type(8n){ background: #e6e6fa; }
#rep div:nth-of-type(9n){ background: #b0e0e6; }
</style>
echo "<div id='rep'>";
for( $i=0; $i < 30; $i++ ) echo "<div>$i</div>";
echo "</div>";
Anyway, just a thought!
or, a better css selector is nth-child perhaps
#rep div:nth-child(3n){background:black;}
if you are interested in this technique this would be a handy guide possibly