Question

Looking at some code written by another developer, I came across this:

for($i=1; $i<=30; $i++)
{
  if($i&1)
    $color = '#fff';
  else
    $color = '#bbb';
}

This $color variable is used for row background colour later in the code. The alternating colours work fine.

If I was writing this, I would have used the modulus operator (%) rather than the bitwise (&) operator.

Why does the bitwise operator work in this case? Is there any advantage of using this method rather than the modulus operator?

Answer 1

The & operator does a bitwise comparison on the number. So if you do

$i & 1

it will then tell you if the ‘1’ flag is set, such as in binary:

001010111010

The last number is the ‘1’ flag (remember, binary goes 1, 2, 4, 8 etc. in reverse order), which in this case is set to 0.

Since 1 is the only odd flag in binary, it will tell you if the number is odd or even.

if $i is 3 for example, then in binary it will be 011 – the last number is a 1 (the 1 flag) and thus $i & 1 will be true.

if $i is 4 for example, then in binary it will be 100 – the last number is a 0 (the 1 flag) and thus $i & 1 will be false.