how does it works "$_ & 1" in perl

Question

I am perl beginner, I am reading upon grep function to filter a list. I came across the following program.

#!/usr/bin/perl

use strict;
use warnings;

# initialize an array
my @array = qw(3 4 5 6 7 8 9);

# first syntax form:
my @subArray = grep { $_ & 1 } @array;

the statement my @subArray = grep { $_ & 1 } @array; returns odd-numbers in @array. I didn't understand how the expression($_ & 1) works. I searched in Google but did not found any useful links.

• Is that any kind of special operator ?

• Are there any other variants of that EXPR ?

Thanks in Advance.

Answer 1

$_ is the variable holding the currently tested value, & is the binary AND operator, and 1 is just the number one. This expression combines all the bits of both $_ and 1 with each other by logical AND. So it returns 1 if the value is odd and 0 if the value is even.

As an example, lets assume $_ is 123 then it's binary representation would be 1111011. The decimal number 1 would be 00000001, so combining all bits by AND you get

123 = 1111011
  1 = 0000001
      - AND -
      0000001 = 1

Another example 200 & 100

200 = 11001000
100 = 01100100
      - AND --
      01000000 = 64