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Perl - left shift operator



The Bitwise left shift operator (<<) takes the two numbers and left shift the bits of first operand by number of place specified by second operand. For example: for left shifting the bits of x by y places, the expression (x<<y) can be used. It is equivalent to multiplying x by 2y.

The example below describes how left shift operator works:

1000 << 2 returns 4000

                      (In Binary)
   1000         ->    1111101000  
   << 2                     |  left shift the bits
   -----                    V  by 2 places
   4000         <-  111110100000 
                      (In Binary) 

The code of using left shift operator (<<) is given below:

$x = 1000;

#left shift operation
$z = $x << 2;

#Displaying the result
print("z = $z");

The output of the above code will be:

z = 4000

Example: Count number of 1 Bits in a positive integer

Consider an integer 1000. In the bit-wise format, it can be written as 1111101000. However, all bits are not written here. A complete representation will be 32 bit representation as given below:

00000000000000000000001111101000  

Bitwise AND operation with 1 at any bit results into 1 if the bit is 1 or 0 if the bit is 0. Performing such operation at every bit, and counting the number of 1 gives the count of 1 bits in the given positive integer. To achieve this bitwise left shift operator can be used as shown in the example below:

sub CountOneBits {
  #passing argument
  $n = $_[0];

  $mask = 1;
  $count = 0;
  
  #performing bitwise AND operation
  #at every bit of the number
  for ($i = 0; $i < 32; $i++) {
    if(($mask & $n) == $mask) {
     $count++; 
    } 
    $mask = $mask << 1;
  }

  return $count;  
}

print("CountOneBits(1000) = ".CountOneBits(1000)."\n");
print("CountOneBits(1023) = ".CountOneBits(1023)."\n");

The above code will give the following output:

CountOneBits(1000) = 6
CountOneBits(1023) = 10

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