Rust - 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 2^y.

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:

fn main() {
  let x = 1000;

  //left shift operation
  let z = x << 2;

  //Displaying the result
  println!("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:

fn count_one_bits(n: i32) -> i32{
  let mut mask = 1;
  let mut count = 0;
  
  //performing bitwise AND operation
  //at every bit of the number
  for i in 0..31 {
    if((mask & n) == mask) {
      count += 1;
    } 
    mask = mask << 1;
  }
  return count; 
}

fn main() {
  println!("count_one_bits(1000) = {}", count_one_bits(1000));
  println!("count_one_bits(1023) = {}", count_one_bits(1023));
}

The above code will give the following output:

count_one_bits(1000) = 6
count_one_bits(1023) = 10