CODEWITHSUNDEEP
pythonbitwise-operators
SAKURA
SAKURA

Reputation: 986

Python Bit-wise Operation

I found the following codes in a book:

def count_bits(x):
   num_bits = 0
   while x:
      num_bits += x&1
      x >>= 1
return num_bits

print(count_bits(12))

I don't understand this line (num_bits += x&1)

Let's say I input 12 (1100), the first character ("1") gets counted. But then there is a right shift and 1100 becomes 0110. If the counter moves to the second character, won't 1 be counted twice?

Upvotes: 4

Views: 1503

Answers (3)

Luke Smith
Luke Smith

Reputation: 977

The code 

num_bits += x&1

Checks if the least significant bit is set, if it is set 1 is added to num_bits.

x >>= 1

Then shifts the number right one bit, shifting out the least significant bit.

Finally, the loop condition

while x:

Checks if there are still any bits set in the number, this check fails as the final set bit is shifted out.

Upvotes: 0

racsar
racsar

Reputation: 66

x&1 checks if the rightmost bit is 1

so for your example it would do:

1100 & 0001 # 0
0110 & 0001 # 0
0011 & 0001 # 1
0001 & 0001 # 1

and correctly return 2. By right shifting, you count the bits from right to left until the last shift results in 0000 and breaks the loop

Upvotes: 4

Ignacio Vazquez-Abrams
Ignacio Vazquez-Abrams

Reputation: 799470

1 is 0b0001. ANDing it with 0b1100 results in 0. There is never any duplicated counting.

Upvotes: 2

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