python finding the 1's in a bitmask

Question

I have a number of nodes that can be grouped to respond to commands via a bitmask. For example: NodeA is in groups 1 and 5. When asked which groups it belongs to, it answers with 17 of which the binary equivalent is '0b10001'. A node in groups 2, 7 and 9 would tell me it belongs to group 322 ('0b101000010'). I need a way to display to the user which group a specified node belongs to. There are a possibility of 16 groups. My code will give me a 'string index out of range' error if the binary is not 16 characters long. I know there is a better way:

def xref(grp):
    a = bin(grp)
    d = str(a)
    if d[-1] == '1':
        print "Group 1"
    if d[-2] == '1':
        print "Group 2"
    if d[-3] == '1':
        print "Group 3"
repeat for 16 groups

Answer 1

You just need to use some basic bitwise operators.

Here's an example:

def findbits(num):
  for i in range(16):
    if num & 1 << i:
      print("Group {0}".format(i + 1))

And the results:

>>> findbits(0b10001) 
Group 1
Group 5

>>> findbits(0b10100010)
Group 2
Group 6
Group 8

>>> findbits(0b101000010)
Group 2 
Group 7 
Group 9

What this does is loop through the 16 bits you want to look at.

• 1 << i shifts the number 1 by i bits, e.g. 1 << 4 would be 0b10000
• num & whatever does a bitwise AND - each bit of the number is set to 1 if the bits of the two operands are 1.

So what this does is compare your values against 0b1, 0b10, 0b100, etc.