how to get last n bits by bit-op?

10,377

Solution 1

This one should work:

mask = (1 << n) - 1
little = num & mask

1 shifted left by n adds n zero bits after 1:

>>> bin(0b1 << 4)
'0b10000'

If you subtract 1 from this, you will have a mask that has n last bits set:

>>> bin((0b1 << 4) - 1)
'0b1111'

Solution 2

Use bits - 1:

>>> num = 0b111111111
>>> n = 8
>>> bits = 1 << n
>>> num & (bits - 1)  # bits - 1 => 0b1111111 because bits is 0b100000000
255
>>> bin(num & (bits - 1))
'0b11111111'
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10,377
roger
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roger

Updated on June 04, 2022

Comments

  • roger
    roger almost 2 years

    I want to get last n bit of number, for example:

    num = 0b111111111
    # if I want to get last 8 bits, it will be 0b11111111
    # if I want to get last 2 bits, it will be 0b11
    

    I thought this can be ok:

    bits = 1 << n
    little = num & (~bits)
    

    but this is wrong, if n = 8, it get 0b110111111