python - Bitwise operation and usage -


consider code:

x = 1        # 0001 x << 2       # shift left 2 bits: 0100 # result: 4  x | 2        # bitwise or: 0011 # result: 3  x & 1        # bitwise and: 0001 # result: 1

i can understand arithmetic operators in python (and other languages), never understood 'bitwise' operators quite well. in above example (from python book), understand left-shift not other two.

also, bitwise operators used for? i'd appreciate examples.

bitwise operators operators work on multi-bit values, conceptually 1 bit @ time.

  • and 1 if both of inputs 1, otherwise it's 0.
  • or 1 if one or both of inputs 1, otherwise it's 0.
  • xor 1 if exactly one of inputs 1, otherwise it's 0.
  • not 1 if input 0, otherwise it's 0.

these can best shown truth tables. input possibilities on top , left, resultant bit 1 of 4 (two in case of not since has 1 input) values shown @ intersection of inputs.

and | 0 1     or | 0 1     xor | 0 1    not | 0 1 ----+-----    ---+----     ----+----    ----+----  0  | 0 0      0 | 0 1       0 | 0 1        | 1 0  1  | 0 1      1 | 1 1       1 | 1 0

one example if want lower 4 bits of integer, , 15 (binary 1111) so:

    201: 1100 1001 ,  15: 0000 1111 ------------------    9  0000 1001

the 0 bits in 15 in case act filter, forcing bits in result 0 well.

in addition, >> , << included bitwise operators, , "shift" value respectively right , left number of bits, throwing away bits roll of end you're shifting towards, , feeding in 0 bits @ other end.

so, example:

1001 0101 >> 2 gives 0010 0101 1111 1111 << 4 gives 1111 0000

note left shift in python unusual in it's not using fixed width bits discarded - while many languages use fixed width based on data type, python expands width cater bits. in order discarding behaviour in python, can follow left shift bitwise and such in 8-bit value shifting left 4 bits:

bits8 = (bits8 << 4) & 255

with in mind, example of bitwise operators if have 2 4-bit values want pack 8-bit one, can use 3 of operators (left-shift, and , or):

packed_val = ((val1 & 15) << 4) | (val2 & 15)
  • the & 15 operation make sure both values have lower 4 bits.
  • the << 4 4-bit shift left move val1 top 4 bits of 8-bit value.
  • the | combines these 2 together.

if val1 7 , val2 4:

                val1            val2                 ====            ====  & 15 (and)   xxxx-0111       xxxx-0100  & 15  << 4 (left)  0111-0000           |                   |               |                   +-------+-------+                           | | (or)                0111-0100

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