Python按位移位无法按预期工作

Question

Here is my code

http://pastebin.com/xRAS7qLH

This creates an infinite loop so don't run it.

        intMask = 0
        while(temp != 0):
            print bin(temp)[2:].zfill(32)
            temp = long(temp << 1)
            intMask += 1

When I look at the output it looks like this

11111111111111111111111100000000

111111111111111111111111000000000

1111111111111111111111110000000000

What I was expecting was something more like this

11111111111111111111111100000000

11111111111111111111111000000000

11111111111111111111110000000000

etc

There must be something different with python, or I need to typecast something? What am I missing here?

Answer 1

Python's integers have arbitrary precision, which besides being difficult to spell, means that as you bit shift, it keeps getting bigger. If you want to limit the size, you'd want to do something like

temp = long(temp << 1) & (0xffffffff)

which should zero out.

Edit: Specifically, the & does a bitwise AND; 0xffffffff (8 f's) is 32 1's (in binary) so anything within the 32 bits will be preserved; anything beyond will be set to zero). For more details, you could do something like:

temp = long(temp << 1) print "Before AND: ",bin(temp) temp = temp & 0xffffffff print "After AND: ", bin(temp)