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In my program (written in Python 3.4) I have a variable which contains various flags, so for example:
FLAG_ONE = 0b1 FLAG_TWO = 0b10 FLAG_THREE = 0b100 status = FLAG_ONE | FLAG_TWO | FLAG_THREE Setting another flag can easily be done with
status |= FLAG_FOUR But what if I explicitly want to clear a flag? I'd do
status &= ~FLAG_THREE Is this approach safe? As the size of an integer in Python is not defined, what if status and FLAG_THREE differ in size?
(status needs to be a bit field because I need this value for a hardware protocol.)
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int.bit_length()Returns the number of bits required to represent an integer in binary, excluding the sign and leading zeros.
If you want to use bits, you can build it up yourself like this: unsigned int key = (1<<9)|(1<<8)|(1<<7)|(1<<6)|(1<<5)|(1<<4)|(1<<0);
Setting a bit Use the bitwise OR operator ( | ) to set a bit. number |= 1UL << n; That will set the n th bit of number . n should be zero, if you want to set the 1 st bit and so on upto n-1 , if you want to set the n th bit.
*/ Step 1 : first convert the number into its binary form using bin(). Step 2 : remove the first two character. Step 3 : then extracting k bits from starting position pos from right.so, the ending index of the extracting substring is e=len(bi)-pos and starting index=e-k+1 Step 4 : extract k bit sub-string.
You should be safe using that approach, yes.
~ in Python is simply implemented as -(x+1) (cf. the CPython source) and negative numbers are treated as if they have any required number of 1s padding the start. From the Python Wiki:
Of course, Python doesn't use 8-bit numbers. It USED to use however many bits were native to your machine, but since that was non-portable, it has recently switched to using an INFINITE number of bits. Thus the number -5 is treated by bitwise operators as if it were written "...1111111111111111111011".
In other words, with bitwise-and & you're guaranteed that those 1s will pad the length of ~FLAG (a negative integer) to the length of status. For example:
100000010000 # status & ~10000 # ~FLAG is treated as
100000010000 & 111111101111 = 100000000000 # new status This behaviour is described in a comment in the source here.
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