On Sun, Dec 2, 2012 at 2:32 AM, Steven D'Aprano <st...@pearwood.info> wrote: > >> ~i returns the value (-i - 1): > > Assuming certain implementation details about how integers are stored, > namely that they are two-compliment rather than one-compliment or > something more exotic.
Yes, the result is platform dependent, at least for the 2.x int type. I saw it in someone else's code or blog a while ago and thought I'd pass it along as a novelty and something to keep an eye out for. A multiprecision long might qualify as exotic. It uses sign-magnitude form. The sign of the number and the length of ob_digit are both stored in ob_size. For the invert op, it adds 1 and negates the sign to emulate 2's complement: http://hg.python.org/cpython/file/70274d53c1dd/Objects/longobject.c#l3566 Further along is more 2's complement emulation for bitwise &, |, and ^: http://hg.python.org/cpython/file/70274d53c1dd/Objects/longobject.c#l3743 > Okay, just about every computer made since 1960 uses two-compliment > integers, but still, the effect of ~i depends on the way integers are > represented internally rather than some property of integers as an > abstract number. That makes it a code smell. It relies on integer modulo arithmetic. The internal base is arbitrary and not apparent. It could be 10s complement on some hypothetical base 10 computer. In terms of a Python sequence, you could use unsigned indices such as [0,1,2,3,4,5,6,7] or the N=8 complement indices [0,1,2,3,-4,-3,-2,-1], where -1 % 8 == 7, and so on. The invert op can be generalized as N-1-i for any N-length window on the integers (e.g. 5-digit base 10, where N=10**5, subtract i from N-1 == 99999), which just inverts the sequence order. The interpretation of this as negative number depends on a signed type that represents negative values as modulo N. That's common because it's a simple shift of the window to be symmetric about 0 (well, almost symmetric for even N); the modulo arithmetic is easy and there's no negative 0. However, with a multiprecision integer type, it's simpler to use a sign magnitude representation. That said, I don't want to give the impression that I disagree with you. You're right that it isn't generally advisable to use a single operation instead of two or three if it sacrifices clarity and portability. It didn't jump out at me as a problem since I take 2s complement for granted and have a bias to favor symmetry and minimalism. _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
