"Kent Johnson" <[EMAIL PROTECTED]> wrote >> to simply shift the bits right or left using >> and << and use >> bitwise and/or operations than do all this multiplication and >> addition malarky. (Its also a lot faster!) > > Are you sure about that? With Python 2.5 on a MacBook Pro it seems > to > be *slightly* faster: > > src $ python -m timeit "5 << 21; 10 << 21; 50 << 21; 247 << 21" > 10000000 loops, best of 3: 0.0917 usec per loop > > src $ python -m timeit "5 * 0x200000; 10 * 0x200000; 50 * 0x200000; > 247 > * 0x200000" > 10000000 loops, best of 3: 0.0924 usec per loop
OK, I really should learn not to make assumptions about how Python works speed wise, its usually too clever for me. In past lives I've found multiplication/addition to be a factor of 2 or 3 times slower than shifting/ORing. I still think it's faster to write since if you just want to shift 7 places right you don't have to think "what's 2 ** 7?" and then multiply by that value. And ORing things together with a bitwise OR is usually just a single operation whereas adding them together is usually many clock cycles. But, lo and behold, when I try timing that in Python addition looks to be about 10% faster than bitwise OR which is astonishing! Alan [EMAIL PROTECTED] ~ $ python -m timeit "128 | 7" 10000000 loops, best of 3: 0.146 usec per loop Alan [EMAIL PROTECTED] ~ $ python -m timeit "128 + 7" 10000000 loops, best of 3: 0.115 usec per loop I guess this relates to our discussion about moving from C to Python, its best not to think too much about the low level workings, just write the clearest code and trust Python to do the right thing! But in this case I still think the bitwise operations are more obviously related to the task than using arithmetic operations. Alan G. _______________________________________________ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
