J. Cliff Dyer wrote: > Dotan Cohen wrote: > >> FIrst of all, how is the % symbol (as in 70%6=4) called in English? >> >> Second, in Turbo C -111%10=-1 however in python -111%10=9. Is one or >> the other in error? Is this a known gotcha? I tried to google the >> subject however one cannot google the symbol %. Thanks in advance. >> >> Dotan Cohen >> >> > > The % operator is called "modulo" in English. I don't think the > difference in implementation is an error. It's just a difference of > calculation method. > > Python will always yield a number x = m%n such that 0 <= x < n, but > Turbo C will always yield a number such that if x = m%n -x = -m%n. That > is, since 111 % 10 = 1, -111 % 10 = -1. The two values will always > differ by n (as used above). > > I'm sure there are mathematicians on the list who can give you a more > technical, precise explanation of the reasons for the different results. > > It's a long time ago, but if I remember well, "modulo" was introduced as an operator in Galois fields theory, and Galois fields only exists of the whole numbers 0 .. N-1. So the Python implementation seems the only correct one. Most languages handle it wrong (I didn't know until now Python was so correct ;-) so you always have to add N (sometimes even more than once) manually (to get valid results) x = (N + (some kind of formula that can have a negative result) ) % N
cheers, Stef -- http://mail.python.org/mailman/listinfo/python-list