Tom Anderson wrote: > On Sun, 3 Jul 2005, George Sakkis wrote: > >> "Tom Anderson" <[EMAIL PROTECTED]> wrote: >> >>>>> And finally, does Guido know something about arithmetic that i >>>>> don't, or >>>>> is this expression: >>>>> >>>>> -1.0 ** 0.5 >>>>> >>>>> Evaluated wrongly? >>>> >>>> >>>> No, it is evaluated according to the rules of precedence. It is >>>> equivalent to -(1.0**0.5). You are confusing it with (-1.0)**0.5 which >>>> fails as expected. >>> >>> >>> Ah. My mistake. I submit that this is also a bug in python's grammar. >>> There's probably some terribly good reason for it, though. >> >> >> How about 'conformance with standard mathematic notation', does this >> count for a terribly good reason? > > > Yes. However, it's an excellent reason why python's precedence rules are > wrong - in conventional mathematical notation, the unary minus, used to > denote the sign of a literal number, does indeed have higher precedence > than exponentiation: -1^2 evaluates to 1, not -1.
No... that would mean that -x^2 == x^2 always, and we certainly expect the lhs to always be negative or zero (for real x), and the rhs to be non-negative. Unary minus has lower precendence than exponentiation, in conventional mathematics. You'll often see (-1)^n written in mathematics when we want the alternating sequence +1, -1, +1, -1, ..., as -1^n wouldn't do it (giving -1, -1, -1, -1). -1^n evaluates to -1, for all non-zero integral n. > >> What would one expect to be the result of 5^2 - 2^2, 29 or 21? > > > 21. True, unambiguously; there are no negative numbers involved at any time, just a simple binary minus. > >> Would you expect 5^2 + - 2^2 to be different, even if you write it as >> 5^2 + -2 ^ 2 ? > > > Yes: 5^2 + -2^2 is 29, however you write it. *If* you take -2 as a number, but not if you take the number as 2 and the unary minus as an operator with lower precedence than exponentiation. > >> White space is not significant in math AFAIK ;-) > > > No, but there's a very big difference between unary and binary minus. Not in this respect. However, the question is whether that's a unary -, which binds lower than exponentiation in most systems, or part of the negative number minus 2. -- James -- http://mail.python.org/mailman/listinfo/python-list
