On Mon, Apr 21, 2008 at 12:07 AM, Alan Gauld <[EMAIL PROTECTED]> wrote:
> > >>> pow(20, 0.3333333) > 2.7144173455393048 > >>> pow(-20, 0.3333333) > Traceback (most recent call last): > File "<stdin>", line 1, in <module> > ValueError: negative number cannot be raised to a fractional power > >>> -20**0.3333333 > -2.7144173455393048 > >>> > > So the exponentiation operator seems happy but pow isn't. > > Very strange and potentially a bug in pow()? > Unless someone knows a reason for it? I think you're confusing the order of operations. math.pow(-20, (1.0/3.0)) and -20**(1.0/3.0) are not equivalent Whereas, as john mentioned, -20**(1.0/3.0) is actually -(20**(1.0/3.0)), math.pow(-20, (1.0/3.0)) is (-20)**(1.0/3.0) The behavior of ** is appropriate since http://docs.python.org/ref/summary.html shows that exponentiation has higher precedence over positive, negative. Also, I found this from http://www.python.org/download/releases/2.0/: *Raise ZeroDivisionError when raising zero to a negative number, e.g. 0.0 ** > -2.0. Note that math.pow is unrelated to the builtin power operator and the > result of math.pow(0.0, -2.0) will vary by platform. On Linux, it raises a > ValueError.* which is related to why some people would get nan and others a ValueError. And, just to clarify (although i doubt there was any confusion over it), -20**(1/3) = -1 * (20**0) = -1 due to integer division magic.
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