math.nroot [was Re: A brief question.]

[Tom Anderson]

On Sun, 3 Jul 2005, George Sakkis wrote:

> "Tom Anderson" <twic at urchin.earth.li> 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.

> What would one expect to be the result of 5^2 - 2^2, 29 or 21?

21.

> 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.

> White space is not significant in math AFAIK ;-)

No, but there's a very big difference between unary and binary minus.

tom

-- 
When you mentioned INSERT-MIND-INPUT ... did they look at you like this?