You are definitely correct about assert versus try/except. The assert is a
debugging-stage tool, and once the code is fully debugged, some asserts
will disappear and others will become try/catch, for the purpose of making
a human-readable error message.
I did not open a ticket. I wanted to wait a few days for everyone to be
able to say what they want and for a consensus to form. I think we have a
consensus that we should do *something* but unless I am very much mistaken,
the suggestion from Vincent Delecroix and Nils Bruin that we make a
symbolic function has advantages. In fact, I think Nils Bruin has posted
code to Sage-devel.
I have to confess that I don't understand Nils's code. Actually, I wasn't
able to get the following to work, but I have a feeling that I am making a
minor and stupid mistake with the syntax.
################
from sage.symbolic.function import SymbolicFunction
class nth_root(SymbolicFunction):
def __init__(self):
SymbolicFunction.__init__(self, 'nth_root', nargs=2)
def _evalf_(self, n, x, parent=None):
return parent(x).nth_root(n)
plot( nth_root( x, 3), -5, 5 )
################
Maybe someone here could explain exactly what the distinction is or what
the pros/cons are for my approach versus Nils's? I have no idea.
I will also post this on Sage-Devel, but perhaps all future replies should
be restricted to Sage-Devel and not Sage-Edu?
---Greg
On Tuesday, July 8, 2014 10:30:09 PM UTC-4, kcrisman wrote:
>
> Ah, I missed all this. Did you ever open a ticket (with this or Nils' or
> other code)? I guess it's too late for your book (well, the 1st print
> edition, on the internet revision means something different, as a recent
> article I read on lit crit was pointing out...)
>
>
>
>
>> def nth_real_root( x, n ):
>> """Note: n must be a positive integer. However, x is any real number.
>> (Otherwise this explanation will make no sense.)
>> For odd values of n, we return the unique real number y such
>> that y^n = x.
>> For even values of n, if x<0, there is no real number y such
>> that y^n = x and so we throw an exception.
>> For even values of n, if x=>0, then we return the unique real
>> number y such that y^n = x."""
>>
>> if ((n in ZZ)==false):
>> raise RuntimeError('nth_real_root(x,n) requires n to be a
>> positive integer. Your n is not an integer.')
>>
>> if (n<=0):
>> raise RuntimeError('nth_real_root(x,n) requires n to be a
>> positive integer. Your n is not positive.')
>>
>> assert (n in ZZ)
>> assert (n>0)
>>
>> if ((n%2) == 0):
>> # n is even
>> if (x<0):
>> raise RuntimeError('There is no nth real root (of a
>> negative number x) when n is even.')
>> else:
>> return x^(1/n)
>>
>> assert ((n%2)==1)
>> # n is odd
>> return sign(x)*(abs(x))^(1/n)
>>
>>
> By the way, apparently it's better to use a try/except clause than an
> assert, so that we have a meaningful error message, or so I have heard, I'm
> not a PEP guru.
>
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