How do I separate a RealNumber from an Expression, once it is constructed?
sage: b = RealField(200)(5)
sage: type(b)
<type 'sage.rings.real_mpfr.RealNumber'>
sage: type((b^x).operands()[0])
<type 'sage.symbolic.expression.Expression'>
You can use the pyobject method:
sage: b = RealField(200)(5)
sage: bb = (b^x).operands()[0]
sage: bb.pyobject(), type(bb.pyobject())
(5.0000000000000000000000000000000000000000000000000000000000, <type 'sage.rings.real_mpfr.RealNumber'>)
That's much easier, thank you! I don't think `pyobject` is a good name for this method though.
Yeah, the name make s more sense when you're familiar with the underlying pynac library. Can you think of a better name that we could use as an alias?
By explicit conversion:
sage: expr = (0.5^x).operands()[0]
sage: expr
0.500000000000000
sage: type(expr)
<type 'sage.symbolic.expression.Expression'>
As a machine double precision floating point number:
sage: RDF(expr)
0.5
As a 200 bit software floating point number:
sage: RealField(200)(expr)
0.50000000000000000000000000000000000000000000000000000000000
As a interval arithmetic 200 bit floating point number:
sage: RealIntervalField(200)(expr)
0.50000000000000000000000000000000000000000000000000000000000000?
And so on...
Thanks, but I was looking for a way to maintain the original precision. It seems that `RealNumber(expr)` works, but for whatever reason there is a minor loss in precision; `RealNumber(RealField(200)(5)).prec()` = 196. Any idea why?
As well, is there a way to detect what type `expr` used to be? To detect that `expr` was a `RealNumber`, for example, or that `(4^x).operands()[0]` was an `Integer`.
I found http://ask.sagemath.org/question/411/substituting-expressions-for-numbers. That looks like it could work for what I want to do. I'll post another answer if it does.
Asked: 2012-06-13 00:55:45 +0200
Seen: 318 times
Last updated: Jun 13 '12
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