Opened 22 months ago
Closed 21 months ago
#22894 closed enhancement (fixed)
Symbolic expression.is_exact()
Reported by:  rws  Owned by:  

Priority:  major  Milestone:  sage8.0 
Component:  symbolics  Keywords:  
Cc:  tmonteil  Merged in:  
Authors:  Marcelo Forets  Reviewers:  Ralf Stephan 
Report Upstream:  N/A  Work issues:  
Branch:  adf17b5 (Commits)  Commit:  adf17b5dd863867e4bf578b5053fbd5898080d15 
Dependencies:  Stopgaps: 
Description
Expressions can contain inexact elements which makes themselves inexact. This status could be made queriable e.g. to prevent integration attempts.
Change History (9)
comment:1 Changed 22 months ago by
comment:2 Changed 22 months ago by
Apparently ring exactness can be tainted if it can possibly contain inexact elements. So we would not check the exactness of symbols/functions etc. which leaves those Python objects that are wrapped by SR (into Expression
) and called "numeric". We can get the pure object via Expression.pyobject()
, e.g.
sage: f1 = x  1 sage: [op.is_numeric() and not op.pyobject().base_ring().is_exact() for op in f1 ....: .operands()] [False, False] sage: f2 = x  1.0 sage: [op.is_numeric() and not op.pyobject().base_ring().is_exact() for op in f2 ....: .operands()] [False, True]
But maybe not every of these rings has an is_exact
member?
comment:3 Changed 22 months ago by
 Cc tmonteil added
@rws : nice. and one can consider more complicated expressions (fractions, etc) by recursively applying the operands()
method to each piece (until you get []).
But maybe not every of these rings has an is_exact member?
I know the basic ones: (QQ, QQbar, ZZ) have is_exact=True
while (RDF, RR, CC) give is_exact=False
.. But Thierry has a comprehensive list so I'm CC'ing him!
comment:4 Changed 22 months ago by
Regarding real and complex numbers only, basically, numerical representations are inexact:
RealDoubleField() RealField(prec) RealIntervalField(prec) RealBallField(prec) ComplexDoubleField() ComplexField(prec) ComplexIntervalField(prec) ComplexBallField(prec)
Algebraic representations are exact:
AlgebraicRealField() NumberField(poly) QuadraticField(n) RationalField() AlgebraicField() UniversalCyclotomicField() CyclotomicField(n)
Symbolic ring is a mixture, so it is inexact.
The main problem are RealLazyField()
and ComplexLazyField()
which claim to be exact, but they are not:
sage: a = RLF(0.1) sage: a 0.10000000000000001? sage: a._value 0.100000000000000 sage: a._value.parent() Real Field with 53 bits of precision sage: RLF.is_exact() True
Perhaps should we fix this.
comment:5 Changed 22 months ago by
See #22960.
comment:6 Changed 21 months ago by
 Branch set to u/mforets/22894
 Commit set to c011cfada9c95d0ded5057c8e88b1b1cfd9d7d31
 Status changed from new to needs_review
@Thierry: that is indeed a comprehensive list :0
comment:7 Changed 21 months ago by
 Commit changed from c011cfada9c95d0ded5057c8e88b1b1cfd9d7d31 to adf17b5dd863867e4bf578b5053fbd5898080d15
Branch pushed to git repo; I updated commit sha1. New commits:
adf17b5  add is_exact method to SR

comment:8 Changed 21 months ago by
 Reviewers set to Ralf Stephan
 Status changed from needs_review to positive_review
LGTM.
comment:9 Changed 21 months ago by
 Branch changed from u/mforets/22894 to adf17b5dd863867e4bf578b5053fbd5898080d15
 Resolution set to fixed
 Status changed from positive_review to closed
how do you approach this problem? since numeric coefficients are cast into SR which is inexact, one has: