Ask Your Question

Why do mpmath mpf values compare strangely to Integers?

asked 2016-02-23 15:53:21 +0200

this post is marked as community wiki

This post is a wiki. Anyone with karma >750 is welcome to improve it.

With both sage 6.8 and 7.1.beta3 on OSX 10.10.5 I get this odd result:

sage: import mpmath
sage: max(Integer(0), mpmath.mpf(-1))
sage: max(mpmath.mpf(-1), Integer(0))
sage: Integer(0) < mpmath.mpf(-1)

The maximum of 0 and -1 should be 0, although I'm not sure if it should be Integer(0) or mpf(0).

Am I missing something about how sage expects me to handle arbitrary-precision floating-point numbers?

edit retag flag offensive close merge delete

1 Answer

Sort by ยป oldest newest most voted

answered 2016-02-24 12:45:16 +0200

slelievre gravatar image

The max function in Sage is the Python built-in function max, which relies on comparisons.

This stackoverflow answer explains roughly how it works. To compute max(a, b),

  1. Provisionally set the result to the first value, a.
  2. If b > a, change the result to b.
  3. Return the result.

In step 2, comparing objects of different types is tricky.

One would expect objects representing real numbers to compare well even if they are of different types. In your example, this fails.

As a practical suggestion: to compare numbers using < and >, first take them to a common parent where such comparison is defined properly.

In your case, if a = Integer(0) and b = mpmath.mpf(-1), you could compare RDF(a)and RDF(b) instead of comparing a and b. (Note: RDF stands for "real double field", a fast and reliable approximation to the field of real numbers, recommended in many contexts.)

sage: import mpmath
sage: a, b = Integer(0), mpmath.mpf(-1)
sage: max(a, b)
sage: max(b, a)
sage: max(RDF(a), RDF(b))
sage: max(RDF(b), RDF(a))
edit flag offensive delete link more

Your Answer

Please start posting anonymously - your entry will be published after you log in or create a new account.

Add Answer

Question Tools


Asked: 2016-02-23 15:53:21 +0200

Seen: 577 times

Last updated: Feb 24 '16