Bug in continued fraction doctest

[kliem]

Part of #32544:

sage -t --long --random-seed=77478494819088915365500074763386376542 src/sage/rings/continued_fraction.py
**********************************************************************
File "src/sage/rings/continued_fraction.py", line 265, in sage.rings.continued_fraction.rat_interval_cf_list
Failed example:
    for prec in range(10,54):
        R = RealIntervalField(20)
        for _ in range(100):
            x = R.random_element() * R.random_element() + R.random_element() / 100
            l = x.lower().exact_rational()
            u = x.upper().exact_rational()
            cf = rat_interval_cf_list(l,u)
            a = continued_fraction(cf).value()
            b = continued_fraction(cf+[1]).value()
            if a > b:
                a,b = b,a
            assert a <= l
            assert b >= u
Exception raised:
    Traceback (most recent call last):
      File "/home/jonathan/Applications/sage/local/lib/python3.8/site-packages/sage/doctest/forker.py", line 694, in _run
        self.compile_and_execute(example, compiler, test.globs)
      File "/home/jonathan/Applications/sage/local/lib/python3.8/site-packages/sage/doctest/forker.py", line 1088, in compile_and_execute
        exec(compiled, globs)
      File "<doctest sage.rings.continued_fraction.rat_interval_cf_list[2]>", line 8, in <module>
        a = continued_fraction(cf).value()
      File "/home/jonathan/Applications/sage/local/lib/python3.8/site-packages/sage/rings/continued_fraction.py", line 2625, in continued_fraction
        x1, x2 = check_and_reduce_pair(x)
      File "/home/jonathan/Applications/sage/local/lib/python3.8/site-packages/sage/rings/continued_fraction.py", line 2254, in check_and_reduce_pair
        raise ValueError("continued fraction can not represent infinity")
    ValueError: continued fraction can not represent infinity
**********************************************************************
1 item had failures:
   1 of   4 in sage.rings.continued_fraction.rat_interval_cf_list
    [437 tests, 1 failure, 5.09 s]
-------------------------------------------------------------

Component: basic arithmetic

Author: Jonathan Kliem

Branch/Commit: e1664ff

Reviewer: Kwankyu Lee

Issue created by migration from https://trac.sagemath.org/ticket/32774

[kliem]

Branch: public/32544

[kliem]

Author: Jonathan Kliem

[kliem]

comment:2

The doctest assumes that two rational numbers that are close enough have the same floor, which needs not to hold.

[kliem]

Changed branch from public/32544 to public/32774

[kliem]

New commits:

fcf0f14

fix doctest of rat_interval_cf_list

[kliem]

Commit: fcf0f14

[kwankyu]

comment:4

The doctest is still strange in that prec is not used.

[sagetrac-git]

Changed commit from fcf0f14 to e1664ff

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

e1664ff

use different precisions in doctest

[kwankyu]

Reviewer: Kwankyu Lee

[kwankyu]

comment:7

Looks good.

[kliem]

comment:8

Thank you.

[vbraun]

Changed branch from public/32774 to e1664ff