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boost::numeric::interval constructor seems not creating an interval enclosing its arguments

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
cpp
Domain
data

Research direction

Start by reproducing the examples with boost/numeric/interval.hpp, Boost 1.85.0, and -frounding-math, then compare the constructor and += behavior with the interval documentation. Determine whether the observed bounds match the documented rounding guarantees and identify what existing tests cover these cases. Done means the behavior is explained and any required implementation or documentation change is covered by tests.

Written by the indexing model from the issue text.

Description

In the document of boost interval https://www.boost.org/doc/libs/1_86_0/libs/numeric/interval/doc/interval.htm#interval
If the interval constructor has only one argument, "The constructors create an interval enclosing their arguments."
But in the following code, seems not working like this, it just sets interval's upper and lower bounds as the same value of my input. This is NOT what we expect for interval arithmetic, since the input may not be precisely stored in a computer.

#include <iostream>
#include <boost/numeric/interval.hpp>

using namespace std;

int main() {
    cout << setprecision(20);
    boost::numeric::interval<double> I = boost::numeric::interval<double>(0.1);
    cout << I.upper() << endl; // print: 0.10000000000000000555
    cout << I.lower() << endl; // print: 0.10000000000000000555
}

another strange thing is when I add a double number, sometimes it gives the desired bound, but sometimes NOT! See below, adding 0.1 to the interval and adding 1.1 to the interval gives different type of the result. adding 1.1 gives the desired result.

#include <iostream>
#include <boost/numeric/interval.hpp>

using namespace std;

int main() {
    cout << setprecision(20);
    boost::numeric::interval<double> I = boost::numeric::interval<double>(0.1);
    I += 0.1 // add 0.1
    cout << I.upper() << endl; // print: 0.2000000000000000111
    cout << I.lower() << endl; // print: 0.2000000000000000111
}

#include <iostream>
#include <boost/numeric/interval.hpp>

using namespace std;

int main() {
    cout << setprecision(20);
    boost::numeric::interval<double> I = boost::numeric::interval<double>(0.1);
    I += 1.1 // add 1.1
    cout << I.upper() << endl; // print: 1.2000000000000001776
    cout << I.lower() << endl; // print: 1.1999999999999999556
}

When compiling I add -frounding-math, and the boost version is v1.85.0.

Dominant language
C++
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Avg merge
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Merged PRs (30d)
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