## Bug #13134

### Rational() inconsistency

**Description**

`Rational()`

parses a float, an integer divided by an integer, and a float divided by an integer.

```
Rational("3.1") #=> (31/10)
Rational("3/2") #=> (3/2)
Rational("3.1/2") #=> (31/20)
```

But a float is not allowed as a denominator.

```
Rational("3.1/2.0") #=> ArgumentError
```

I'd expect the last also passes and results in `(31/20)`

, or the third also raises an `ArgumentError`

A patch to let all pass.

https://github.com/ruby/ruby/compare/trunk...nobu:parse_rat

#### Updated by mrkn (Kenta Murata) over 3 years ago

**Assignee**set to*mrkn (Kenta Murata)***Status**changed from*Open*to*Assigned*

#### Updated by snood1205 (Eli Sadoff) over 3 years ago

I propose leaving the behavior the way it is. A float denominator is filled with possible unexpected behavior. For example, one would hypothesize that `(a/b).to_r == Rational("#{a}/#{b}")`

yet `(3.1/2.0).to_r != Rational('3.1/2.0')`

. The imprecision that can come with float denominators is why this behavior should not be allowed. Either we would have to allow for `(a/b).to_r == Rational("#{a}/#{b}")`

to not always be true, or we would turn `Rational('3.1/2.0')`

into `(6980579422424269/4503599627370496)`

which defeats the purpose of the rational class.

#### Updated by shyouhei (Shyouhei Urabe) over 3 years ago

I don't think `Rational("3.1/2.0")`

should introduce imprecision because the argument string has nothing to do with inexact numbers (at least seems to me).

So Eli's argument is ultimately "should we hold (a/b).to_r == Rational("#{a}/#{b}") ?" and I think this is a valid argument. No opinion on this point though.

#### Updated by stomar (Marcus Stollsteimer) over 3 years ago

The imprecision that can come with float denominators is why this behavior should not be allowed.

Note there already __is__ this kind of "unexpected" behavior:

2.4.0 :001 > (3.1/2).to_r == Rational("3.1/2") # => false

or even

2.4.0 :002 > 1.2.to_r == Rational("1.2") # => false

It's not caused by float denominators at all. So I don't see why strings with float denominator should raise an exception.

#### Updated by Eregon (Benoit Daloze) over 3 years ago

Floating-point numbers in String are exact, Float literal are inexact by nature.

So I would not expect any relation with to_r, even more so when float division is involved.

A Floating-point denominator in String sounds fine to me.

The equation that should hold is

Rational("#{a}/#{b}") == Rational(a) / Rational(b)

with a and b floating-point numbers as Strings.

Note that Float#rationalize produces a nicer-looking Rational, but there is no guarantee to how much precision might be lost on floating-point operations (e.g. 1.1-1.0),

so it is moot to expect a relation between those.

#### Updated by nobu (Nobuyoshi Nakada) over 3 years ago

**Status**changed from*Assigned*to*Closed*

Applied in changeset r57990.

rational.c: float denom

- rational.c (parse_rat): allow float as a denominator as well as a numerator. [ruby-core:79104] [Bug #13134]