Simplify Rational Numbers in Python

The real number system: rational numbers (with natural, whole, and integers nested inside) alongside irrational numbers.

What Is a Rational Number?

A rational number is any number that can be expressed as a fraction of two integers, where the numerator is a whole number and the denominator is a non-zero integer. The decimal form of a rational number either ends (terminates) or repeats infinitely. ^{[1, 2, 3, 4]}

Key Characteristics and Forms

Rational numbers encompass a wide variety of numbers, which can always be categorized into these four main forms:

• Integers: All whole numbers, both positive and negative, are rational because they can be written as a fraction over 1 (e.g., 5 = 5/1, -3 = -3/1).
• Fractions: Any standard or mixed fraction where the top and bottom are integers (with a denominator not equal to 0) is rational (e.g., 3/4, 2 1/2).
• Terminating Decimals: Decimals that eventually stop are rational because they can be converted into fractions (e.g., 0.75 = 3/4).
• Repeating Decimals: Decimals that go on forever in a predictable, repeating pattern are rational and can be converted into fractions (e.g., 0.333... = 1/3). ^{[3, 4, 6]}

Non-Examples (Irrational Numbers)

If a number cannot be written as a fraction, and its decimal portion neither terminates nor repeats, it is considered irrational.

• Examples: π (Pi), the square root of a non-perfect square (e.g., √2), or Euler's number (e). ^{[1, 2, 4, 6, 7]}

References

1. Wikipedia: Rational number
2. YouTube: What is a rational number
3. Mathnasium: What is a rational number
4. Math is Fun: Rational Numbers
5. YouTube Shorts: Rational numbers
6. Mathnasium: Rational number (term)
7. Mometrix: Rational Numbers

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How the Code Works

The notebook below converts a floating-point number into an exact fraction, and shows why the obvious approach can be surprising. It uses Python's built-in fractions.Fraction together with SymPy's Rational and nsimplify.

• Imports: from fractions import Fraction and from sympy import Rational, nsimplify bring in both the standard-library fraction type and the symbolic-math tools.
• Fraction(1.1) and Fraction.from_float(1.1): these return the exact value the computer actually stores for 1.1. Because 1.1 cannot be represented exactly in binary floating point, the result is a long, unexpected fraction rather than 11/10.
• Rational(1.1): SymPy's Rational applied to a float behaves the same way, reflecting the exact binary value of the float.
• nsimplify(1.1): nsimplify works backwards from the float to find the simple rational a human most likely meant, recovering 11/10.
• to_frational(x): a small helper that prints both Fraction(x) and nsimplify(x) side by side, so you can compare the literal floating-point fraction against the cleaned-up rational for any input.

The takeaway: converting a float straight to a fraction preserves the floating-point rounding error, while nsimplify recovers the intended simple fraction.

In [1]:

from fractions import Fraction
from sympy import Rational, nsimplify

In [2]:

Fraction.from_float(1.1)

Out[2]:

Fraction(2476979795053773, 2251799813685248)

In [3]:

Fraction(1.1)

Out[3]:

Fraction(2476979795053773, 2251799813685248)

In [4]:

Rational(1.1)

Out[4]:

$\displaystyle \frac{2476979795053773}{2251799813685248}$

In [5]:

nsimplify(1.1)

Out[5]:

$\displaystyle \frac{11}{10}$

In [6]:

def to_frational(x):
  print(Fraction(x))
  print(nsimplify(x))
  print()

In [7]:

vals = [0.0,
        1.2879011017187576,
        2.5905203907920316e-16,
        -0.4714045207910316,
        1.4802973661668753e-16,
        0.3450920601366943]

In [8]:

for v in vals:
  to_frational(v)

0
0

725023865223831/562949953421312
32197527542969/25000000000000

2627099782632789/10141204801825835211973625643008
259052039079203/1000000000000000000000000000000

-8492068896701029/18014398509481984
-sqrt(2)/3

6004799503160661/40564819207303340847894502572032
9251858538543/62500000000000000000000000000

6216625893760533/18014398509481984
172546030068347/500000000000000

This post pairs a refresher on the real number system, how rational, integer, whole, and natural numbers nest inside each other, with simplifying fractions in Python. A rational number is any value that can be written as a ratio of two integers.

Simplifying a fraction means dividing the numerator and denominator by their greatest common divisor, which Python's math.gcd makes straightforward, while the fractions module can reduce them automatically.