fromString static method

Rational fromString(String value)

Parses a string into a Rational object, supporting fractions, mixed numbers, integers, decimals, and scientific notation.

This method extends the functionality of Rational.parse by handling fraction formats produced by Rational.toString (e.g., "7/4") as well mixed fractions (e.g., "1 3/4"). It ignores whitespace and accepts positive and negative:

• Mixed numbers (e.g., "1 3/4", " - 2 1/2 ") with optional whole parts and whitespace
• Simple fractions (e.g., "3/4", " - 5/6 ") with optional whitespace
• Integers, decimals, and scientific notation (e.g., "5", "0.75", "1e-3"), passed to Rational.parse

Example:

print(RationalParsing.fromString("1 3/4"));      // Outputs: 7/4
print(RationalParsing.fromString("-2 1/2"));     // Outputs: -5/2
print(RationalParsing.fromString(" 3/4 "));      // Outputs: 3/4
print(RationalParsing.fromString(" - 5/6 "));    // Outputs: -5/6
print(RationalParsing.fromString("5"));          // Outputs: 5
print(RationalParsing.fromString("0.75"));       // Outputs: 3/4
print(RationalParsing.fromString("1e-3"));       // Outputs: 1/1000

• value: The string to parse into a Rational
• Returns: A Rational object representing the parsed value
• Throws: FormatException if the string is not a valid number or fraction format, or if the denominator of a fraction is zero

Implementation

static Rational fromString(String value) {
  // Case 1: Mixed number or simple fraction (e.g., " 1 3/4 ", " - 3/4 ")
  if (fractionRegex.hasMatch(value)) {
    final match = fractionRegex.firstMatch(value)!;

    // Extract the negative sign (group 1)
    final isNegative = match.group(1) != null;

    // Whole part might be null if there's no whole number (e.g., "3/4")
    final whole = match.group(2);
    final numerator = int.parse(match.group(3)!);
    final denominator = int.parse(match.group(4)!);

    if (denominator == 0) {
      throw FormatException('Denominator cannot be zero in fraction: $value');
    }

    // If there's a whole part, convert mixed number to improper fraction
    // Otherwise, use just the fraction part
    final wholePart = denominator * (whole != null ? int.parse(whole) : 0);
    final result = Rational.fromInt(wholePart + numerator, denominator);
    return isNegative ? -result : result;
  }
  // Case 2: Decimal or Integer (e.g., " 0.75 ", " 5 ")
  else {
    try {
      // Rational.parse doesn't handle whitespace, but we do
      return Rational.parse(value.trim());
    } catch (e) {
      throw FormatException('Invalid number format: $value');
    }
  }
}