TheAlgorithms/C-Sharp · error · ArgumentException

An automorphic number must always be positive.

Error message

An automorphic number must always be positive.

What it means

IsAutomorphic(int number) checks whether a number's square ends in the number's own digits. Automorphism is defined only for positive integers, so any input < 1 (zero or negative) throws ArgumentException. The library rejects non-positive values up front instead of returning false, because the mathematical property is meaningless for them.

Solutions

1. Check number >= 1 before calling IsAutomorphic and skip or handle non-positive values yourself.
2. Filter the input sequence to positive values before applying the predicate, e.g. nums.Where(n => n > 0 && IsAutomorphic(n)).
3. If zero/negative input is expected in your domain, wrap the call in try/catch or use your own automorphic check that returns false instead of throwing.

Example fix

// before
bool isAuto = AutomorphicNumber.IsAutomorphic(value); // throws when value <= 0
// after
bool isAuto = value >= 1 && AutomorphicNumber.IsAutomorphic(value);

bool isAuto = number >= 1 && AutomorphicNumber.IsAutomorphic(number);

try
{
    ok = AutomorphicNumber.IsAutomorphic(n);
}
catch (ArgumentException)
{
    ok = false; // non-positive input is trivially not automorphic
}

When it happens

Trigger: Calling AutomorphicNumber.IsAutomorphic(0) or IsAutomorphic(-5); passing a value from a computation that can yield zero (e.g. a subtraction or an uninitialized default int).

Common situations: Filtering a list that contains 0 or negatives (e.g. list.Where(IsAutomorphic) over an unfiltered dataset); feeding zero from a default-initialized variable; porting code that expected a false return for non-positives.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• cannot be negative
• The lower bound must be less than or equal to the upper…
• num ≥ k ≥ 0
• should be greater than zero
• Invalid parameter settings for Ascon Hash

AI-assisted analysis of TheAlgorithms/C-Sharp@96e2905cab (2026-09-13). Data as JSON: /api/errors/fdb2f596cea7de56. Report an issue: GitHub.

Appendix: source

Thrown at Algorithms/Numeric/AutomorphicNumber.cs:49

        if (lowerBound > upperBound)
        {
            throw new ArgumentException($"The lower bound must be less than or equal to the upper bound.");
        }

        return Enumerable.Range(lowerBound, upperBound).Where(IsAutomorphic);
    }

    /// <summary>
    /// Checks if a given natural number is automorphic or not.
    /// </summary>
    /// <param name="number">The number to check.</param>
    /// <returns>True if the number is automorphic, false otherwise.</returns>
    /// <exception cref="ArgumentException">If the number is non-positive.</exception>
    public static bool IsAutomorphic(int number)
    {
        if (number < 1)
        {
            throw new ArgumentException($"An automorphic number must always be positive.");
        }

        BigInteger square = BigInteger.Pow(number, 2);

        // Extract the last digits of both numbers
        while (number > 0)
        {
            if (number % 10 != square % 10)
            {
                return false;
            }

            number /= 10;
            square /= 10;
        }

        return true;
    }

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