TheAlgorithms/C-Sharp · error · ArgumentException

is not a positive integer

Error message

{0} is not a positive integer

What it means

ModularPow computes (b^e) mod m using a naive loop; the modulus must be a positive integer because modular exponentiation is undefined for m <= 0 (and division by zero would occur for m = 0). The library throws ArgumentException with a formatted message naming the modulus. Base and exponent are not range-checked by this guard.

Solutions

1. Ensure the modulus m is a positive integer (m >= 1) before calling.
2. Check argument order: the modulus is the third parameter.
3. If a zero modulus is legitimate in your domain, guard with a special case (result defined as 0) instead of calling.

Example fix

// before
var r = ModularPow(baseVal, exp, mod); // mod may be 0
// after
if (mod <= 0) throw new ArgumentOutOfRangeException(nameof(mod));
var r = ModularPow(baseVal, exp, mod);

if (m <= 0) throw new ArgumentOutOfRangeException(nameof(m), "Modulus must be a positive integer");
var r = ModularPow(b, e, m);

try { var r = ModularPow(b, e, m); }
catch (ArgumentException ex) when (ex.Message.EndsWith("is not a positive integer")) { /* supply m >= 1 or define domain result */ }

When it happens

Trigger: Calling ModularPow(b, e, m) with m <= 0, e.g. ModularPow(2, 10, 0) or ModularPow(2, 10, -5).

Common situations: A modulus read from config defaulting to 0, an unsigned/signed conversion bug yielding negative m, or mistakenly passing the exponent as the third argument.

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

• Invalid parameter settings for Ascon Hash
• Cash flows list cannot be empty
• The parameters 'listOfAs' and 'listOfNs' must not be null…
• The value for some n_i is smaller than or equal to 1.
• The value for some a_i is smaller than 0.

AI-assisted analysis of TheAlgorithms/C-Sharp@96e2905cab (2026-09-13). Data as JSON: /api/errors/c5a0958b4e7b51f1. Report an issue: GitHub.

Appendix: source

Thrown at Algorithms/Numeric/ModularExponentiation.cs:30

    /// </summary>
    /// <param name="b">Base.</param>
    /// <param name="e">Exponent.</param>
    /// <param name="m">Modulus.</param>
    /// <returns>Modular Exponential.</returns>
    public int ModularPow(int b, int e, int m)
    {
        // initialize result in variable res
        int res = 1;
        if (m == 1)
        {
            // 1 divides every number
            return 0;
        }

        if (m <= 0)
        {
            // exponential not defined in this case
            throw new ArgumentException(string.Format("{0} is not a positive integer", m));
        }

        for (int i = 0; i < e; i++)
        {
            res = (res * b) % m;
        }

        return res;
    }
}

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