Why Does My Prime Number Detection Program Fail, and How Can I Fix It?

Debugging a Faulty Prime Number Detector

A program designed to identify prime numbers within a range defined by a long variable is failing to produce any output. Analysis of the code reveals two critical flaws:

• Unending Loop: The inner loop within the prime_num() function contains a flawed condition (i), causing an infinite loop.
• Flawed Prime Number Logic: The prime check is incorrect. A prime number is only divisible by 1 and itself. The condition if (i != j && i % j == 0) incorrectly classifies numbers with additional divisors as prime.

An Enhanced Prime Number Finding Algorithm

A more efficient solution employs a "trial division sieve" method:

<code class="language-csharp">Enumerable.Range(0, Math.Floor(2.52*Math.Sqrt(num)/Math.Log(num))).Aggregate(
    Enumerable.Range(2, num-1).ToList(), 
    (result, index) => { 
        var bp = result[index]; var sqr = bp * bp;
        result.RemoveAll(i => i >= sqr && i % bp == 0); 
        return result; 
    }
);</code>

This improved algorithm uses an approximation to minimize the number of prime candidates tested, resulting in significant performance gains.

The above is the detailed content of Why Does My Prime Number Detection Program Fail, and How Can I Fix It?. For more information, please follow other related articles on the PHP Chinese website!