Honaker primes: Difference between revisions

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=={{header|C#}}==

{{trans|Java}}

<syntaxhighlight lang="C#">

using System;

using System.Collections.Generic;

using System.Linq;

public class HonakerPrimes

{

private static List<int> primes = new List<int>();

private static int honakerIndex = 0;

private static int primeIndex = 0;

public static void Main(string[] args)

{

SievePrimes(5_000_000);

Console.WriteLine("The first 50 Honaker primes (honaker index: prime index, prime):");

for (int i = 1; i <= 50; i++)

{

Console.Write($"{NextHonakerPrime()}{(i % 5 == 0 ? "\n" : " ")}");

}

for (int i = 51; i < 10_000; i++)

{

NextHonakerPrime();

}

Console.WriteLine();

Console.WriteLine($"The 10,000th Honaker prime is: {NextHonakerPrime()}");

}

private static HonakerPrime NextHonakerPrime()

{

honakerIndex++;

primeIndex++;

while (DigitalSum(primeIndex) != DigitalSum(primes[primeIndex - 1]))

{

primeIndex++;

}

return new HonakerPrime(honakerIndex, primeIndex, primes[primeIndex - 1]);

}

private static int DigitalSum(int number)

{

return number.ToString().Select(c => c - '0').Sum();

}

private static void SievePrimes(int limit)

{

primes.Add(2);

var halfLimit = (limit + 1) / 2;

bool[] composite = new bool[halfLimit];

for (int i = 1, p = 3; i < halfLimit; p += 2, i++)

{

if (!composite[i])

{

primes.Add(p);

for (int a = i + p; a < halfLimit; a += p)

{

composite[a] = true;

}

}

}

}

private class HonakerPrime

{

public int HonakerIndex { get; }

public int PrimeIndex { get; }

public int Prime { get; }

public HonakerPrime(int honakerIndex, int primeIndex, int prime)

{

HonakerIndex = honakerIndex;

PrimeIndex = primeIndex;

Prime = prime;

}

public override string ToString() => $"({HonakerIndex}: {PrimeIndex}, {Prime})";

}

}

</syntaxhighlight>

{{out}}

<pre>

The first 50 Honaker primes (honaker index: prime index, prime):

(1: 32, 131) (2: 56, 263) (3: 88, 457) (4: 175, 1039) (5: 176, 1049)

(6: 182, 1091) (7: 212, 1301) (8: 218, 1361) (9: 227, 1433) (10: 248, 1571)

(11: 293, 1913) (12: 295, 1933) (13: 323, 2141) (14: 331, 2221) (15: 338, 2273)

(16: 362, 2441) (17: 377, 2591) (18: 386, 2663) (19: 394, 2707) (20: 397, 2719)

(21: 398, 2729) (22: 409, 2803) (23: 439, 3067) (24: 446, 3137) (25: 457, 3229)

(26: 481, 3433) (27: 499, 3559) (28: 508, 3631) (29: 563, 4091) (30: 571, 4153)

(31: 595, 4357) (32: 599, 4397) (33: 635, 4703) (34: 637, 4723) (35: 655, 4903)

(36: 671, 5009) (37: 728, 5507) (38: 751, 5701) (39: 752, 5711) (40: 755, 5741)

(41: 761, 5801) (42: 767, 5843) (43: 779, 5927) (44: 820, 6301) (45: 821, 6311)

(46: 826, 6343) (47: 827, 6353) (48: 847, 6553) (49: 848, 6563) (50: 857, 6653)

The 10,000th Honaker prime is: (10000: 286069, 4043749)