Neighbour primes: Difference between revisions

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

How about some other offsets besides <code>+ 2</code> ?

<lang fsharp>using System; using System.Collections.Generic;

using System.Linq; using static System.Console; using System.Collections;

class Program {

static void Main(string[] args) {

WriteLine ("Multiply two consecutive prime numbers, add an even number," +

" see if the result is a prime number (up to a limit).");

int c, lim = 500; var pr = PG.Primes(lim * lim).ToList();

pr = pr.TakeWhile(x => x < lim).ToList();

var Lst = new[]{ Tuple.Create(2, 2), Tuple.Create(-20, 20) };

foreach (var pair in Lst) {

bool sho = pair.Item1 == pair.Item2;

for (int ofs = pair.Item1; ofs <= pair.Item2; ofs += ofs == -2 ? 4 : 2) {

c = 0; string s = ofs.ToString("+0;-#").Insert(1, " ");

for (int i = 0, j = 1, k; j < pr.Count; i = j++)

if (PG.isPr(k = pr[i] * pr[j] + ofs))

if (sho) WriteLine (" {0,3} * {1,3} {2} = {3,-6}",

pr[i], pr[j], s, k, c++);

else c++;

WriteLine("{0,2} found under {1} for \" {2} \"", c, lim, s);

} WriteLine (); } } }

class PG { static bool[] flags; public static bool isPr(int x) {

if (x < 2) return false; return !flags[x]; }

public static IEnumerable<int> Primes(int lim) {

flags = new bool[lim + 1]; int j = 3;

for (int d = 8, sq = 9; sq <= lim; j += 2, sq += d += 8)

if (!flags[j]) { yield return j;

for (int k = sq, i=j<<1; k<=lim; k += i) flags[k] = true; }

for (; j <= lim; j += 2) if (!flags[j]) yield return j; } }</lang>

{{out}}

<pre>Multiply two consecutive prime numbers, add an even number, see if the result is a prime number (up to a limit).

3 * 5 + 2 = 17

5 * 7 + 2 = 37

7 * 11 + 2 = 79

13 * 17 + 2 = 223

19 * 23 + 2 = 439

67 * 71 + 2 = 4759

149 * 151 + 2 = 22501

179 * 181 + 2 = 32401

229 * 233 + 2 = 53359

239 * 241 + 2 = 57601

241 * 251 + 2 = 60493

269 * 271 + 2 = 72901

277 * 281 + 2 = 77839

307 * 311 + 2 = 95479

313 * 317 + 2 = 99223

397 * 401 + 2 = 159199

401 * 409 + 2 = 164011

419 * 421 + 2 = 176401

439 * 443 + 2 = 194479

487 * 491 + 2 = 239119

20 found under 500 for " + 2 "

5 found under 500 for " - 20 "

26 found under 500 for " - 18 "

22 found under 500 for " - 16 "

10 found under 500 for " - 14 "

22 found under 500 for " - 12 "

21 found under 500 for " - 10 "

13 found under 500 for " - 8 "

32 found under 500 for " - 6 "

20 found under 500 for " - 4 "

5 found under 500 for " - 2 "

20 found under 500 for " + 2 "

9 found under 500 for " + 4 "

36 found under 500 for " + 6 "

18 found under 500 for " + 8 "

11 found under 500 for " + 10 "

27 found under 500 for " + 12 "

20 found under 500 for " + 14 "

8 found under 500 for " + 16 "

17 found under 500 for " + 18 "

25 found under 500 for " + 20 "</pre>