# Twin primes

Twin primes are pairs of natural numbers   (P1  and  P2)   that satisfy the following:

Twin primes
You are encouraged to solve this task according to the task description, using any language you may know.
1.     P1   and   P2   are primes
2.     P1  +  2   =   P2

Task

Write a program that displays the number of pairs of twin primes that can be found under a user-specified number
(P1 < user-specified number & P2 < user-specified number).

Extension
1. Find all twin prime pairs under 100000, 10000000 and 1000000000.
2. What is the time complexity of the program? Are there ways to reduce computation time?

Examples
```> Search Size: 100
> 8 twin prime pairs.
```
```> Search Size: 1000
> 35 twin prime pairs.
```

Also see

## ALGOL 68

Works with: ALGOL 68G version Any - tested with release 2.8.3.win32

Simplifies array bound checking by using the equivalent definition of twin primes: p and p - 2.

```BEGIN
# count twin primes (where p and p - 2 are prime)                             #
PR heap=128M PR # set heap memory size for Algol 68G                          #
# sieve of Eratosthenes: sets s[i] to TRUE if i is a prime, FALSE otherwise   #
PROC sieve = ( REF[]BOOL s )VOID:
BEGIN
FOR i TO UPB s DO s[ i ] := TRUE OD;
s[ 1 ] := FALSE;
FOR i FROM 2 TO ENTIER sqrt( UPB s ) DO
IF s[ i ] THEN FOR p FROM i * i BY i TO UPB s DO s[ p ] := FALSE OD FI
OD
END # sieve # ;
# find the maximum number to search for twin primes                           #
INT max;
print( ( "Maximum: " ) );
read( ( max, newline ) );
INT max number = max;
# construct a sieve of primes up to the maximum number                        #
[ 1 : max number ]BOOL primes;
sieve( primes );
# count the twin primes                                                       #
# note 2 cannot be one of the primes in a twin prime pair, so we start at 3   #
INT twin count := 0;
FOR p FROM 3 BY 2 TO max number - 1 DO IF primes[ p ] AND primes[ p - 2 ] THEN twin count +:= 1 FI OD;
print( ( "twin prime pairs below  ", whole( max number, 0 ), ": ", whole( twin count, 0 ), newline ) )
END```
Output:
```Maximum: 10
twin prime pairs below  10: 2
```
```Maximum: 100
twin prime pairs below  100: 8
```
```Maximum: 1000
twin prime pairs below  1000: 35
```
```Maximum: 10000
twin prime pairs below  10000: 205
```
```Maximum: 100000
twin prime pairs below  100000: 1224
```
```Maximum: 1000000
twin prime pairs below  1000000: 8169
```
```Maximum: 10000000
twin prime pairs below  10000000: 58980
```

## Arturo

```pairsOfPrimes: function [upperLim][
count: 0
j: 0
k: 1
i: 0
while [i=<upperLim][
i: (6 * k) - 1
j: i + 2
if and? [prime? i] [prime? j] [
count: count + 1
]
k: k + 1
]
return count + 1
]

ToNum: 10
while [ToNum =< 1000000][
x: pairsOfPrimes ToNum
print ["From 2 to" ToNum ": there are" x "pairs of twin primes"]
ToNum: ToNum * 10
]
```
Output:
```From 2 to 10 : there are 3 pairs of twin primes
From 2 to 100 : there are 9 pairs of twin primes
From 2 to 1000 : there are 35 pairs of twin primes
From 2 to 10000 : there are 205 pairs of twin primes
From 2 to 100000 : there are 1224 pairs of twin primes
From 2 to 1000000 : there are 8169 pairs of twin primes```

## Applesoft BASIC

``` 0  INPUT "SEARCH SIZE: ";S: FOR N = 1 TO S - 3 STEP 2:P = N: GOSUB 3: IF F THEN P = N + 2: GOSUB 3:C = C + F
1 J = J + (N > 5): IF J = 3 THEN N = N + 4:J = 0
2  NEXT N: PRINT C" TWIN PRIME PAIRS.": END
3 F = 0: IF P < 2 THEN  RETURN
4 F = P = 2: IF F THEN  RETURN
5 F = P -  INT (P / 2) * 2: IF  NOT F THEN  RETURN
6  FOR B = 3 TO  SQR (P) STEP 2: IF B >  = P THEN  NEXT B
7  IF B >  = P THEN  RETURN
8 F = 0: FOR I = B TO  SQR (P) STEP 2: IF P -  INT (P / I) * I = 0 THEN  RETURN
9  NEXT I:F = 1: RETURN
```

## AWK

```# syntax: GAWK -f TWIN_PRIMES.AWK
BEGIN {
n = 1
for (i=1; i<=6; i++) {
n *= 10
printf("twin prime pairs < %8s : %d\n",n,count_twin_primes(n))
}
exit(0)
}
function count_twin_primes(limit,  count,i,p1,p2,p3) {
p1 = 0
p2 = p3 = 1
for (i=5; i<=limit; i++) {
p3 = p2
p2 = p1
p1 = is_prime(i)
if (p3 && p1) {
count++
}
}
return(count)
}
function is_prime(x,  i) {
if (x <= 1) {
return(0)
}
for (i=2; i<=int(sqrt(x)); i++) {
if (x % i == 0) {
return(0)
}
}
return(1)
}
```
Output:
```twin prime pairs <       10 : 2
twin prime pairs <      100 : 8
twin prime pairs <     1000 : 35
twin prime pairs <    10000 : 205
twin prime pairs <   100000 : 1224
twin prime pairs <  1000000 : 8169
```

## BASIC256

Translation of: FreeBASIC
```function isPrime(v)
if v < 2 then return False
if v mod 2 = 0 then return v = 2
if v mod 3 = 0 then return v = 3
d = 5
while d * d <= v
if v mod d = 0 then return False else d += 2
end while
return True
end function

function paresDePrimos(limite)
p1 = 0
p2 = 1
p3 = 1
cont = 0
for i = 5 to limite
p3 = p2
p2 = p1
p1 = isPrime(i)
if (p3 and p1) then cont += 1
next i
return cont
end function

n = 1
for i = 1 to 6
n = n * 10
print "pares de primos gemelos por debajo de < "; n; " : "; paresDePrimos(n)
next i
end```
Output:
```Similar a la entrada de FreeBASIC.
```

## C

```#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>

bool isPrime(int64_t n) {
int64_t i;

if (n < 2)       return false;
if (n % 2 == 0)  return n == 2;
if (n % 3 == 0)  return n == 3;
if (n % 5 == 0)  return n == 5;
if (n % 7 == 0)  return n == 7;
if (n % 11 == 0) return n == 11;
if (n % 13 == 0) return n == 13;
if (n % 17 == 0) return n == 17;
if (n % 19 == 0) return n == 19;

for (i = 23; i * i <= n; i += 2) {
if (n % i == 0) return false;
}

return true;
}

int countTwinPrimes(int limit) {
int count = 0;

//       2          3          4
int64_t p3 = true, p2 = true, p1 = false;
int64_t i;

for (i = 5; i <= limit; i++) {
p3 = p2;
p2 = p1;
p1 = isPrime(i);
if (p3 && p1) {
count++;
}
}
return count;
}

void test(int limit) {
int count = countTwinPrimes(limit);
printf("Number of twin prime pairs less than %d is %d\n", limit, count);
}

int main() {
test(10);
test(100);
test(1000);
test(10000);
test(100000);
test(1000000);
test(10000000);
test(100000000);
return 0;
}
```
Output:
```Number of twin prime pairs less than 10 is 2
Number of twin prime pairs less than 100 is 8
Number of twin prime pairs less than 1000 is 35
Number of twin prime pairs less than 10000 is 205
Number of twin prime pairs less than 100000 is 1224
Number of twin prime pairs less than 1000000 is 8169
Number of twin prime pairs less than 10000000 is 58980
Number of twin prime pairs less than 100000000 is 440312```

## C++

Library: Primesieve

The primesieve library includes the functionality required for this task. RC already has plenty of C++ examples showing how to generate prime numbers from scratch. (The module Math::Primesieve, which is used by the Raku example on this page, is implemented on top of this library.)

```#include <cstdint>
#include <iostream>
#include <string>
#include <primesieve.hpp>

void print_twin_prime_count(long long limit) {
std::cout << "Number of twin prime pairs less than " << limit
<< " is " << (limit > 0 ? primesieve::count_twins(0, limit - 1) : 0) << '\n';
}

int main(int argc, char** argv) {
std::cout.imbue(std::locale(""));
if (argc > 1) {
// print number of twin prime pairs less than limits specified
// on the command line
for (int i = 1; i < argc; ++i) {
try {
print_twin_prime_count(std::stoll(argv[i]));
} catch (const std::exception& ex) {
std::cerr << "Cannot parse limit from '" << argv[i] << "'\n";
}
}
} else {
// if no limit was specified then show the number of twin prime
// pairs less than powers of 10 up to 100 billion
uint64_t limit = 10;
for (int power = 1; power < 12; ++power, limit *= 10)
print_twin_prime_count(limit);
}
return 0;
}
```
Output:
```Number of twin prime pairs less than 10 is 2
Number of twin prime pairs less than 100 is 8
Number of twin prime pairs less than 1,000 is 35
Number of twin prime pairs less than 10,000 is 205
Number of twin prime pairs less than 100,000 is 1,224
Number of twin prime pairs less than 1,000,000 is 8,169
Number of twin prime pairs less than 10,000,000 is 58,980
Number of twin prime pairs less than 100,000,000 is 440,312
Number of twin prime pairs less than 1,000,000,000 is 3,424,506
Number of twin prime pairs less than 10,000,000,000 is 27,412,679
Number of twin prime pairs less than 100,000,000,000 is 224,376,048
```

### Without external libraries

```#include <bitset>
#include <cstdint>
#include <iomanip>
#include <iostream>
#include <vector>

const uint32_t limit = 1'000'000'000;
std::bitset<limit + 1> primes;

void sieve_primes(uint32_t limit) {
primes.set();
primes.reset(0); primes.reset(1);

for ( uint32_t p = 2; p * p <= limit; ++p ) {
if ( primes.test(p) ) {
for ( uint32_t i = p * p; i <= limit; i += p ) {
primes.reset(i);
}
}
}
}

int main() {
sieve_primes(limit);

uint32_t target = 10;
uint32_t count = 0;
bool last = false;
bool first = true;

for ( uint32_t index = 5; index <= limit; index += 2 ) {
last = first;
first = primes[index];
if ( last && first ) {
count += 1;
}
if ( index + 1 == target ) {
std::cout << std::setw(8) << count << " twin primes below " << index + 1 << std::endl;
target *= 10;
}
}
}
```
Output:
```       2 twin primes below 10
8 twin primes below 100
35 twin primes below 1000
205 twin primes below 10000
1224 twin primes below 100000
8169 twin primes below 1000000
58980 twin primes below 10000000
440312 twin primes below 100000000
3424506 twin primes below 1000000000
```

## C#

### Conventional

```using System;

class Program {

static uint[] res = new uint[10];
static uint ri = 1, p = 10, count = 0;

static void TabulateTwinPrimes(uint bound) {
if (bound < 5) return; count++;
uint cl = (bound - 1) >> 1, i = 1, j,
limit = (uint)(Math.Sqrt(bound) - 1) >> 1;
var comp = new bool[cl]; bool lp;
for (j = 3; j < cl; j += 3) comp[j] = true;
while (i < limit) {
if (lp = !comp[i]) {
uint pr = (i << 1) + 3;
for (j = (pr * pr - 2) >> 1; j < cl; j += pr)
comp[j] = true;
}
if (!comp[++i]) {
uint pr = (i << 1) + 3;
if (lp) {
if (pr > p) {
res[ri++] = count;
p *= 10;
}
count++;
i++;
}
for (j = (pr * pr - 2) >> 1; j < cl; j += pr)
comp[j] = true;
}
}
cl--;
while (i < cl) {
lp = !comp[i++];
if (!comp[i] && lp) {
if ((i++ << 1) + 3 > p) {
res[ri++] = count;
p *= 10;
}
count++;
}
}
res[ri] = count;
}

static void Main(string[] args) {
var sw = System.Diagnostics.Stopwatch.StartNew();
string fmt = "{0,9:n0} twin primes below {1,-13:n0}";
TabulateTwinPrimes(1_000_000_000);
sw.Stop();
p = 1;
for (var j = 1; j <= ri; j++)
Console.WriteLine(fmt, res[j], p *= 10);
Console.Write("{0} sec", sw.Elapsed.TotalSeconds);
}
}
```
Output @ Tio.run:
```        2 twin primes below 10
8 twin primes below 100
35 twin primes below 1,000
205 twin primes below 10,000
1,224 twin primes below 100,000
8,169 twin primes below 1,000,000
58,980 twin primes below 10,000,000
440,312 twin primes below 100,000,000
3,424,506 twin primes below 1,000,000,000
17.8284822 sec```

### Using C# 8 features

Works with: C sharp version 8

Runs in about 18 seconds.

```using System;
using System.Linq;
using System.Collections;
using System.Collections.Generic;

public static class TwinPrimes
{
public static void Main()
{
CountTwinPrimes(Enumerable.Range(1, 9).Select(i => (int)Math.Pow(10, i)).ToArray());
}

private static void CountTwinPrimes(params int[] bounds)
{
Array.Sort(bounds);
int b = 0;
int count = 0;
string format = "There are {0:N0} twin primes below {1:N0}";
foreach (var twin in FindTwinPrimes(bounds[^1])) {
if (twin.p2 >= bounds[b]) {
Console.WriteLine(format, count, bounds[b]);
b++;
}
count++;
}
Console.WriteLine(format, count, bounds[b]);
}

private static IEnumerable<(int p1, int p2)> FindTwinPrimes(int bound) =>
PrimeSieve(bound).Pairwise().Where(pair => pair.p1 + 2 == pair.p2);

private static IEnumerable<int> PrimeSieve(int bound)
{
if (bound < 2) yield break;
yield return 2;

var composite = new BitArray((bound - 1) / 2);
int limit = (int)(Math.Sqrt(bound) - 1) / 2;
for (int i = 0; i < limit; i++) {
if (composite[i]) continue;
int prime = 2 * i + 3;
yield return prime;
for (int j = (prime * prime - 2) / 2; j < composite.Count; j += prime) {
composite[j] = true;
}
}
for (int i = limit; i < composite.Count; i++) {
if (!composite[i]) yield return 2 * i + 3;
}
}

private static IEnumerable<(T p1, T p2)> Pairwise<T>(this IEnumerable<T> source)
{
using var e = numbers.GetEnumerator();
if (!e.MoveNext()) yield break;
T p1 = e.Current;
while (e.MoveNext()) {
T p2 = e.Current;
yield return (p1, p2);
p1 = p2;
}
}
}
```
Output:
```There are 2 twin primes below 10
There are 8 twin primes below 100
There are 35 twin primes below 1,000
There are 205 twin primes below 10,000
There are 1,224 twin primes below 100,000
There are 8,169 twin primes below 1,000,000
There are 58,980 twin primes below 10,000,000
There are 440,312 twin primes below 100,000,000
There are 3,424,506 twin primes below 1,000,000,000```

## Delphi

Translation of: Wren
```program Primes;

{\$APPTYPE CONSOLE}

{\$R *.res}

uses
System.SysUtils;

function IsPrime(a: UInt64): Boolean;
var
d: UInt64;
begin
if (a < 2) then
exit(False);

if (a mod 2) = 0 then
exit(a = 2);

if (a mod 3) = 0 then
exit(a = 3);

d := 5;

while (d * d <= a) do
begin
if (a mod d = 0) then
Exit(false);
inc(d, 2);

if (a mod d = 0) then
Exit(false);
inc(d, 4);
end;

Result := True;
end;

function Sieve(limit: UInt64): TArray<Boolean>;
var
p, p2, i: UInt64;
begin
inc(limit);
SetLength(Result, limit);
FillChar(Result[2], sizeof(Boolean) * limit - 2, 0); // all false except 1,2
FillChar(Result[0], sizeof(Boolean) * 2, 1); // 1,2 are true

p := 3;
while true do
begin
p2 := p * p;
if p2 >= limit then
break;

i := p2;
while i < limit do
begin
Result[i] := true;
inc(i, 2 * p);
end;

while true do
begin
inc(p, 2);
if not Result[p] then
Break;
end;
end;
end;

function Commatize(const n: UInt64): string;
var
str: string;
digits: Integer;
i: Integer;
begin
Result := '';
str := n.ToString;
digits := str.Length;

for i := 1 to digits do
begin
if ((i > 1) and (((i - 1) mod 3) = (digits mod 3))) then
Result := Result + ',';
Result := Result + str[i];
end;
end;

var
limit, start, twins: UInt64;
c: TArray<Boolean>;
i, j: UInt64;

begin

c := Sieve(Trunc(1e9 - 1));
limit := 10;
start := 3;
twins := 0;
for i := 1 to 9 do
begin
j := start;
while j < limit do
begin
if (not c[j]) and (not c[j - 2]) then
inc(twins);
inc(j, 2);
end;
Writeln(Format('Under %14s there are %10s pairs of twin primes.', [commatize
(limit), commatize(twins)]));

start := limit + 1;
limit := 10 * limit;
end;

readln;

end.
```
Output:
```Under             10 there are          2 pairs of twin primes.
Under            100 there are          8 pairs of twin primes.
Under          1,000 there are         35 pairs of twin primes.
Under         10,000 there are        205 pairs of twin primes.
Under        100,000 there are      1,224 pairs of twin primes.
Under      1,000,000 there are      8,169 pairs of twin primes.
Under     10,000,000 there are     58,980 pairs of twin primes.
Under    100,000,000 there are    440,312 pairs of twin primes.
Under  1,000,000,000 there are  3,424,506 pairs of twin primes.
```

## EasyLang

Translation of: AWK
```fastfunc isprim num .
if num mod 2 = 0 and num > 2
return 0
.
i = 3
while i <= sqrt num
if num mod i = 0
return 0
.
i += 2
.
return 1
.
func count limit .
p2 = 1
p3 = 1
for i = 5 to limit
p3 = p2
p2 = p1
p1 = isprim i
if p3 = 1 and p1 = 1
cnt += 1
.
.
return cnt
.
n = 1
for i = 1 to 6
n *= 10
print "twin prime pairs < " & n & " : " & count n
.```

## F#

This task uses Extensible Prime Generator (F#)

```printfn "twin primes below 100000: %d" (primes64()|>Seq.takeWhile(fun n->n<=100000L)|>Seq.pairwise|>Seq.filter(fun(n,g)->g=n+2L)|>Seq.length)
printfn "twin primes below 1000000: %d" (primes64()|>Seq.takeWhile(fun n->n<=1000000L)|>Seq.pairwise|>Seq.filter(fun(n,g)->g=n+2L)|>Seq.length)
printfn "twin primes below 10000000: %d" (primes64()|>Seq.takeWhile(fun n->n<=10000000L)|>Seq.pairwise|>Seq.filter(fun(n,g)->g=n+2L)|>Seq.length)
printfn "twin primes below 100000000: %d" (primes64()|>Seq.takeWhile(fun n->n<=100000000L)|>Seq.pairwise|>Seq.filter(fun(n,g)->g=n+2L)|>Seq.length)
printfn "twin primes below 1000000000: %d" (primes64()|>Seq.takeWhile(fun n->n<=1000000000L)|>Seq.pairwise|>Seq.filter(fun(n,g)->g=n+2L)|>Seq.length)
printfn "twin primes below 10000000000: %d" (primes64()|>Seq.takeWhile(fun n->n<=10000000000L)|>Seq.pairwise|>Seq.filter(fun(n,g)->g=n+2L)|>Seq.length)
printfn "twin primes below 100000000000: %d" (primes64()|>Seq.takeWhile(fun n->n<=100000000000L)|>Seq.pairwise|>Seq.filter(fun(n,g)->g=n+2L)|>Seq.length)
```
Output:
```twin primes below 100000: 1224
Real: 00:00:00.003, CPU: 00:00:00.015, GC gen0: 0, gen1: 0, gen2: 0

twin primes below 1000000: 8169
Real: 00:00:00.021, CPU: 00:00:00.031, GC gen0: 3, gen1: 3, gen2: 0

twin primes below 10000000: 58980
Real: 00:00:00.154, CPU: 00:00:00.171, GC gen0: 19, gen1: 19, gen2: 0

twin primes below 100000000: 440312
Real: 00:00:01.400, CPU: 00:00:01.406, GC gen0: 162, gen1: 162, gen2: 0

twin primes below 1000000000: 3424506
Real: 00:00:12.682, CPU: 00:00:12.671, GC gen0: 1428, gen1: 1426, gen2: 1

twin primes below 10000000000: 27412679
Real: 00:02:04.441, CPU: 00:02:04.406, GC gen0: 12771, gen1: 12768, gen2: 2

twin primes below 100000000000: 224376048
Real: 00:23:00.853, CPU: 00:23:00.125, GC gen0: 115562, gen1: 115536, gen2: 14
```

## Factor

Works with: Factor version 0.99 2020-07-03
```USING: io kernel math math.parser math.primes.erato math.ranges
sequences tools.memory.private ;

: twin-pair-count ( n -- count )
[ 5 swap 2 <range> ] [ sieve ] bi
[ over 2 - over [ marked-prime? ] 2bi@ and ] curry count ;

"Search size: " write flush readln string>number
twin-pair-count commas write " twin prime pairs." print
```
Output:
```Search size: 100,000
1,224 twin prime pairs.
```
```Search size: 10,000,000
58,980 twin prime pairs.
```
```Search size: 1,000,000,000
3,424,506 twin prime pairs.
```

## FreeBASIC

Translation of: AWK
```Function isPrime(Byval ValorEval As Integer) As Boolean
If ValorEval <=1 Then Return False
For i As Integer = 2 To Int(Sqr(ValorEval))
If ValorEval Mod i = 0 Then Return False
Next i
Return True
End Function

Function paresDePrimos(limite As Uinteger) As Uinteger
Dim As Uinteger p1 = 0, p2 = 1, p3 = 1, count = 0
For i As Uinteger = 5 To limite
p3 = p2
p2 = p1
p1 = isPrime(i)
If (p3 And p1) Then count += 1
Next i
Return count
End Function

Dim As Uinteger n = 1
For i As Byte = 1 To 6
n *= 10
Print Using "pares de primos gemelos por debajo de < ####### : ####"; n; paresDePrimos(n)
Next i
Print !"\n--- terminado, pulsa RETURN---"
Sleep
```
Output:
```pares de primos gemelos por debajo de <      10 :    2
pares de primos gemelos por debajo de <     100 :    8
pares de primos gemelos por debajo de <    1000 :   35
pares de primos gemelos por debajo de <   10000 :  205
pares de primos gemelos por debajo de <  100000 : 1224
pares de primos gemelos por debajo de < 1000000 : 8169
```

## Frink

```upper = eval[input["Enter upper bound:"]]
countTwins[upper]
countTwins[100000]
countTwins[10000000]
countTwins[1000000000]

countTwins[upper] :=
{
count = 0
for n = primes[2, upper-2]
if isPrime[n+2]
count = count + 1

println["\$count twin primes under \$upper"]
}```
Output:
```35 twin primes under 1000
1224 twin primes under 100000
58980 twin primes under 10000000
3424506 twin primes under 1000000000
```

## Go

Translation of: Wren
```package main

import "fmt"

func sieve(limit uint64) []bool {
limit++
// True denotes composite, false denotes prime.
c := make([]bool, limit) // all false by default
c[0] = true
c[1] = true
// no need to bother with even numbers over 2 for this task
p := uint64(3) // Start from 3.
for {
p2 := p * p
if p2 >= limit {
break
}
for i := p2; i < limit; i += 2 * p {
c[i] = true
}
for {
p += 2
if !c[p] {
break
}
}
}
return c
}

func commatize(n int) string {
s := fmt.Sprintf("%d", n)
if n < 0 {
s = s[1:]
}
le := len(s)
for i := le - 3; i >= 1; i -= 3 {
s = s[0:i] + "," + s[i:]
}
if n >= 0 {
return s
}
return "-" + s
}

func main() {
c := sieve(1e10 - 1)
limit := 10
start := 3
twins := 0
for i := 1; i < 11; i++ {
for i := start; i < limit; i += 2 {
if !c[i] && !c[i-2] {
twins++
}
}
fmt.Printf("Under %14s there are %10s pairs of twin primes.\n", commatize(limit), commatize(twins))
start = limit + 1
limit *= 10
}
}
```
Output:
```Under             10 there are          2 pairs of twin primes.
Under            100 there are          8 pairs of twin primes.
Under          1,000 there are         35 pairs of twin primes.
Under         10,000 there are        205 pairs of twin primes.
Under        100,000 there are      1,224 pairs of twin primes.
Under      1,000,000 there are      8,169 pairs of twin primes.
Under     10,000,000 there are     58,980 pairs of twin primes.
Under    100,000,000 there are    440,312 pairs of twin primes.
Under  1,000,000,000 there are  3,424,506 pairs of twin primes.
Under 10,000,000,000 there are 27,412,679 pairs of twin primes.
```

### Alternative using primegen package

```package main

import (
"fmt"
"github.com/jbarham/primegen.go"
)

func main() {
p := primegen.New()
count := 0
previous := uint64(0)
power := 1
limit := uint64(10)
for {
prime := p.Next()
if prime >= limit {
fmt.Printf("Number of twin prime pairs less than %d: %d\n", limit, count)
power++
if power > 10 {
break
}
limit *= 10
}
if previous > 0 && prime == previous + 2 {
count++
}
previous = prime
}
}
```
Output:
```Number of twin prime pairs less than 10: 2
Number of twin prime pairs less than 100: 8
Number of twin prime pairs less than 1000: 35
Number of twin prime pairs less than 10000: 205
Number of twin prime pairs less than 100000: 1224
Number of twin prime pairs less than 1000000: 8169
Number of twin prime pairs less than 10000000: 58980
Number of twin prime pairs less than 100000000: 440312
Number of twin prime pairs less than 1000000000: 3424506
Number of twin prime pairs less than 10000000000: 27412679
```

## Java

BigInteger Implementation:

```import java.math.BigInteger;
import java.util.Scanner;

public class twinPrimes {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.println("Search Size: ");
BigInteger max = input.nextBigInteger();
int counter = 0;
for(BigInteger x = new BigInteger("3"); x.compareTo(max) <= 0; x = x.add(BigInteger.ONE)){
BigInteger sqrtNum = x.sqrt().add(BigInteger.ONE);
if(x.add(BigInteger.TWO).compareTo(max) <= 0) {
counter += findPrime(x.add(BigInteger.TWO), x.add(BigInteger.TWO).sqrt().add(BigInteger.ONE)) && findPrime(x, sqrtNum) ? 1 : 0;
}
}
System.out.println(counter + " twin prime pairs.");
}
public static boolean findPrime(BigInteger x, BigInteger sqrtNum){
for(BigInteger divisor = BigInteger.TWO; divisor.compareTo(sqrtNum) <= 0; divisor = divisor.add(BigInteger.ONE)){
if(x.remainder(divisor).compareTo(BigInteger.ZERO) == 0){
return false;
}
}
return true;
}
}
```
Output:
```> Search Size:
> 100
> 8 twin prime pairs.
```
```> Search Size:
> 1000
> 35 twin prime pairs.
```

### Extended Task

```import java.util.BitSet;

public final class TwinPrimes {

public static void main(String[] args) {
final int limit = 1_000_000_000;
sievePrimes(limit);

int target = 10;
int count = 0;
boolean last = false;
boolean first = true;

for ( int index = 5; index <= limit; index += 2 ) {
last = first;
first = primes.get(index);
if ( last && first ) {
count += 1;
}
if ( index + 1 == target ) {
System.out.println(String.format("%8d%s%d", count, " twin primes below ", index + 1));
target *= 10;
}
}
}

private static void sievePrimes(int aLimit) {
primes = new BitSet(aLimit + 1);
primes.set(2, aLimit + 1);
for ( int i = 2; i <= Math.sqrt(aLimit); i = primes.nextSetBit(i + 1) ) {
for ( int j = i * i; j <= aLimit; j += i ) {
primes.clear(j);
}
}
}

private static BitSet primes;

}
```
Output:
```       2 twin primes below 10
8 twin primes below 100
35 twin primes below 1000
205 twin primes below 10000
1224 twin primes below 100000
8169 twin primes below 1000000
58980 twin primes below 10000000
440312 twin primes below 100000000
3424506 twin primes below 1000000000
```

## J

```tp=: (_2 +/@:= 2 -/\ i.&.(p:inv))"0

NB. i.&.(p:inv) generate a list of primes below the given limit
NB. 2 -/\       pairwise subtract adjacent primes (create a list of differences)
NB. _2 +/@:=    compare these differences to -2, and
NB.             sum up the resulting boolean list (get the number of twin pairs)
```
Output:
```   tp 10 ^ >: i. 7
2 8 35 205 1224 8169 58980

NB. test larger limits, and get their time/space usage
tp 1e8
440312
timespacex 'tp 1e8'
0.657576 2.01377e8
tp 1e9
3424506
timespacex 'tp 1e9'
8.59713 1.61071e9
```

## jq

Slightly modified from the C entry

```def odd_gt2_is_prime:
. as \$n
| if   (\$n % 3 == 0)  then \$n == 3
elif (\$n % 5 == 0)  then \$n == 5
elif (\$n % 7 == 0)  then \$n == 7
elif (\$n % 11 == 0) then \$n == 11
elif (\$n % 13 == 0) then \$n == 13
elif (\$n % 17 == 0) then \$n == 17
elif (\$n % 19 == 0) then \$n == 19
else {i:23}
| until( (.i * .i) > \$n or (\$n % .i == 0); .i += 2)
| .i * .i > \$n
end;

def twin_primes(\$max):
{count:0, i:3, isprime:true}
| until(.i >= \$max;
.i += 2
| if .isprime
then if .i|odd_gt2_is_prime then .count+=1 else .isprime = false end
else .isprime = (.i|odd_gt2_is_prime)
end )
| .count;

pow(10; range(1;8)) | "Number of twin primes less than \(.) is \(twin_primes(.))."```
Output:
```Number of twin primes less than 10 is 2.
Number of twin primes less than 100 is 8.
Number of twin primes less than 1000 is 35.
Number of twin primes less than 10000 is 205.
Number of twin primes less than 100000 is 1224.
Number of twin primes less than 1000000 is 8169.
Number of twin primes less than 10000000 is 58980.
```

## Julia

```using Formatting, Primes

function counttwinprimepairsbetween(n1, n2)
npairs, t = 0, nextprime(n1)
while t < n2
p = nextprime(t + 1)
if p - t == 2
npairs += 1
end
t = p
end
return npairs
end

for t2 in (10).^collect(2:8)
paircount = counttwinprimepairsbetween(1, t2)
println("Under", lpad(format(t2, commas=true), 12), " there are",
lpad(format(paircount, commas=true), 8), " pairs of twin primes.")
end
```
Output:
```Under         100 there are       8 pairs of twin primes.
Under       1,000 there are      35 pairs of twin primes.
Under      10,000 there are     205 pairs of twin primes.
Under     100,000 there are   1,224 pairs of twin primes.
Under   1,000,000 there are   8,169 pairs of twin primes.
Under  10,000,000 there are  58,980 pairs of twin primes.
Under 100,000,000 there are 440,312 pairs of twin primes.
```

### Extension to large n and other tuples

Task Extension: to get primes up to a billion, it becomes important to cache the results so that large numbers do not need to be factored more than once. This trades memory for speed. The time complexity is dominated by the prime sieve time used to create the primes mask, which is n log(log n).

We can generalize pairs to reflect any tuple of integer differences between the first prime and the successive primes: see http://www.rosettacode.org/wiki/Successive_prime_differences.

If we ignore the first difference from the index prime with itself (always 0), we can express a prime pair as a difference tuple of (2,), and a prime quadruplet such as [11, 13, 17, 19] as the tuple starting with 11 of type (2, 6, 8).

```using Formatting, Primes

const PMAX = 1_000_000_000
const pmb = primesmask(PMAX)
const primestoabillion = [i for i in 2:PMAX if pmb[i]]

tuplefitsat(k, tup, arr) = all(i -> arr[k + i] - arr[k] == tup[i], 1:length(tup))

function countprimetuples(tup, n)
arr =  filter(i -> i <= n, primestoabillion)
return count(k -> tuplefitsat(k, tup, arr), 1:length(arr) - length(tup))
end

println("Count of prime pairs from 1 to 1 billion: ",
format(countprimetuples((2,), 1000000000), commas=true))
println("Count of a form of prime quads from 1 to 1 million: ",
format(countprimetuples((2, 6, 8), 1000000), commas=true))
println("Count of a form of prime octets from 1 to 1 million: ",
format(countprimetuples((2, 6, 12, 14, 20, 24, 26), 1000000), commas=true))
```
Output:
```Count of prime pairs from 1 to 1 billion: 3,424,506
Count of a form of prime quads from 1 to 1 million: 166
Count of a form of prime octets from 1 to 1 million: 3
```

## Kotlin

Translation of: Java
```import java.math.BigInteger
import java.util.*

fun main() {
val input = Scanner(System.`in`)
println("Search Size: ")
val max = input.nextBigInteger()
var counter = 0
var x = BigInteger("3")
while (x <= max) {
val sqrtNum = x.sqrt().add(BigInteger.ONE)
if (x.add(BigInteger.TWO) <= max) {
counter += if (findPrime(
x.add(BigInteger.TWO),
x.add(BigInteger.TWO).sqrt().add(BigInteger.ONE)
) && findPrime(x, sqrtNum)
) 1 else 0
}
x = x.add(BigInteger.ONE)
}
println("\$counter twin prime pairs.")
}

fun findPrime(x: BigInteger, sqrtNum: BigInteger?): Boolean {
var divisor = BigInteger.TWO
while (divisor <= sqrtNum) {
if (x.remainder(divisor).compareTo(BigInteger.ZERO) == 0) {
return false
}
divisor = divisor.add(BigInteger.ONE)
}
return true
}
```

## Mathematica /Wolfram Language

```ClearAll[TwinPrimeCount]
TwinPrimeCount[mx_] := Module[{pmax, min, max, total},
pmax = PrimePi[mx];
total = 0;
Do[
min = 10^6 i;
min = Max[min, 1];
max = 10^6 (i + 1);
max = Min[max, pmax];
total += Count[Differences[Prime[Range[min, max]]], 2]
,
{i, 0, Ceiling[pmax/10^6]}
];
total
]
Do[Print[{10^i, TwinPrimeCount[10^i]}], {i, 9}]
```
Output:
```{10,2}
{100,8}
{1000,35}
{10000,205}
{100000,1224}
{1000000,8169}
{10000000,58980}
{100000000,440312}
{1000000000,3424506}```

## Maxima

Using built-in predicate function primep to detect primes and the fact that all but the first pair are of the form [6n-1,6n+1].

```/* Function to count the number of pairs below n */
twin_primes_under_n(n):=block(
L:makelist([6*i-1,6*i+1],i,1,floor(n/6)),
caching:length(L),
L1:[],
for i from 1 thru caching do if map(primep,L[i])=[true,true] then push(L[i],L1),
append([[3,5]],reverse(L1)),
length(%%));

/* Test cases */
twin_primes_under_n(10);
twin_primes_under_n(100);
twin_primes_under_n(1000);
twin_primes_under_n(10000);
twin_primes_under_n(100000);
```
Output:
```2
8
35
205
1224
```

## Nim

We use a sieve of Erathostenes which needs a lot of memory. It is possible to reduce memory usage by using bit strings for the sieve (one bit per boolean instead of eight bits), but the price is a significant loss of performance.

As, except for the pair (3, 5), all twins pairs are composed of a number congruent to 2 modulo 3 and a number congruent to 1 modulo 3, we can save some time by using a step of 6. Unfortunately, this is the filling of the sieve which is the most time consuming, so the gain is not very important (on our computer, half a second on a total time of 8.3 s).

```import math, strformat, strutils

const N = 1_000_000_000

proc sieve(n: Positive): seq[bool] =
## Build and fill a sieve of Erathosthenes.
result.setLen(n + 1)  # Default to false which means prime.
result[0] = true
result[1] = true
for n in countup(3, sqrt(N.toFloat).int, 2):
if not result[n]:
for k in countup(n * n, N, 2 * n):
result[k] = true

let composite = sieve(N)

proc findTwins(composite: openArray[bool]) =
var
lim = 10
count = 1     # Start with 3, 5 which is a special case.
n = 7         # First prime congruent to 1 modulo 3.
while true:
if not composite[n] and not composite[n - 2]: inc count
inc n, 6      # Next odd number congruent to 1 modulo 3.
if n > lim:
echo &"There are {insertSep(\$count)} pairs of twin primes under {insertSep(\$lim)}."
lim *= 10
if lim > N: break

composite.findTwins()
```
Output:
```There are 2 pairs of twin primes under 10.
There are 8 pairs of twin primes under 100.
There are 35 pairs of twin primes under 1_000.
There are 205 pairs of twin primes under 10_000.
There are 1_224 pairs of twin primes under 100_000.
There are 8_169 pairs of twin primes under 1_000_000.
There are 58_980 pairs of twin primes under 10_000_000.
There are 440_312 pairs of twin primes under 100_000_000.
There are 3_424_506 pairs of twin primes under 1_000_000_000.```

## Perl

```use strict;
use warnings;

use Primesieve;

sub comma { reverse ((reverse shift) =~ s/(.{3})/\$1,/gr) =~ s/^,//r }

printf "Twin prime pairs less than %14s: %s\n", comma(10**\$_), comma count_twins(1, 10**\$_) for 1..10;
```
Output:
```Twin prime pairs less than             10: 2
Twin prime pairs less than            100: 8
Twin prime pairs less than          1,000: 35
Twin prime pairs less than         10,000: 205
Twin prime pairs less than        100,000: 1,224
Twin prime pairs less than      1,000,000: 8,169
Twin prime pairs less than     10,000,000: 58,980
Twin prime pairs less than    100,000,000: 440,312
Twin prime pairs less than  1,000,000,000: 3,424,506
Twin prime pairs less than 10,000,000,000: 27,412,679```

## Phix

Added both parameter to reflect the recent task specification changes, as shown for a limit of 6 you can count {3,5} and {5,7} as one pair (the default, matching task description) or two. Obviously delete the "6 --" if you actually want a prompt.

The time complexity here is all about building a table of primes. It turns out that using the builtin get_prime() is actually faster than using an explicit sieve (as per Delphi/Go/Wren) due to retaining all the intermediate 0s, not that I particularly expect this to win any performance trophies.

```with javascript_semantics
atom t0 = time()
function twin_primes(integer maxp, bool both=true)
integer n = 0,  -- result
pn = 2, -- next prime index
p,      -- a prime, <= maxp
prev_p = 2
while true do
p = get_prime(pn)
if both and p>=maxp then exit end if
n += (prev_p = p-2)
if (not both) and p>=maxp then exit end if
prev_p = p
pn += 1
end while
return n
end function
integer mp = 6 -- prompt_number("Enter limit:")
printf(1,"Twin prime pairs less than %,d: %,d\n",{mp,twin_primes(mp)})
printf(1,"Twin prime pairs less than %,d: %,d\n",{mp,twin_primes(mp,false)})
for p=1 to 9 do
integer p10 = power(10,p)
printf(1,"Twin prime pairs less than %,d: %,d\n",{p10,twin_primes(p10)})
end for
?elapsed(time()-t0)
```
Output:
```Twin prime pairs less than 6: 1
Twin prime pairs less than 6: 2
Twin prime pairs less than 10: 2
Twin prime pairs less than 100: 8
Twin prime pairs less than 1,000: 35
Twin prime pairs less than 10,000: 205
Twin prime pairs less than 100,000: 1,224
Twin prime pairs less than 1,000,000: 8,169
Twin prime pairs less than 10,000,000: 58,980
Twin prime pairs less than 100,000,000: 440,312
Twin prime pairs less than 1,000,000,000: 3,424,506
"16.2s"
```

### using primesieve

Windows 64-bit only, unless you can find/make a suitable dll/so.
Note that unlike the above this version of twin_primes() carries on from where it left off.

```requires(WINDOWS)
requires(64,true)
include builtins/primesieve.e
atom t0 = time()
integer n = 0, p = 2, prev_p = 2
function twin_primes(integer maxp, bool both=true)
while true do
if both and p>=maxp then exit end if
n += (prev_p = p-2)
if (not both) and p>=maxp then exit end if
prev_p = p
p = primesieve_next_prime()
end while
return n
end function
for i=1 to 11 do
integer p10 = power(10,i)
printf(1,"Twin prime pairs less than %,d: %,d\n",{p10,twin_primes(p10)})
end for
?elapsed(time()-t0)
```
Output:

Same as above, which it completes in 7.2s (and 1e10 in 1 min 6s), less the 6's and plus two more lines:

```Twin prime pairs less than 10: 2
Twin prime pairs less than 100: 8
Twin prime pairs less than 1,000: 35
Twin prime pairs less than 10,000: 205
Twin prime pairs less than 100,000: 1,224
Twin prime pairs less than 1,000,000: 8,169
Twin prime pairs less than 10,000,000: 58,980
Twin prime pairs less than 100,000,000: 440,312
Twin prime pairs less than 1,000,000,000: 3,424,506
Twin prime pairs less than 10,000,000,000: 27,412,679
Twin prime pairs less than 100,000,000,000: 224,376,048
"10 minutes and 25s"
```

## PureBasic

Translation of: FreeBASIC
```Procedure isPrime(v.i)
If     v <= 1    : ProcedureReturn #False
ElseIf v < 4     : ProcedureReturn #True
ElseIf v % 2 = 0 : ProcedureReturn #False
ElseIf v < 9     : ProcedureReturn #True
ElseIf v % 3 = 0 : ProcedureReturn #False
Else
Protected r = Round(Sqr(v), #PB_Round_Down)
Protected f = 5
While f <= r
If v % f = 0 Or v % (f + 2) = 0
ProcedureReturn #False
EndIf
f + 6
Wend
EndIf
ProcedureReturn #True
EndProcedure

Procedure paresDePrimos(limite.d)
p1.i = 0
p2.i = 1
p3.i = 1
count.i = 0
For i.i = 5 To limite
p3 = p2
p2 = p1
p1 = isPrime(i)
If p3 And p1
count + 1
EndIf
Next i
ProcedureReturn count
EndProcedure

OpenConsole()
n.i = 1
For i.i = 1 To 6
n = n * 10
PrintN("pares de primos gemelos por debajo de < " + Str(n) + " : " + Str(paresDePrimos(n)))
Next i
PrintN(#CRLF\$ + "--- terminado, pulsa RETURN---"): Input()
CloseConsole()
End
```
Output:
```Similar a la entrada de FreeBASIC.
```

## Python

```primes = [2, 3, 5, 7, 11, 13, 17, 19]

def count_twin_primes(limit: int) -> int:
global primes
if limit > primes[-1]:
ram_limit = primes[-1] + 90000000 - len(primes)
reasonable_limit = min(limit, primes[-1] ** 2, ram_limit) - 1

while reasonable_limit < limit:
ram_limit = primes[-1] + 90000000 - len(primes)
if ram_limit > primes[-1]:
reasonable_limit = min(limit, primes[-1] ** 2, ram_limit)
else:
reasonable_limit = min(limit, primes[-1] ** 2)

sieve = list({x for prime in primes for x in
range(primes[-1] + prime - (primes[-1] % prime), reasonable_limit, prime)})
primes += [x - 1 for i, x in enumerate(sieve) if i and x - 1 != sieve[i - 1] and x - 1 < limit]

count = len([(x, y) for (x, y) in zip(primes, primes[1:]) if x + 2 == y])

return count

def test(limit: int):
count = count_twin_primes(limit)
print(f"Number of twin prime pairs less than {limit} is {count}\n")

test(10)
test(100)
test(1000)
test(10000)
test(100000)
test(1000000)
test(10000000)
test(100000000)
```
Output:
```Number of twin prime pairs less than 10 is 2
Number of twin prime pairs less than 100 is 8
Number of twin prime pairs less than 1000 is 35
Number of twin prime pairs less than 10000 is 205
Number of twin prime pairs less than 100000 is 1224
Number of twin prime pairs less than 1000000 is 8169
Number of twin prime pairs less than 10000000 is 58980
Number of twin prime pairs less than 100000000 is 440312
```

## Quackery

`primes` and `eratosthenes` are defined at Sieve of Eratosthenes#Quackery.

```  [ dup dip
[ eratosthenes
0 1 primes share ]
bit 1 - & 1 >>
[ dup while
dup 5 & 5 = if
[ rot 1+ unrot ]
2 >>
dip [ 2 + ]
again ]
2drop ]                is twinprimes ( n --> [ )

5 times
[ say "Number of twin primes below "
10 i^ 1+ ** dup echo
say " is "
twinprimes echo say "." cr ]```
Output:
```Number of twin primes below 10 is 2.
Number of twin primes below 100 is 8.
Number of twin primes below 1000 is 35.
Number of twin primes below 10000 is 205.
Number of twin primes below 100000 is 1224.
```

## Raku

Works with: Rakudo version 2020.07
```use Lingua::EN::Numbers;

use Math::Primesieve;

my \$p = Math::Primesieve.new;

printf "Twin prime pairs less than %17s: %s\n", comma(10**\$_), comma \$p.count(10**\$_, :twins) for 1 .. 12;
say (now - INIT now).round(.01) ~ ' seconds';
```
Output:
```Twin prime pairs less than                10: 2
Twin prime pairs less than               100: 8
Twin prime pairs less than             1,000: 35
Twin prime pairs less than            10,000: 205
Twin prime pairs less than           100,000: 1,224
Twin prime pairs less than         1,000,000: 8,169
Twin prime pairs less than        10,000,000: 58,980
Twin prime pairs less than       100,000,000: 440,312
Twin prime pairs less than     1,000,000,000: 3,424,506
Twin prime pairs less than    10,000,000,000: 27,412,679
Twin prime pairs less than   100,000,000,000: 224,376,048
Twin prime pairs less than 1,000,000,000,000: 1,870,585,220
6.89 seconds```

## REXX

### straight-forward prime generator

The   genP   function could be optimized for higher specifications of the limit(s).

```/*REXX pgm counts the number of twin prime pairs under a specified number N (or a list).*/
parse arg \$ .                                    /*get optional number of primes to find*/
if \$='' | \$=","  then \$= 10 100 1000 10000 100000 1000000 10000000  /*No \$? Use default.*/
w= length( commas( word(\$, words(\$) ) ) )        /*get length of the last number in list*/
@found= ' twin prime pairs found under '         /*literal used in the showing of output*/

do i=1  for words(\$);       x= word(\$, i) /*process each N─limit in the  \$  list.*/
say right( commas(genP(x)), 20)     @found     right(commas(x), max(length(x), w) )
end   /*i*/
exit 0                                           /*stick a fork in it,  we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
commas:  parse arg _;  do ?=length(_)-3  to 1  by -3; _=insert(',', _, ?); end;   return _
/*──────────────────────────────────────────────────────────────────────────────────────*/
genP: parse arg y; @.1=2;  @.2=3;  @.3=5;  @.4=7;  @.5=11;  @.6=13; #= 6; tp= 2; sq.6= 169
if y>10  then tp= tp+1
do j=@.#+2  by 2  for max(0, y%2-@.#%2-1)      /*find odd primes from here on. */
parse var  j   ''  -1  _                /*obtain the last digit of the  J  var.*/
if    _==5  then iterate;  if j// 3==0  then iterate  /*J ÷ by 5?   J ÷ by  3? */
if j//7==0  then iterate;  if j//11==0  then iterate  /*" "  " 7?   " "  " 11? */
/* [↓]  divide by the primes.   ___    */
do k=6  to #  while  sq.k<=j      /*divide  J  by other primes ≤ √ J     */
if j//@.k == 0  then iterate j    /*÷ by prev. prime?  ¬prime     ___    */
end   /*k*/                       /* [↑]   only divide up to     √ J     */
prev= @.#;  #= #+1;  sq.#= j*j;  @.#= j /*save prev. P; bump # primes; assign P*/
if j-2==prev   then tp= tp + 1          /*This & previous prime twins? Bump TP.*/
end         /*j*/;            return tp
```
output   when using the default inputs:
```                   2  twin prime pairs found under          10
8  twin prime pairs found under         100
35  twin prime pairs found under       1,000
205  twin prime pairs found under      10,000
1,224  twin prime pairs found under     100,000
8,169  twin prime pairs found under   1,000,000
58,980  twin prime pairs found under  10,000,000
```

### optimized prime number generator

This REXX version has some optimization for prime generation.

This version won't return a correct value (for the number of twin pairs) for a limit < 73   (because of the manner in
which low primes are generated from a list),   however,   the primes are returned from the function.

```/*REXX pgm counts the number of twin prime pairs under a specified number N (or a list).*/
parse arg \$ .                                    /*get optional number of primes to find*/
if \$='' | \$=","  then \$= 100 1000 10000 100000 1000000 10000000    /*No \$?  Use default.*/
w= length( commas( word(\$, words(\$) ) ) )        /*get length of the last number in list*/
@found= ' twin prime pairs found under '         /*literal used in the showing of output*/

do i=1  for words(\$);       x= word(\$, i) /*process each N─limit in the  \$  list.*/
say right( commas(genP(x)), 20)     @found     right(commas(x), max(length(x), w) )
end   /*i*/
exit 0                                           /*stick a fork in it,  we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
commas:  parse arg _;  do ?=length(_)-3  to 1  by -3; _=insert(',', _, ?); end;   return _
/*──────────────────────────────────────────────────────────────────────────────────────*/
genP: arg y; _= 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97 101
tp=8;          #= words(_); sq.103=103*103 /*#: number of prims; TP: # twin pairs.*/
do aa=1  for #;  @.aa= word(_, aa)       /*assign some low primes for quick ÷'s.*/
end   /*aa*/

do j=@.#+2  by 2  while j<y              /*continue with the next prime past 101*/
parse var  j  ''  -1  _                  /*obtain the last digit of the  J  var.*/
if _    ==5       then iterate           /*is this integer a multiple of five?  */
if j//3 ==0       then iterate           /* "   "     "    "     "     " three? */

do a=4  for 23                        /*divide low primes starting with seven*/
if j//@.a ==0  then iterate j         /*is integer a multiple of a low prime?*/
end           /*a*/
/* [↓]  divide by the primes.   ___    */
do k=27  to #  while sq.k<= j /*divide  J  by other primes ≤ √ J     */
if j//@.k ==0  then iterate j /*÷ by prev. prime?  ¬prime     ___    */
end   /*k*/                   /* [↑]   only divide up to     √ J     */
prev= @.#;  #= #+1;  sq.#= j*j;   @.#= j /*save prev. P; bump # primes; assign P*/
if j-2==prev  then tp= tp + 1            /*This & previous prime twins? Bump TP.*/
end              /*j*/;        return tp
```

## Ring

```load "stdlib.ring"

limit = list(7)
for n = 1 to 7
limit[n] = pow(10,n)
next

TwinPrimes = []

for n = 1 to limit[7]-2
bool1 = isprime(n)
bool2 = isprime(n+2)
bool = bool1 and bool2
if bool =1
add(TwinPrimes,[n,n+2])
ok
next

numTwin = list(7)
len = len(TwinPrimes)

for n = 1 to len
for p = 1 to 6
if TwinPrimes[n][2] < pow(10,p) and TwinPrimes[n+1][1] > pow(10,p)-2
numTwin[p] = n
ok
next
next

numTwin[7] = len

for n = 1 to 7
see "Maximum: " + pow(10,n) + nl
see "twin prime pairs below " + pow(10,n) + ": " + numTwin[n] + nl + nl
next```

Output:

```Maximum: 10
twin prime pairs below 10: 2

Maximum: 100
twin prime pairs below 100: 8

Maximum: 1000
twin prime pairs below 1000: 35

Maximum: 10000
twin prime pairs below 10000: 205

Maximum: 100000
twin prime pairs below  100000: 1224

Maximum: 1000000
twin prime pairs below  1000000: 8169

Maximum: 10000000
twin prime pairs below  10000000: 58980
```

## RPL

Works with: HP version 49
```« → m
« 0 2
WHILE DUP m < REPEAT
DUP NEXTPRIME DUP ROT - 2 == ROT + SWAP
END DROP
» » ‘TWINP’ STO
```
```100000 TWINP
```
Output:
```1: 1224
```

## Ruby

```require 'prime'

(1..8).each do |n|
count = Prime.each(10**n).each_cons(2).count{|p1, p2| p2-p1 == 2}
puts "Twin primes below 10**#{n}: #{count}"
end
```
Output:
```Twin primes below 10**1: 2
Twin primes below 10**2: 8
Twin primes below 10**3: 35
Twin primes below 10**4: 205
Twin primes below 10**5: 1224
Twin primes below 10**6: 8169
Twin primes below 10**7: 58980
Twin primes below 10**8: 440312
```

## Rust

Limits can be specified on the command line, otherwise the twin prime counts for powers of ten from 1 to 10 are shown.

```// [dependencies]
// primal = "0.3"
// num-format = "0.4"

use num_format::{Locale, ToFormattedString};

fn twin_prime_count_for_powers_of_ten(max_power: u32) {
let mut count = 0;
let mut previous = 0;
let mut power = 1;
let mut limit = 10;
for prime in primal::Primes::all() {
if prime > limit {
println!(
"Number of twin prime pairs less than {} is {}",
limit.to_formatted_string(&Locale::en),
count.to_formatted_string(&Locale::en)
);
limit *= 10;
power += 1;
if power > max_power {
break;
}
}
if previous > 0 && prime == previous + 2 {
count += 1;
}
previous = prime;
}
}

fn twin_prime_count(limit: usize) {
let mut count = 0;
let mut previous = 0;
for prime in primal::Primes::all().take_while(|x| *x < limit) {
if previous > 0 && prime == previous + 2 {
count += 1;
}
previous = prime;
}
println!(
"Number of twin prime pairs less than {} is {}",
limit.to_formatted_string(&Locale::en),
count.to_formatted_string(&Locale::en)
);
}

fn main() {
let args: Vec<String> = std::env::args().collect();
if args.len() > 1 {
for i in 1..args.len() {
if let Ok(limit) = args[i].parse::<usize>() {
twin_prime_count(limit);
} else {
eprintln!("Cannot parse limit from string {}", args[i]);
}
}
} else {
twin_prime_count_for_powers_of_ten(10);
}
}
```
Output:
```Number of twin prime pairs less than 10 is 2
Number of twin prime pairs less than 100 is 8
Number of twin prime pairs less than 1,000 is 35
Number of twin prime pairs less than 10,000 is 205
Number of twin prime pairs less than 100,000 is 1,224
Number of twin prime pairs less than 1,000,000 is 8,169
Number of twin prime pairs less than 10,000,000 is 58,980
Number of twin prime pairs less than 100,000,000 is 440,312
Number of twin prime pairs less than 1,000,000,000 is 3,424,506
Number of twin prime pairs less than 10,000,000,000 is 27,412,679
```

## Sidef

```func twin_primes_count(upto) {
var count = 0
var p1 = 2
each_prime(3, upto, {|p2|
if (p2 - p1 == 2) {
++count
}
p1 = p2
})
return count
}

for n in (1..9) {
var count = twin_primes_count(10**n)
say "There are #{count} twin primes <= 10^#{n}"
}
```
Output:
```There are 2 twin primes <= 10^1
There are 8 twin primes <= 10^2
There are 35 twin primes <= 10^3
There are 205 twin primes <= 10^4
There are 1224 twin primes <= 10^5
There are 8169 twin primes <= 10^6
There are 58980 twin primes <= 10^7
There are 440312 twin primes <= 10^8
There are 3424506 twin primes <= 10^9
```

## Visual Basic

Works with: Visual Basic version 4
Works with: Visual Basic version 5
Works with: Visual Basic version 6
```Function IsPrime(x As Long) As Boolean
Dim i As Long
If x Mod 2 = 0 Then
Exit Function
Else
For i = 3 To Int(Sqr(x)) Step 2
If x Mod i = 0 Then Exit Function
Next i
End If
IsPrime = True
End Function

Function TwinPrimePairs(max As Long) As Long
Dim p1 As Boolean, p2 As Boolean, count As Long, i As Long
p2 = True
For i = 5 To max Step 2
p1 = p2
p2 = IsPrime(i)
If p1 And p2 Then count = count + 1
Next i
TwinPrimePairs = count
End Function

Sub Test(x As Long)
Debug.Print "Twin prime pairs below" + Str(x) + ":" + Str(TwinPrimePairs(x))
End Sub

Sub Main()
Test 10
Test 100
Test 1000
Test 10000
Test 100000
Test 1000000
Test 10000000
End Sub
```
Output:
```Twin prime pairs below 10: 2
Twin prime pairs below 100: 8
Twin prime pairs below 1000: 35
Twin prime pairs below 10000: 205
Twin prime pairs below 100000: 1224
Twin prime pairs below 1000000: 8169
Twin prime pairs below 10000000: 58980```

## Wren

Library: Wren-math
Library: Wren-fmt
```import "./math" for Int
import "./fmt" for Fmt

var c = Int.primeSieve(1e8-1, false)
var limit = 10
var start = 3
var twins = 0
for (i in 1..8) {
var j = start
while (j < limit) {
if (!c[j] && !c[j-2]) twins = twins + 1
j = j + 2
}
Fmt.print("Under \$,11d there are \$,7d pairs of twin primes.", limit, twins)
start = limit + 1
limit = limit * 10
}
```
Output:
```Under         100 there are       8 pairs of twin primes.
Under       1,000 there are      35 pairs of twin primes.
Under      10,000 there are     205 pairs of twin primes.
Under     100,000 there are   1,224 pairs of twin primes.
Under   1,000,000 there are   8,169 pairs of twin primes.
Under  10,000,000 there are  58,980 pairs of twin primes.
Under 100,000,000 there are 440,312 pairs of twin primes.
```

## XPL0

100 million takes 150 seconds on Pi4.

```func IsPrime(N);        \Return 'true' if N is prime
int  N, I;
[if N <= 2 then return N = 2;
if (N&1) = 0 then \even >2\ return false;
for I:= 3 to sqrt(N) do
[if rem(N/I) = 0 then return false;
I:= I+1;
];
return true;
];

func Twins(Limit);
int  Limit, C, N;
[C:= 0;  N:= 3;
repeat  if IsPrime(N) then
loop [N:= N+2;
if N >= Limit then return C;
if not IsPrime(N) then quit;
C:= C+1;
];
N:= N+2;
until   N >= Limit;
return C;
];

[IntOut(0, Twins(100_000));  CrLf(0);
IntOut(0, Twins(10_000_000));  CrLf(0);
IntOut(0, Twins(100_000_000));  CrLf(0);
]```
Output:
```1224
58980
440312
```

## Yabasic

Translation of: FreeBASIC
```sub isPrime(v)
if v < 2 then return False : fi
if mod(v, 2) = 0 then return v = 2 : fi
if mod(v, 3) = 0 then return v = 3 : fi
d = 5
while d * d <= v
if mod(v, d) = 0 then return False else d = d + 2 : fi
wend
return True
end sub

sub paresDePrimos(limite)
p1 = 0 : p2 = 1 : p3 = 1 : count = 0
for i = 5 to limite
p3 = p2
p2 = p1
p1 = isPrime(i)
if (p3 and p1) then count = count + 1 : fi
next i
return count
end sub

n = 1
for i = 1 to 6
n = n * 10
print "pares de primos gemelos por debajo de < ", n, " : ", paresDePrimos(n)
next i
end```
Output:
```Igual que la entrada de FreeBASIC.
```