Long primes

From Rosetta Code
Task
Long primes
You are encouraged to solve this task according to the task description, using any language you may know.


A   long prime   (the definition that will be used here)   are primes whose reciprocals   (in decimal)   have a   period length   of one less than the prime number   (also expressed in decimal).


Long primes   are also known as:

  •   base ten cyclic numbers
  •   full reptend primes
  •   golden primes
  •   long period primes
  •   maximal period primes
  •   proper primes


Example

7   is the first long prime,   the reciprocal of seven is   1/7,   which is equal to the repeating decimal fraction   0.142857142857···

The length of the   repeating   part of the decimal fraction is six,   (the underlined part)   which is one less than the (decimal) prime number   7.
Thus   7   is a long prime.


There are other (more) general definitions of a   long prime   which include wording/verbiage for other bases other than ten.


Task
  •   Show all long primes up to   500   (preferably on one line).
  •   Show the   number   of long primes up to       500
  •   Show the   number   of long primes up to     1,000
  •   Show the   number   of long primes up to     2,000
  •   Show the   number   of long primes up to     4,000
  •   Show the   number   of long primes up to     8,000
  •   Show the   number   of long primes up to   16,000
  •   Show the   number   of long primes up to   32,000
  •   Show the   number   of long primes up to   64,000   (optional)
  •   Show all output here.


Also see



C[edit]

Translation of: Go
#include <stdio.h>
#include <stdlib.h>
 
#define TRUE 1
#define FALSE 0
 
typedef int bool;
 
void sieve(int limit, int primes[], int *count) {
bool *c = calloc(limit + 1, sizeof(bool)); // composite = TRUE
// no need to process even numbers
int i, p = 3, p2, n = 0;
while (TRUE) {
p2 = p * p;
if (p2 > limit) break;
for (i = p2; i <= limit; i += 2 * p) c[i] = TRUE;
while (TRUE) {
p += 2;
if (!c[p]) break;
}
}
for (i = 3; i <= limit; i += 2) {
if (!c[i]) primes[n++] = i;
}
*count = n;
free(c);
}
 
// finds the period of the reciprocal of n
int findPeriod(int n) {
int i, r = 1, rr, period = 0;
for (i = 1; i <= n + 1; ++i) {
r = (10 * r) % n;
}
rr = r;
while (TRUE) {
r = (10 * r) % n;
period++;
if (r == rr) break;
}
return period;
}
 
int main() {
int i, prime, count = 0, index = 0, primeCount, longCount = 0;
int *primes, *longPrimes;
int numbers[] = {500, 1000, 2000, 4000, 8000, 16000, 32000, 64000};
int totals[8];
primes = calloc(6500, sizeof(int));
longPrimes = calloc(2500, sizeof(int));
sieve(64000, primes, &primeCount);
for (i = 0; i < primeCount; ++i) {
prime = primes[i];
if (findPeriod(prime) == prime - 1) {
longPrimes[longCount++] = prime;
}
}
for (i = 0; i < longCount; ++i) {
if (longPrimes[i] > numbers[index]) {
totals[index++] = count;
}
count++;
}
totals[7] = count;
printf("The long primes up to 500 are:\n");
printf("[");
for (i = 0; i < totals[0]; ++i) {
printf("%d ", longPrimes[i]);
}
printf("\b]\n");
 
printf("\nThe number of long primes up to:\n");
for (i = 0; i < 8; ++i) {
printf("  %5d is %d\n", numbers[i], totals[i]);
}
free(longPrimes);
free(primes);
return 0;
}
Output:
The long primes up to 500 are:
[7 17 19 23 29 47 59 61 97 109 113 131 149 167 179 181 193 223 229 233 257 263 269 313 337 367 379 383 389 419 433 461 487 491 499]

The number of long primes up to:
    500 is 35
   1000 is 60
   2000 is 116
   4000 is 218
   8000 is 390
  16000 is 716
  32000 is 1300
  64000 is 2430

Go[edit]

package main
 
import "fmt"
 
func sieve(limit int) []int {
var primes []int
c := make([]bool, limit+1) // composite = true
// no need to process even numbers
p := 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
}
}
}
for i := 3; i <= limit; i += 2 {
if !c[i] {
primes = append(primes, i)
}
}
return primes
}
 
// finds the period of the reciprocal of n
func findPeriod(n int) int {
r := 1
for i := 1; i <= n+1; i++ {
r = (10 * r) % n
}
rr := r
period := 0
for {
r = (10 * r) % n
period++
if r == rr {
break
}
}
return period
}
 
func main() {
primes := sieve(64000)
var longPrimes []int
for _, prime := range primes {
if findPeriod(prime) == prime-1 {
longPrimes = append(longPrimes, prime)
}
}
numbers := []int{500, 1000, 2000, 4000, 8000, 16000, 32000, 64000}
index := 0
count := 0
totals := make([]int, len(numbers))
for _, longPrime := range longPrimes {
if longPrime > numbers[index] {
totals[index] = count
index++
}
count++
}
totals[len(numbers)-1] = count
fmt.Println("The long primes up to 500 are: ")
fmt.Println(longPrimes[:totals[0]])
 
fmt.Println("\nThe number of long primes up to: ")
for i, total := range totals {
fmt.Printf("  %5d is %d\n", numbers[i], total)
}
}
Output:
The long primes up to 500 are: 
[7 17 19 23 29 47 59 61 97 109 113 131 149 167 179 181 193 223 229 233 257 263 269 313 337 367 379 383 389 419 433 461 487 491 499]

The number of long primes up to: 
    500 is 35
   1000 is 60
   2000 is 116
   4000 is 218
   8000 is 390
  16000 is 716
  32000 is 1300
  64000 is 2430

J[edit]

   NB. define the verb long
   NB. long is true iff the prime input greater than 2
   NB. is a rosettacode long prime.
   NB. 0 is false, 1 is true.

   long =: ( <:@:[ = #@[email protected]( [: }. ( | 10&* )^:( <@[ ) ) )&1&>
   

   NB. demonstration of the long verb
   NB. long applied to integers 3 through 9 inclusively

   (,: long) 3 4 5 6 7 8 9
3 4 5 6 7 8 9
0 0 0 0 1 0 0

 
   NB. find the number of primes through 64000

  [ N =: p:^:_1 ] 64000
6413

 
   NB. copy the long primes, excluding 2, the first.

   LONG_PRIMES =: (#~ long) p: >: i. N


   NB. those less than 500

   ( #~ <&500) LONG_PRIMES
7 17 19 23 29 47 59 61 97 109 113 131 149 167 179 181 193 223 229 233 257 263 269 313 337 367 379 383 389 419 433 461 487 491 499


   NB. counts

   [ MEASURE =: 500 * 2 ^ i. 8
500 1000 2000 4000 8000 16000 32000 64000


   LONG_PRIMES ( ] ,: [: +/ </ ) MEASURE
500 1000 2000 4000 8000 16000 32000 64000
35   60  116  218  390   716  1300  2430

Kotlin[edit]

Translation of: Go
// Version 1.2.60
 
fun sieve(limit: Int): List<Int> {
val primes = mutableListOf<Int>()
val c = BooleanArray(limit + 1) // composite = true
// no need to process even numbers
var p = 3
while (true) {
val p2 = p * p
if (p2 > limit) break
for (i in p2..limit step 2 * p) c[i] = true
while (true) {
p += 2
if (!c[p]) break
}
}
for (i in 3..limit step 2) {
if (!c[i]) primes.add(i)
}
return primes
}
 
// finds the period of the reciprocal of n
fun findPeriod(n: Int): Int {
var r = 1
for (i in 1..n + 1) r = (10 * r) % n
val rr = r
var period = 0
while (true) {
r = (10 * r) % n
period++
if (r == rr) break
}
return period
}
 
fun main(args: Array<String>) {
val primes = sieve(64000)
val longPrimes = mutableListOf<Int>()
for (prime in primes) {
if (findPeriod(prime) == prime - 1) {
longPrimes.add(prime)
}
}
val numbers = listOf(500, 1000, 2000, 4000, 8000, 16000, 32000, 64000)
var index = 0
var count = 0
val totals = IntArray(numbers.size)
for (longPrime in longPrimes) {
if (longPrime > numbers[index]) {
totals[index++] = count
}
count++
}
totals[numbers.lastIndex] = count
println("The long primes up to 500 are:")
println(longPrimes.take(totals[0]))
 
println("\nThe number of long primes up to:")
for ((i, total) in totals.withIndex()) {
System.out.printf("  %5d is %d\n", numbers[i], total)
}
}
Output:
The long primes up to 500 are:
[7, 17, 19, 23, 29, 47, 59, 61, 97, 109, 113, 131, 149, 167, 179, 181, 193, 223, 229, 233, 257, 263, 269, 313, 337, 367, 379, 383, 389, 419, 433, 461, 487, 491, 499]

The number of long primes up to:
    500 is 35
   1000 is 60
   2000 is 116
   4000 is 218
   8000 is 390
  16000 is 716
  32000 is 1300
  64000 is 2430

Pascal[edit]

first post.old program modified. Using Euler Phi

  www . arndt-bruenner.de/mathe/scripts/periodenlaenge.htm
 
PROGRAM Periode;
 
{$IFDEF FPC}
{$MODE Delphi}
 
{$OPTIMIZATION ON}
{$OPTIMIZATION Regvar}
{$OPTIMIZATION Peephole}
{$OPTIMIZATION cse}
{$OPTIMIZATION asmcse}
{$else}
{$Apptype Console}
{$ENDIF}
 
uses
sysutils;
 
const
cBASIS = 10;
PRIMFELDOBERGRENZE = 6542;
{Das sind alle Primzahlen bis 2^16}
{Das reicht fuer al8le Primzahlen bis 2^32}
TESTZAHL = 500;//429496709;//High(Dword) DIV cBasis;
type
tPrimFeld = array [1..PRIMFELDOBERGRENZE] of Word;
tFaktorPotenz = record
Faktor,
Potenz : DWord;
end;
//2*3*5*7*11*13*17*19*23 *29 > DWord also maximal 9 Faktoren
tFaktorFeld = array [1..9] of TFaktorPotenz;//DWord
// tFaktorFeld = array [1..15] of TFaktorPotenz;//QWord
tFaktorisieren = class(TObject)
private
fFakZahl : DWord;
fFakBasis : DWord;
fFakAnzahl : Dword;
fAnzahlMoeglicherTeiler : Dword;
fEulerPhi : DWORD;
fStartPeriode : DWORD;
fPeriodenLaenge : DWORD;
fTeiler : array of DWord;
fFaktoren : tFaktorFeld;
fBasFakt : tFaktorFeld;
fPrimfeld : tPrimFeld;
 
procedure PrimFeldAufbauen;
procedure Fakteinfuegen(var Zahl:Dword;inFak:Dword);
function BasisPeriodeExtrahieren(var inZahl:Dword):DWord;
procedure NachkommaPeriode(var OutText: String);
public
constructor create; overload;
function Prim(inZahl:Dword):Boolean;
procedure AusgabeFaktorfeld(n : DWord);
procedure Faktorisierung (inZahl: DWord);
procedure TeilerErmitteln;
procedure PeriodeErmitteln(inZahl:Dword);
function BasExpMod( b, e, m : Dword) : DWord;
 
property
EulerPhi : Dword read fEulerPhi;
property
PeriodenLaenge: DWord read fPeriodenLaenge ;
property
StartPeriode: DWord read fStartPeriode ;
end;
 
constructor tFaktorisieren.create;
begin
inherited;
PrimFeldAufbauen;
 
fFakZahl := 0;
fFakBasis := cBASIS;
Faktorisierung(fFakBasis);
fBasFakt := fFaktoren;
 
fFakZahl := 0;
fEulerPhi := 1;
fPeriodenLaenge :=0;
fFakZahl := 0;
fFakAnzahl := 0;
fAnzahlMoeglicherTeiler := 0;
end;
 
function tFaktorisieren.Prim(inZahl:Dword):Boolean;
{Testet auf PrimZahl}
var
Wurzel,
pos : Dword;
Begin
if fFakZahl = inZahl then
begin
result := (fAnzahlMoeglicherTeiler = 2);
exit;
end;
result := false;
if inZahl >1 then
begin
result := true;
Pos := 1;
Wurzel:= trunc(sqrt(inZahl));
While fPrimFeld[Pos] <= Wurzel do
begin
if (inZahl mod fPrimFeld[Pos])=0 then
begin
result := false;
break;
end;
inc(Pos);
IF Pos > High(fPrimFeld) then
break;
end;
end;
end;
 
Procedure tFaktorisieren.PrimFeldAufbauen;
{Baut die Liste der Primzahlen bis Obergrenze auf}
const
MAX = 65536;
var
TestaufPrim,
Zaehler,delta : Dword;
 
begin
Zaehler := 1;
fPrimFeld[Zaehler] := 2;
inc(Zaehler);
fPrimFeld[Zaehler] := 3;
 
delta := 2;
TestAufPrim:=5;
repeat
if prim(TestAufPrim) then
begin
inc(Zaehler);
fPrimFeld[Zaehler] := TestAufPrim;
end;
inc(TestAufPrim,delta);
delta := 6-delta; // 2,4,2,4,2,4,2,
until (TestAufPrim>=MAX);
 
end; {PrimfeldAufbauen}
 
 
procedure tFaktorisieren.Fakteinfuegen(var Zahl:Dword;inFak:Dword);
var
i : DWord;
begin
inc(fFakAnzahl);
with fFaktoren[fFakAnzahl] do
begin
fEulerPhi := fEulerPhi*(inFak-1);
Faktor :=inFak;
Potenz := 0;
while (Zahl mod inFak) = 0 do
begin
Zahl := Zahl div inFak;
inc(Potenz);
end;
For i := 2 to Potenz do
fEulerPhi := fEulerPhi*inFak;
end;
fAnzahlMoeglicherTeiler:=fAnzahlMoeglicherTeiler*(1+fFaktoren[fFakAnzahl].Potenz);
end;
 
procedure tFaktorisieren.Faktorisierung (inZahl: DWord);
var
j,
og : longint;
begin
if fFakZahl = inZahl then
exit;
 
fPeriodenLaenge := 0;
fFakZahl := inZahl;
fEulerPhi := 1;
fFakAnzahl := 0;
fAnzahlMoeglicherTeiler :=1;
setlength(fTeiler,0);
 
If inZahl < 2 then
exit;
og := round(sqrt(inZahl)+1.0);
{Suche Teiler von inZahl}
for j := 1 to High(fPrimfeld) do
begin
If fPrimfeld[j]> OG then
Break;
if (inZahl mod fPrimfeld[j])= 0 then
Fakteinfuegen(inZahl,fPrimfeld[j]);
end;
If inZahl>1 then
Fakteinfuegen(inZahl,inZahl);
TeilerErmitteln;
end; {Faktorisierung}
 
procedure tFaktorisieren.AusgabeFaktorfeld(n : DWord);
var
i :integer;
begin
if fFakZahl <> n then
Faktorisierung(n);
write(fAnzahlMoeglicherTeiler:5,' Faktoren ');
 
For i := 1 to fFakAnzahl-1 do
with fFaktoren[i] do
IF potenz >1 then
write(Faktor,'^',Potenz,'*')
else
write(Faktor,'*');
with fFaktoren[fFakAnzahl] do
IF potenz >1 then
write(Faktor,'^',Potenz)
else
write(Faktor);
 
writeln(' Euler Phi: ',fEulerPhi:12,PeriodenLaenge:12);
end;
 
procedure tFaktorisieren.TeilerErmitteln;
var
Position : DWord;
i,j : DWord;
procedure FaktorAufbauen(Faktor: DWord;n: DWord);
var
i,
Pot : DWord;
begin
Pot := 1;
i := 0;
repeat
IF n > Low(fFaktoren) then
FaktorAufbauen(Pot*Faktor,n-1)
else
begin
FTeiler[Position] := Pot*Faktor;
inc(Position);
end;
Pot := Pot*fFaktoren[n].Faktor;
inc(i);
until i > fFaktoren[n].Potenz;
end;
 
begin
Position:= 0;
setlength(FTeiler,fAnzahlMoeglicherTeiler);
FaktorAufbauen(1,fFakAnzahl);
//Sortieren
For i := Low(fTeiler) to fAnzahlMoeglicherTeiler-2 do
begin
j := i;
while (j>=Low(fTeiler)) AND (fTeiler[j]>fTeiler[j+1]) do
begin
Position := fTeiler[j];
fTeiler[j] := fTeiler[j+1];
fTeiler[j+1]:= Position;
dec(j);
end;
end;
end;
 
function tFaktorisieren.BasisPeriodeExtrahieren(var inZahl:Dword):DWord;
var
i,cnt,
Teiler: Dword;
begin
cnt := 0;
result := 0;
For i := Low(fBasFakt) to High(fBasFakt) do
begin
with fBasFakt[i] do
begin
IF Faktor = 0 then
BREAK;
Teiler := Faktor;
For cnt := 2 to Potenz do
Teiler := Teiler*Faktor;
end;
cnt := 0;
while (inZahl<> 0) AND (inZahl mod Teiler = 0) do
begin
inZahl := inZahl div Teiler;
inc(cnt);
end;
IF cnt > result then
result := cnt;
end;
end;
 
procedure tFaktorisieren.PeriodeErmitteln(inZahl:Dword);
var
i,
TempZahl,
TempPhi,
TempPer,
TempBasPer: DWord;
begin
Faktorisierung(inZahl);
TempZahl := inZahl;
//Die Basis_Nicht_Periode ermitteln
TempBasPer := BasisPeriodeExtrahieren(TempZahl);
TempPer := 0;
IF TempZahl >1 then
begin
Faktorisierung(TempZahl);
TempPhi := fEulerPhi;
IF (TempPhi > 1) then
begin
Faktorisierung(TempPhi);
i := 0;
repeat
TempPer := fTeiler[i];
IF BasExpMod(fFakBasis,TempPer,TempZahl)= 1 then
Break;
inc(i);
until i >= Length(fTeiler);
IF i >= Length(fTeiler) then
TempPer := inZahl-1;
end;
end;
 
Faktorisierung(inZahl);
fPeriodenlaenge := TempPer;
fStartPeriode := TempBasPer;
end;
 
procedure tFaktorisieren.NachkommaPeriode(var OutText: String);
var
i,
limit : integer;
 
Rest,
Rest1,
Divi,
basis: DWord;
pText : pChar;
 
procedure Ziffernfolge(Ende: longint);
var
j : longint;
begin
j := i-Ende;
 
while j < 0 do
begin
Rest := Rest *Basis;
Rest1:= Rest Div Divi;
Rest := Rest-Rest1*Divi;//== Rest1 Mod Divi
 
pText^ := chr(Rest1+Ord('0'));
inc(pText);
 
inc(j);
end;
 
i := Ende;
end;
 
begin
limit:= fStartPeriode+fPeriodenlaenge;
 
setlength(OutText,limit+2+2+5);
OutText[1]:='0';
OutText[2]:='.';
pText := @OutText[3];
 
Rest := 1;
Divi := fFakZahl;
Basis := fFakBasis;
 
i := 0;
Ziffernfolge(fStartPeriode);
if fPeriodenlaenge = 0 then
begin
setlength(OutText,fStartPeriode+2);
EXIT;
end;
 
pText^ := '_'; inc(pText);
Ziffernfolge(limit);
pText^ := '_'; inc(pText);
 
Ziffernfolge(limit+5);
end;
 
type
tZahl = integer;
tRestFeld = array[0..31] of integer;
 
VAR
F : tFaktorisieren;
 
function tFaktorisieren.BasExpMod( b, e, m : Dword) : DWord;
begin
Result := 1;
IF m = 0 then
exit;
Result := 1;
while ( e > 0 ) do
begin
if (e AND 1) <> 0 then
Result := (Result * int64(b)) mod m;
b := (int64(b) * b ) mod m;
e := e shr 1;
end;
end;
 
procedure start;
VAR
Limit,
Testzahl : DWord;
longPrimCount : int64;
t1,t0: TDateTime;
BEGIN
 
Limit := 500;
Testzahl := 2;
longPrimCount := 0;
t0 := time;
 
repeat
write(Limit:8,': ');
repeat
if F.Prim(Testzahl) then
begin
F.PeriodeErmitteln(Testzahl);
if F.PeriodenLaenge = Testzahl-1 then
Begin
inc(longPrimCount);
IF Limit = 500 then
write(TestZahl,',');
end
end;
inc(Testzahl);
until TestZahl = Limit;
inc(Limit,Limit);
write(' .. count ',longPrimCount:8,' ');
t1:= time;
If (t1-t0)>1/864000 then
write(FormatDateTime('HH:NN:SS.ZZZ',t1-T0));
writeln;
until Limit > 10*1000*1000;
 
t1 := time;
writeln;
writeln('count of long primes ',longPrimCount);
writeln('Benoetigte Zeit ',FormatDateTime('HH:NN:SS.ZZZ',T1-T0));
 
END;
 
BEGIN
F := tFaktorisieren.create;
writeln('Start');
start;
writeln('Fertig.');
F.free;
readln;
end.
Output:
sh-4.4# ./Periode
Start
     500: 7,17,19,23,29,47,59,61,97,109,113,131,149,167,179,181,193,223,229,233,257,263,269,313,337,367,379,383,389,419,433,461,487,491,499,  .. count       35 
    1000:   .. count       60 
    2000:   .. count      116 
    4000:   .. count      218 
    8000:   .. count      390 
   16000:   .. count      716 
   32000:   .. count     1300 
   64000:   .. count     2430 
  128000:   .. count     4498 
  256000:   .. count     8434 00:00:00.100
  512000:   .. count    15920 00:00:00.220
 1024000:   .. count    30171 00:00:00.494
 2048000:   .. count    57115 00:00:01.140
 4096000:   .. count   108381 00:00:02.578
 8192000:   .. count   206594 00:00:06.073

count of long primes 206594
Benoetigte Zeit 00:00:06.073
Fertig.

Perl[edit]

Translation of: Sidef
use ntheory qw/divisors powmod/;
 
sub is_long_prime {
my($p) = @_;
for my $d (divisors($p-1)) {
return $d+1 == $p if powmod(10, $d, $p)== 1;
}
0;
}
 
print "Long primes ≤ 500:\n";
print (join ' ', grep {is_long_prime($_) } 1 .. 500). "\n\n";
 
for $n (500, 1000, 2000, 4000, 8000, 16000, 32000, 64000) {
printf "Number of long primes ≤ $n: %d\n", scalar grep { is_long_prime($_) } 1 .. $n;
}
Output:
Long primes ≤ 500:
7 17 19 23 29 47 59 61 97 109 113 131 149 167 179 181 193 223 229 233 257 263 269 313 337 367 379 383 389 419 433 461 487 491 499

Number of long primes ≤ 500: 35
Number of long primes ≤ 1000: 60
Number of long primes ≤ 2000: 116
Number of long primes ≤ 4000: 218
Number of long primes ≤ 8000: 390
Number of long primes ≤ 16000: 716
Number of long primes ≤ 32000: 1300
Number of long primes ≤ 64000: 2430

Perl 6[edit]

Works with: Rakudo version 2018.06

Not very fast as the numbers get larger. 64000 takes a little over 15 minutes on my computer. 😕

my @long-primes = lazy (1..*).grep(*.is-prime).hyper(:8degree, :8batch).grep({1+(1/$_).base-repeating[1].chars == $_});
 
put "Long primes ≤ 500:\n", @long-primes[^(@long-primes.first: * > 500, :k)];
 
say "\nNumber of long primes ≤ $_: ", +@long-primes[^(@long-primes.first: * > $_, :k)]
for 500, 1000, 2000, 4000, 8000, 16000, 32000, 64000;
Output:
Long primes ≤ 500:
7 17 19 23 29 47 59 61 97 109 113 131 149 167 179 181 193 223 229 233 257 263 269 313 337 367 379 383 389 419 433 461 487 491 499

Number of long primes ≤ 500: 35

Number of long primes ≤ 1000: 60

Number of long primes ≤ 2000: 116

Number of long primes ≤ 4000: 218

Number of long primes ≤ 8000: 390

Number of long primes ≤ 16000: 716

Number of long primes ≤ 32000: 1300

Number of long primes ≤ 64000: 2430

Python[edit]

Translation of: Kotlin
def sieve(limit):
primes = []
c = [False] * (limit + 1) # composite = true
# no need to process even numbers
p = 3
while True:
p2 = p * p
if p2 > limit: break
for i in range(p2, limit, 2 * p): c[i] = True
while True:
p += 2
if not c[p]: break
 
for i in range(3, limit, 2):
if not c[i]: primes.append(i)
return primes
 
# finds the period of the reciprocal of n
def findPeriod(n):
r = 1
for i in range(1, n): r = (10 * r) % n
rr = r
period = 0
while True:
r = (10 * r) % n
period += 1
if r == rr: break
return period
 
primes = sieve(64000)
longPrimes = []
for prime in primes:
if findPeriod(prime) == prime - 1:
longPrimes.append(prime)
numbers = [500, 1000, 2000, 4000, 8000, 16000, 32000, 64000]
count = 0
index = 0
totals = [0] * len(numbers)
for longPrime in longPrimes:
if longPrime > numbers[index]:
totals[index] = count
index += 1
count += 1
totals[-1] = count
print('The long primes up to 500 are:')
print(str(longPrimes[:totals[0]]).replace(',', ''))
print('\nThe number of long primes up to:')
for (i, total) in enumerate(totals):
print('  %5d is %d' % (numbers[i], total))
Output:
The long primes up to 500 are:
[7 17 19 23 29 47 59 61 97 109 113 131 149 167 179 181 193 223 229 233 257 263 269 313 337 367 379 383 389 419 433 461 487 491 499]

The number of long primes up to:
    500 is 35
   1000 is 60
   2000 is 116
   4000 is 218
   8000 is 390
  16000 is 716
  32000 is 1300
  64000 is 2430

REXX[edit]

For every   doubling   of the limit, it takes about roughly   5   times longer to compute the long primes.

uses odd numbers[edit]

/*REXX pgm calculates/displays base ten  long primes  (AKA golden primes, proper primes,*/
/*───────────────────── maximal period primes, long period primes, full reptend primes).*/
parse arg a /*obtain optional argument from the CL.*/
if a='' | a="," then a='500 -500 -1000 -2000 -4000 -8000 -16000 -32000 -64000' /*deflt?*/
do k=1 for words(a); H=word(a, k) /*step through the list of high limits.*/
neg= H<1 /*used as an indicator to display count*/
H= abs(H) /*obtain the absolute value of H. */
$= /*the list of long primes (so far). */
do j=7 to H by 2 /*start with 7, just use odd integers.*/
if .len(j) + 1 \== j then iterate /*Period length wrong? Then skip it. */
$=$ j /*add the long prime to the $ list.*/
end /*j*/
say
if neg then do; say 'number of long primes ≤ ' H " is: " words($); end
else do; say 'list of long primes ≤ ' H":"; say strip($); end
end /*k*/
exit /*stick a fork in it, we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
.len: procedure; parse arg x; r=1; do x; r= 10*r // x; end /*x*/
rr=r; do p=1 until r==rr; r= 10*r // x; end /*p*/
return p
output   when using the internal default inputs:
list of long primes ≤  500:
7 17 19 23 29 47 59 61 97 109 113 131 149 167 179 181 193 223 229 233 257 263 269 313 337 367 379 383 389 419 433 461 487 491 499

number of long primes ≤  500  is:  35

number of long primes ≤  1000  is:  60

number of long primes ≤  2000  is:  116

number of long primes ≤  4000  is:  218

number of long primes ≤  8000  is:  390

number of long primes ≤  16000  is:  716

number of long primes ≤  32000  is:  2430

uses odd numbers, some prime tests[edit]

This REXX version is about   2   times faster than the 1st REXX version   (because it does some primality testing).

/*REXX pgm calculates/displays base ten  long primes  (AKA golden primes, proper primes,*/
/*───────────────────── maximal period primes, long period primes, full reptend primes).*/
parse arg a /*obtain optional argument from the CL.*/
if a='' | a="," then a='500 -500 -1000 -2000 -4000 -8000 -16000 -32000 -64000' /*deflt?*/
do k=1 for words(a); H=word(a, k) /*step through the list of high limits.*/
neg= H<1 /*used as an indicator to display count*/
H= abs(H) /*obtain the absolute value of H. */
$= /*the list of long primes (so far). */
do j=7 to H by 2; parse var j '' -1 _ /*start with 7, just use odd integers.*/
if _==5 then iterate /*last digit a five? Then not a prime.*/
if j// 3==0 then iterate /*Is divisible by 3? " " " " */
if j\==11 then if j//11==0 then iterate /* " " " 11? " " " " */
if j\==13 then if j//13==0 then iterate /* " " " 13? " " " " */
if j\==17 then if j//17==0 then iterate /* " " " 17? " " " " */
if j\==19 then if j//19==0 then iterate /* " " " 19? " " " " */
if .len(j) + 1 \== j then iterate /*Period length wrong? Then skip it. */
$=$ j /*add the long prime to the $ list.*/
end /*j*/
say
if neg then do; say 'number of long primes ≤ ' H " is: " words($); end
else do; say 'list of long primes ≤ ' H":"; say strip($); end
end /*k*/
exit /*stick a fork in it, we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
.len: procedure; parse arg x; r=1; do x; r= 10*r // x; end /*x*/
rr=r; do p=1 until r==rr; r= 10*r // x; end /*p*/
return p
output   is identical to the 1st REXX version.

uses primes[edit]

This REXX version is about   5   times faster than the 1st REXX version   (because it only tests primes).

/*REXX pgm calculates/displays base ten  long primes  (AKA golden primes, proper primes,*/
/*───────────────────── maximal period primes, long period primes, full reptend primes).*/
parse arg a /*obtain optional argument from the CL.*/
if a='' | a="," then a='500 -500 -1000 -2000 -4000 -8000 -16000 -32000 -64000' /*deflt?*/
m=0; aa=words(a) /* [↑] two list types of low primes. */
do j=1 for aa; m= max(m, abs(word(a, j))) /*find the maximum argument in the list*/
end /*j*/
call genP /*go and generate some primes. */
do k=1 for aa; H=word(a, k) /*step through the list of high limits.*/
neg= H<1 /*used as an indicator to display count*/
H= abs(H) /*obtain the absolute value of H. */
$= /*the list of long primes (so far). */
do j=7 to H by 2
if \@.j then iterate /*Is J not a prime? Then skip it. */
if .len(j) + 1 \== j then iterate /*Period length wrong? " " " */
$=$ j /*add the long prime to the $ list.*/
end /*j*/ /* [↑] some pretty weak prime testing.*/
say
if neg then do; say 'number of long primes ≤ ' H " is: " words($); end
else do; say 'list of long primes ≤ ' H":"; say strip($); end
end /*k*/
exit /*stick a fork in it, we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
genP: @.=0; @.2=1; @.3=1; @.5=1; @.7=1; @.11=1;  !.=0; !.1=2; !.2=3; !.3=5; !.4=7; !.5=11
#=5 /*the number of primes (so far). */
do g=!.#+2 by 2 until #>=m /*gen enough primes to satisfy max A. */
do d=2 until !.d**2 > g /*only divide up to square root of X. */
if g // !.d == 0 then iterate g /*Divisible? Then skip this integer. */
end /*d*/ /* [↓] a spanking new prime was found.*/
#=#+1; @.g=1;  !.#=g /*bump P counter; assign P, add to P's.*/
end /*g*/
return
/*──────────────────────────────────────────────────────────────────────────────────────*/
.len: procedure; parse arg x; r=1; do x; r= 10*r // x; end /*x*/
rr=r; do p=1 until r==rr; r= 10*r // x; end /*p*/
return p
output   is identical to the 1st REXX version.


Sidef[edit]

The smallest divisor d of p-1 such that 10^d = 1 (mod p), is the length of the period of the decimal expansion of 1/p.

func is_long_prime(p) {
 
for d in (divisors(p-1)) {
if (powmod(10, d, p) == 1) {
return (d+1 == p)
}
}
 
return false
}
 
say "Long primes ≤ 500:"
say primes(500).grep(is_long_prime).join(' ')
 
for n in ([500, 1000, 2000, 4000, 8000, 16000, 32000, 64000]) {
say ("Number of long primes ≤ #{n}: ", primes(n).count_by(is_long_prime))
}
Output:
Long primes ≤ 500:
7 17 19 23 29 47 59 61 97 109 113 131 149 167 179 181 193 223 229 233 257 263 269 313 337 367 379 383 389 419 433 461 487 491 499
Number of long primes ≤ 500: 35
Number of long primes ≤ 1000: 60
Number of long primes ≤ 2000: 116
Number of long primes ≤ 4000: 218
Number of long primes ≤ 8000: 390
Number of long primes ≤ 16000: 716
Number of long primes ≤ 32000: 1300
Number of long primes ≤ 64000: 2430

Alternatively, we can implement the is_long_prime(p) function using the `znorder(a, p)` built-in method, which is considerably faster:

func is_long_prime(p) {
znorder(10, p) == p-1
}

zkl[edit]

Using GMP (GNU Multiple Precision Arithmetic Library, probabilistic primes), because it is easy and fast to generate primes.

var [const] BN=Import("zklBigNum");  // libGMP
primes,p := List.createLong(7_000), BN(3); // one big alloc vs lots of allocs
while(p.nextPrime()<=64_000){ primes.append(p.toInt()) } // 6412 of them, skipped 2
primes.append(p.toInt()); // and one more so tail prime is >64_000
 
longPrimes:=primes.filter(fcn(p){ findPeriod(p)==p-1 }); // yawn
fcn findPeriod(n){
r,period := 1,0;
do(n){ r=(10*r)%n }
rr:=r;
while(True){ // reduce is more concise but 2.5 times slower
r=(10*r)%n;
period+=1;
if(r==rr) break;
}
period
}
fiveHundred:=longPrimes.filter('<(500));
println("The long primes up to 500 are:\n",longPrimes.filter('<(500)).concat(","));
 
println("\nThe number of long primes up to:");
foreach n in (T(500, 1000, 2000, 4000, 8000, 16000, 32000, 64000)){
println("  %5d is %d".fmt( n, longPrimes.filter1n('>(n)) ));
}
Output:
The long primes up to 500 are:
7,17,19,23,29,47,59,61,97,109,113,131,149,167,179,181,193,223,229,233,257,263,269,313,337,367,379,383,389,419,433,461,487,491,499

The number of long primes up to:
    500 is 35
   1000 is 60
   2000 is 116
   4000 is 218
   8000 is 390
  16000 is 716
  32000 is 1300
  64000 is 2430