Safe primes and unsafe primes

Safe primes and unsafe primes
You are encouraged to solve this task according to the task description, using any language you may know.
Definitions
•   A   safe prime   is a prime   p   and where   (p-1)/2   is also prime.
•   The corresponding prime  (p-1)/2   is known as a   Sophie Germain   prime.
•   An   unsafe prime   is a prime   p   and where   (p-1)/2   isn't   a prime.
•   An   unsafe prime   is a prime that   isn't   a   safe   prime.

•   Find and display (on one line) the first   35   safe primes.
•   Find and display the   count   of the safe primes below   1,000,000.
•   Find and display the   count   of the safe primes below 10,000,000.
•   Find and display (on one line) the first   40   unsafe primes.
•   Find and display the   count   of the unsafe primes below   1,000,000.
•   Find and display the   count   of the unsafe primes below 10,000,000.
•   (Optional)   display the   counts   and   "below numbers"   with commas.

Show all output here.

Also see

C#

Works with: C sharp version 7
using static System.Console;
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

public static class SafePrimes
{
public static void Main() {
HashSet<int> primes = Primes(10_000_000).ToHashSet();
WriteLine("First 35 safe primes:");
WriteLine(string.Join(" ", primes.Where(IsSafe).Take(35)));
WriteLine(\$"There are {primes.TakeWhile(p => p < 1_000_000).Count(IsSafe):n0} safe primes below {1_000_000:n0}");
WriteLine(\$"There are {primes.TakeWhile(p => p < 10_000_000).Count(IsSafe):n0} safe primes below {10_000_000:n0}");
WriteLine("First 40 unsafe primes:");
WriteLine(string.Join(" ", primes.Where(IsUnsafe).Take(40)));
WriteLine(\$"There are {primes.TakeWhile(p => p < 1_000_000).Count(IsUnsafe):n0} unsafe primes below {1_000_000:n0}");
WriteLine(\$"There are {primes.TakeWhile(p => p < 10_000_000).Count(IsUnsafe):n0} unsafe primes below {10_000_000:n0}");

bool IsSafe(int prime) => primes.Contains(prime / 2);
bool IsUnsafe(int prime) => !primes.Contains(prime / 2);
}

//Method from maths library
static IEnumerable<int> Primes(int bound) {
if (bound < 2) yield break;
yield return 2;

BitArray 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;
}
}

}
Output:
First 35 safe primes:
5 7 11 23 47 59 83 107 167 179 227 263 347 359 383 467 479 503 563 587 719 839 863 887 983 1019 1187 1283 1307 1319 1367 1439 1487 1523 1619
There are 4,324 safe primes below 1,000,000
There are 30,657 safe primes below 10,000,000
First 40 unsafe primes:
2 3 13 17 19 29 31 37 41 43 53 61 67 71 73 79 89 97 101 103 109 113 127 131 137 139 149 151 157 163 173 181 191 193 197 199 211 223 229 233
There are 74,174 unsafe primes below 1,000,000
There are 633,922 unsafe primes below 10,000,000

F#

This task uses Extensible Prime Generator (F#)

pCache |> Seq.filter(fun n->isPrime((n-1)/2)) |> Seq.take 35 |> Seq.iter (printf "%d ")

Output:
5 7 11 23 47 59 83 107 167 179 227 263 347 359 383 467 479 503 563 587 719 839 863 887 983 1019 1187 1283 1307 1319 1367 1439 1487 1523 1619

printfn "There are %d safe primes less than 1000000" (pCache |> Seq.takeWhile(fun n->n<1000000) |> Seq.filter(fun n->isPrime((n-1)/2)) |> Seq.length)

Output:
There are 4324 safe primes less than 10000000

printfn "There are %d safe primes less than 10000000" (pCache |> Seq.takeWhile(fun n->n<10000000) |> Seq.filter(fun n->isPrime((n-1)/2)) |> Seq.length)

Output:
There are 30657 safe primes less than 10000000

pCache |> Seq.filter(fun n->not (isPrime((n-1)/2))) |> Seq.take 40 |> Seq.iter (printf "%d ")

Output:
2 3 13 17 19 29 31 37 41 43 53 61 67 71 73 79 89 97 101 103 109 113 127 131 137 139 149 151 157 163 173 181 191 193 197 199 211 223 229 233

printfn "There are %d unsafe primes less than 1000000" (pCache |> Seq.takeWhile(fun n->n<1000000) |> Seq.filter(fun n->not (isPrime((n-1)/2))) |> Seq.length);;

Output:
There are 74174 unsafe primes less than 1000000

printfn "There are %d unsafe primes less than 10000000" (pCache |> Seq.takeWhile(fun n->n<10000000) |> Seq.filter(fun n->not (isPrime((n-1)/2))) |> Seq.length);;

Output:
There are 633922 unsafe primes less than 10000000

Factor

Much like the Perl 6 example, this program uses an in-built primes generator to efficiently obtain the first ten million primes. If memory is a concern, it wouldn't be unreasonable to perform primality tests on the (odd) numbers below ten million, however.

USING: fry interpolate kernel literals math math.primes
sequences tools.memory.private ;
IN: rosetta-code.safe-primes

CONSTANT: primes \$[ 10,000,000 primes-upto ]

: safe/unsafe ( -- safe unsafe )
primes [ 1 - 2/ prime? ] partition ;

: count< ( seq n -- str ) '[ _ < ] count commas ;

: seq>commas ( seq -- str ) [ commas ] map " " join ;

: stats ( seq n -- head count1 count2 )
'[ _ head seq>commas ] [ 1e6 count< ] [ 1e7 count< ] tri ;

safe/unsafe [ 35 ] [ 40 ] bi* [ stats ] [email protected]

[I
First 35 safe primes:
\${5}
Safe prime count below 1,000,000: \${4}
Safe prime count below 10,000,000: \${3}

First 40 unsafe primes:
\${2}
Unsafe prime count below 1,000,000: \${1}
Unsafe prime count below 10,000,000: \${}
I]
Output:
First 35 safe primes:
5 7 11 23 47 59 83 107 167 179 227 263 347 359 383 467 479 503 563 587 719 839 863 887 983 1,019 1,187 1,283 1,307 1,319 1,367 1,439 1,487 1,523 1,619
Safe prime count below  1,000,000: 4,324
Safe prime count below 10,000,000: 30,657

First 40 unsafe primes:
2 3 13 17 19 29 31 37 41 43 53 61 67 71 73 79 89 97 101 103 109 113 127 131 137 139 149 151 157 163 173 181 191 193 197 199 211 223 229 233
Unsafe prime count below  1,000,000: 74,174
Unsafe prime count below 10,000,000: 633,922

FreeBASIC

' version 19-01-2019
' compile with: fbc -s console

Const As UInteger max = 10000000
Dim As UInteger i, j, sc1, usc1, sc2, usc2
Dim As String safeprimes, unsafeprimes
Dim As UByte sieve()

ReDim sieve(max)
' 0 = prime, 1 = no prime
sieve(0) = 1 : sieve(1) = 1

For i = 4 To max Step 2
sieve(i) = 1
Next
For i = 3 To Sqr(max) +1 Step 2
If sieve(i) = 0 Then
For j = i * i To max Step i * 2
sieve(j) = 1
Next
End If
Next

usc1 = 1 : unsafeprimes = "2"
For i = 3 To 3001 Step 2
If sieve(i) = 0 Then
If sieve(i \ 2) = 0 Then
sc1 += 1
If sc1 <= 35 Then
safeprimes += " " + Str(i)
End If
Else
usc1 += 1
If usc1 <= 40 Then
unsafeprimes += " " + Str(i)
End If
End If
End If
Next

For i = 3003 To max \ 10 Step 2
If sieve(i) = 0 Then
If sieve(i \ 2) = 0 Then
sc1 += 1
Else
usc1 += 1
End If
End If
Next

sc2 = sc1 : usc2 = usc1
For i = max \ 10 +1 To max Step 2
If sieve(i) = 0 Then
If sieve(i \ 2) = 0 Then
sc2 += 1
Else
usc2 += 1
End If
End If
Next

Print "the first 35 Safeprimes are: "; safeprimes
Print
Print "the first 40 Unsafeprimes are: "; unsafeprimes
Print
Print " Safeprimes Unsafeprimes"
Print " Below ---------------------------"
Print Using "##########, "; max \ 10; sc1; usc1
Print Using "##########, "; max  ; sc2; usc2

' empty keyboard buffer
While Inkey <> "" : Wend
Print : Print "hit any key to end program"
Sleep
End
Output:
the first 35 Safeprimes are:  5 7 11 23 47 59 83 107 167 179 227 263 347 359 383 467 479 503 563 587 719 839 863 887 983 1019 1187 1283 1307 1319 1367 1439 1487 1523 1619

the first 40 Unsafeprimes are:  2 3 13 17 19 29 31 37 41 43 53 61 67 71 73 79 89 97 101 103 109 113 127 131 137 139 149 151 157 163 173 181 191 193 197 199 211 223 229 233

Safeprimes     Unsafeprimes
Below         ---------------------------
1,000,000            4,324           74,174
10,000,000           30,657          633,922

Go

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 = true
c = true
// apart from 2 all even numbers are of course composite
for i := uint64(4); i < limit; i += 2 {
c[i] = true
}
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() {
// sieve up to 10 million
sieved := sieve(1e7)
var safe = make([]int, 35)
count := 0
for i := 3; count < 35; i += 2 {
if !sieved[i] && !sieved[(i-1)/2] {
safe[count] = i
count++
}
}
fmt.Println("The first 35 safe primes are:\n", safe, "\n")

count = 0
for i := 3; i < 1e6; i += 2 {
if !sieved[i] && !sieved[(i-1)/2] {
count++
}
}
fmt.Println("The number of safe primes below 1,000,000 is", commatize(count), "\n")

for i := 1000001; i < 1e7; i += 2 {
if !sieved[i] && !sieved[(i-1)/2] {
count++
}
}
fmt.Println("The number of safe primes below 10,000,000 is", commatize(count), "\n")

unsafe := make([]int, 40)
unsafe = 2 // since (2 - 1)/2 is not prime
count = 1
for i := 3; count < 40; i += 2 {
if !sieved[i] && sieved[(i-1)/2] {
unsafe[count] = i
count++
}
}
fmt.Println("The first 40 unsafe primes are:\n", unsafe, "\n")

count = 1
for i := 3; i < 1e6; i += 2 {
if !sieved[i] && sieved[(i-1)/2] {
count++
}
}
fmt.Println("The number of unsafe primes below 1,000,000 is", commatize(count), "\n")

for i := 1000001; i < 1e7; i += 2 {
if !sieved[i] && sieved[(i-1)/2] {
count++
}
}
fmt.Println("The number of unsafe primes below 10,000,000 is", commatize(count), "\n")
}
Output:
The first 35 safe primes are:
[5 7 11 23 47 59 83 107 167 179 227 263 347 359 383 467 479 503 563 587 719 839 863 887 983 1019 1187 1283 1307 1319 1367 1439 1487 1523 1619]

The number of safe primes below 1,000,000 is 4,324

The number of safe primes below 10,000,000 is 30,657

The first 40 unsafe primes are:
[2 3 13 17 19 29 31 37 41 43 53 61 67 71 73 79 89 97 101 103 109 113 127 131 137 139 149 151 157 163 173 181 191 193 197 199 211 223 229 233]

The number of unsafe primes below 1,000,000 is 74,174

The number of unsafe primes below 10,000,000 is 633,922

J

NB. play around a bit to get primes less than ten million
p:inv 10000000
664579

p:664579
10000019

PRIMES =: p:i.664579
10 {. PRIMES
2 3 5 7 11 13 17 19 23 29

{: PRIMES
9999991

primeQ =: 1&p:
safeQ =: [email protected]:-:@:<:
Filter =: (#~`)(`:6)

SAFE =: safeQ Filter PRIMES

NB. first thirty-five safe primes
(32+3) {. SAFE
5 7 11 23 47 59 83 107 167 179 227 263 347 359 383 467 479 503 563 587 719 839 863 887 983 1019 1187 1283 1307 1319 1367 1439 1487 1523 1619

NB. first forty unsafe primes
(33+7) {. PRIMES -. SAFE
2 3 13 17 19 29 31 37 41 43 53 61 67 71 73 79 89 97 101 103 109 113 127 131 137 139 149 151 157 163 173 181 191 193 197 199 211 223 229 233

NB. tally of safe primes less than ten million
# SAFE
30657

NB. tally of safe primes below a million
# 1000000&>Filter SAFE
4324

NB. tally of perilous primes below ten million
UNSAFE =: PRIMES -. SAFE

# UNSAFE
633922

NB. tally of these below one million
K =: 1 : 'm * 1000'
+/ UNSAFE < 1 K K
74174

Essentially we have

primeQ =: 1&p:
safeQ =: [email protected]:-:@:<:
Filter =: (#~`)(`:6)
K =: adverb def 'm * 1000'
PRIMES =: i.&.:(p:inv) 10 K K
SAFE =: safeQ Filter PRIMES
UNSAFE =: PRIMES -. SAFE

The rest of the display is mere window dressing.

Java

public class SafePrimes {
public static void main(String... args) {
// Use Sieve of Eratosthenes to find primes
int SIEVE_SIZE = 10_000_000;
boolean[] isComposite = new boolean[SIEVE_SIZE];
// It's really a flag indicating non-prime, but composite usually applies
isComposite = true;
isComposite = true;
for (int n = 2; n < SIEVE_SIZE; n++) {
if (isComposite[n]) {
continue;
}
for (int i = n * 2; i < SIEVE_SIZE; i += n) {
isComposite[i] = true;
}
}

int oldSafePrimeCount = 0;
int oldUnsafePrimeCount = 0;
int safePrimeCount = 0;
int unsafePrimeCount = 0;
StringBuilder safePrimes = new StringBuilder();
StringBuilder unsafePrimes = new StringBuilder();
int safePrimesStrCount = 0;
int unsafePrimesStrCount = 0;
for (int n = 2; n < SIEVE_SIZE; n++) {
if (n == 1_000_000) {
oldSafePrimeCount = safePrimeCount;
oldUnsafePrimeCount = unsafePrimeCount;
}
if (isComposite[n]) {
continue;
}
boolean isUnsafe = isComposite[(n - 1) >>> 1];
if (isUnsafe) {
if (unsafePrimeCount < 40) {
if (unsafePrimeCount > 0) {
unsafePrimes.append(", ");
}
unsafePrimes.append(n);
unsafePrimesStrCount++;
}
unsafePrimeCount++;
}
else {
if (safePrimeCount < 35) {
if (safePrimeCount > 0) {
safePrimes.append(", ");
}
safePrimes.append(n);
safePrimesStrCount++;
}
safePrimeCount++;
}
}

System.out.println("First " + safePrimesStrCount + " safe primes: " + safePrimes.toString());
System.out.println("Number of safe primes below 1,000,000: " + oldSafePrimeCount);
System.out.println("Number of safe primes below 10,000,000: " + safePrimeCount);
System.out.println("First " + unsafePrimesStrCount + " unsafe primes: " + unsafePrimes.toString());
System.out.println("Number of unsafe primes below 1,000,000: " + oldUnsafePrimeCount);
System.out.println("Number of unsafe primes below 10,000,000: " + unsafePrimeCount);

return;
}
}
Output:
First 35 safe primes: 5, 7, 11, 23, 47, 59, 83, 107, 167, 179, 227, 263, 347, 359, 383, 467, 479, 503, 563, 587, 719, 839, 863, 887, 983, 1019, 1187, 1283, 1307, 1319, 1367, 1439, 1487, 1523, 1619
Number of safe primes below 1,000,000: 4324
Number of safe primes below 10,000,000: 30657
First 40 unsafe primes: 2, 3, 13, 17, 19, 29, 31, 37, 41, 43, 53, 61, 67, 71, 73, 79, 89, 97, 101, 103, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 173, 181, 191, 193, 197, 199, 211, 223, 229, 233
Number of unsafe primes below 1,000,000: 74174
Number of unsafe primes below 10,000,000: 633922

Julia

using Primes, Formatting

function parseprimelist()
primelist = primes(2, 10000000)
safeprimes = Vector{Int64}()
unsafeprimes = Vector{Int64}()
for p in primelist
if isprime(div(p - 1, 2))
push!(safeprimes, p)
else
push!(unsafeprimes, p)
end
end
println("The first 35 unsafe primes are: ", safeprimes[1:35])
println("There are ", format(sum(map(x -> x < 1000000, safeprimes)), commas=true), " safe primes less than 1 million.")
println("There are ", format(length(safeprimes), commas=true), " safe primes less than 10 million.")
println("The first 40 unsafe primes are: ", unsafeprimes[1:40])
println("There are ", format(sum(map(x -> x < 1000000, unsafeprimes)), commas=true), " unsafe primes less than 1 million.")
println("There are ", format(length(unsafeprimes), commas=true), " unsafe primes less than 10 million.")
end

parseprimelist()

Output:

The first 35 unsafe primes are: [5, 7, 11, 23, 47, 59, 83, 107, 167, 179, 227, 263, 347, 359, 383, 467, 479, 503, 563, 587, 719, 839, 863, 887, 983, 1019, 1187, 1283, 1307, 1319, 1367, 1439, 1487, 1523, 1619] There are 4,324 safe primes less than 1 million. There are 30,657 safe primes less than 10 million. The first 40 unsafe primes are: [2, 3, 13, 17, 19, 29, 31, 37, 41, 43, 53, 61, 67, 71, 73, 79, 89, 97, 101, 103, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 173, 181, 191, 193, 197, 199, 211, 223, 229, 233] There are 74,174 unsafe primes less than 1 million. There are 633,922 unsafe primes less than 10 million.

Kotlin

Translation of: Go
// Version 1.2.70

fun sieve(limit: Int): BooleanArray {
// True denotes composite, false denotes prime.
val c = BooleanArray(limit + 1) // all false by default
c = true
c = true
// apart from 2 all even numbers are of course composite
for (i in 4..limit step 2) c[i] = true
var p = 3 // start from 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
}
}
return c
}

fun main(args: Array<String>) {
// sieve up to 10 million
val sieved = sieve(10_000_000)
val safe = IntArray(35)
var count = 0
var i = 3
while (count < 35) {
if (!sieved[i] && !sieved[(i - 1) / 2]) safe[count++] = i
i += 2
}
println("The first 35 safe primes are:")
println(safe.joinToString(" ","[", "]\n"))

count = 0
for (j in 3 until 1_000_000 step 2) {
if (!sieved[j] && !sieved[(j - 1) / 2]) count++
}
System.out.printf("The number of safe primes below 1,000,000 is %,d\n\n", count)

for (j in 1_000_001 until 10_000_000 step 2) {
if (!sieved[j] && !sieved[(j - 1) / 2]) count++
}
System.out.printf("The number of safe primes below 10,000,000 is %,d\n\n", count)

val unsafe = IntArray(40)
unsafe = 2 // since (2 - 1)/2 is not prime
count = 1
i = 3
while (count < 40) {
if (!sieved[i] && sieved[(i - 1) / 2]) unsafe[count++] = i
i += 2
}
println("The first 40 unsafe primes are:")
println(unsafe.joinToString(" ","[", "]\n"))

count = 1
for (j in 3 until 1_000_000 step 2) {
if (!sieved[j] && sieved[(j - 1) / 2]) count++
}
System.out.printf("The number of unsafe primes below 1,000,000 is %,d\n\n", count)

for (j in 1_000_001 until 10_000_000 step 2) {
if (!sieved[j] && sieved[(j - 1) / 2]) count++
}
System.out.printf("The number of unsafe primes below 10,000,000 is %,d\n\n", count)
}
Output:
The first 35 safe primes are:
[5 7 11 23 47 59 83 107 167 179 227 263 347 359 383 467 479 503 563 587 719 839 863 887 983 1019 1187 1283 1307 1319 1367 1439 1487 1523 1619]

The number of safe primes below 1,000,000 is 4,324

The number of safe primes below 10,000,000 is 30,657

The first 40 unsafe primes are:
[2 3 13 17 19 29 31 37 41 43 53 61 67 71 73 79 89 97 101 103 109 113 127 131 137 139 149 151 157 163 173 181 191 193 197 199 211 223 229 233]

The number of unsafe primes below 1,000,000 is 74,174

The number of unsafe primes below 10,000,000 is 633,922

Maple

showSafePrimes := proc(n::posint)
local prime_list, k;
prime_list := ;
for k to n - 1 do
prime_list := [op(prime_list), NumberTheory:-NextSafePrime(prime_list[-1])];
end do;
return prime_list;
end proc;

showUnsafePrimes := proc(n::posint)
local prime_num, k;
prime_num := ;
for k to n-1 do
prime_num := [op(prime_num), nextprime(prime_num[-1])];
end do;
return remove(x -> member(x, showSafePrimes(n)), prime_num);
end proc:

countSafePrimes := proc(n::posint)
local counts, prime;
counts := 0;
prime := 5;
while prime < n do prime := NumberTheory:-NextSafePrime(prime);
counts := counts + 1;
end do;
return counts;
end proc;

countUnsafePrimes := proc(n::posint)
local safe_counts, total;
safe_counts := countSafePrimes(n);
total := NumberTheory:-PrimeCounting(n);
end proc;

showSafePrimes(35);
showUnsafePrimes(40);
countSafePrimes(1000000);
countSafePrimes(10000000);
countUnsafePrimes(1000000);
countUnsafePrimes(10000000);
Output:
[5, 7, 11, 23, 47, 59, 83, 107, 167, 179, 227, 263, 347, 359, 383, 467, 479, 503, 563, 587, 719, 839, 863, 887, 983, 1019, 1187, 1283, 1307, 1319, 1367, 1439, 1487, 1523, 1619]
[2, 3, 13, 17, 19, 29, 31, 37, 41, 43, 53, 61, 67, 71, 73, 79, 89, 97, 101, 103, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 173]
4324
30657
74174
633922

Pascal

Works with: Free Pascal

Using unit mp_prime of Wolfgang Erhardt ( RIP ) , of which I use two sieve, to simplify things. Generating small primes and checked by the second, which starts to run 2x ahead.Sieving of consecutive prime number is much faster than primality check.

program Sophie;
{ Find and count Sophie Germain primes }
{ uses unit mp_prime out of mparith of Wolfgang Ehrhardt
* http://wolfgang-ehrhardt.de/misc_en.html#mparith
http://wolfgang-ehrhardt.de/mp_intro.html }

{\$APPTYPE CONSOLE}
uses
mp_prime,sysutils;
var
pS0,pS1:TSieve;
procedure SafeOrNoSavePrimeOut(totCnt:NativeInt;CntSafe:boolean);
var
cnt,pr,pSG,testPr : NativeUint;
begin
prime_sieve_reset(pS0,1);
prime_sieve_reset(pS1,1);
cnt := 0;
// memorize prime of the sieve, because sometimes prime_sieve_next(pS1) is to far ahead.
testPr := prime_sieve_next(pS1);
IF CntSafe then
Begin
writeln('First ',totCnt,' safe primes');
repeat
pr := prime_sieve_next(pS0);
pSG := 2*pr+1;
while testPr< pSG do
testPr := prime_sieve_next(pS1);
if pSG = testPr then
begin
write(pSG,',');
inc(cnt);
end;
until cnt >= totCnt
end
else
Begin
writeln('First ',totCnt,' unsafe primes');
repeat
pr := prime_sieve_next(pS0);
pSG := (pr-1) DIV 2;
while testPr< pSG do
testPr := prime_sieve_next(pS1);
if pSG <> testPr then
begin
write(pr,',');
inc(cnt);
end;
until cnt >= totCnt;
end;
writeln(#8,#32);
end;

function CountSafePrimes(Limit:NativeInt):NativeUint;
var
cnt,pr,pSG,testPr : NativeUint;
begin
prime_sieve_reset(pS0,1);
prime_sieve_reset(pS1,1);
cnt := 0;
testPr := 0;
repeat
pr := prime_sieve_next(pS0);
pSG := 2*pr+1;
while testPr< pSG do
testPr := prime_sieve_next(pS1);
if pSG = testPr then
inc(cnt);
until pSG >= Limit;
CountSafePrimes := cnt;
end;

procedure CountSafePrimesOut(Limit:NativeUint);
Begin
writeln('there are ',CountSafePrimes(limit),' safe primes out of ',
primepi32(limit),' primes up to ',Limit);
end;

procedure CountUnSafePrimesOut(Limit:NativeUint);
var
prCnt: NativeUint;
Begin
prCnt := primepi32(limit);
writeln('there are ',prCnt-CountSafePrimes(limit),' unsafe primes out of ',
prCnt,' primes up to ',Limit);
end;

var
T1,T0 : INt64;
begin
T0 :=gettickcount64;
prime_sieve_init(pS0,1);
prime_sieve_init(pS1,1);
//Find and display (on one line) the first 35 safe primes.
SafeOrNoSavePrimeOut(35,true);
//Find and display the count of the safe primes below 1,000,000.
CountSafePrimesOut(1000*1000);
//Find and display the count of the safe primes below 10,000,000.
CountSafePrimesOut(10*1000*1000);
//Find and display (on one line) the first 40 unsafe primes.
SafeOrNoSavePrimeOut(40,false);
//Find and display the count of the unsafe primes below 1,000,000.
CountUnSafePrimesOut(1000*1000);
//Find and display the count of the unsafe primes below 10,000,000.
CountUnSafePrimesOut(10*1000*1000);
writeln;
CountSafePrimesOut(1000*1000*1000);
T1 :=gettickcount64;
writeln('runtime ',T1-T0,' ms');
end.
Output:
First 35 safe primes
5,7,11,23,47,59,83,107,167,179,227,263,347,359,383,467,479,503,563,587,719,839,863,887,983,1019,1187,1283,1307,1319,1367,1439,1487,1523,1619
there are 4324 safe primes out of 78498 primes up to 1000000
there are 30657 safe primes out of 664579 primes up to 10000000
First 40 unsafe primes
2,3,13,17,19,29,31,37,41,43,53,61,67,71,73,79,89,97,101,103,109,113,127,131,137,139,149,151,157,163,173,181,191,193,197,199,211,223,229,233
there are 74174 unsafe primes out of 78498 primes up to 1000000
there are 633922 unsafe primes out of 664579 primes up to 10000000
there are 1775676 safe primes out of 50847534 primes up to 1000000000
runtime 2797 ms

Perl

The module ntheory does fast prime generation and testing.

Library: ntheory
use ntheory qw(forprimes is_prime);

my \$upto = 1e7;
my %class = ( safe => [], unsafe =>  );

forprimes {
push @{\$class{ is_prime((\$_-1)>>1) ? 'safe' : 'unsafe' }}, \$_;
} 3, \$upto;

for (['safe', 35], ['unsafe', 40]) {
my(\$type, \$quantity) = @\$_;
print "The first \$quantity \$type primes are:\n";
print join(" ", map { comma(\$class{\$type}->[\$_-1]) } 1..\$quantity), "\n";
for my \$q (\$upto/10, \$upto) {
my \$n = scalar(grep { \$_ <= \$q } @{\$class{\$type}});
printf "The number of \$type primes up to %s: %s\n", comma(\$q), comma(\$n);
}
}

sub comma {
(my \$s = reverse shift) =~ s/(.{3})/\$1,/g;
\$s =~ s/,(-?)\$/\$1/;
\$s = reverse \$s;
}
Output:
The first 35 safe primes are:
5 7 11 23 47 59 83 107 167 179 227 263 347 359 383 467 479 503 563 587 719 839 863 887 983 1,019 1,187 1,283 1,307 1,319 1,367 1,439 1,487 1,523 1,619
The number of safe primes up to 1,000,000: 4,324
The number of safe primes up to 10,000,000: 30,657
The first 40 unsafe primes are:
2 3 13 17 19 29 31 37 41 43 53 61 67 71 73 79 89 97 101 103 109 113 127 131 137 139 149 151 157 163 173 181 191 193 197 199 211 223 229 233
The number of unsafe primes up to 1,000,000: 74,174
The number of unsafe primes up to 10,000,000: 633,922

Perl 6

Works with: Rakudo version 2018.08

Perl 6 has a built-in method .is-prime to test for prime numbers. It's great for testing individual numbers or to find/filter a few thousand numbers, but when you are looking for millions, it becomes a drag. No fear, the Perl 6 ecosystem has a fast prime sieve module available which can produce 10 million primes in a few seconds. Once we have the primes, it is just a small matter of filtering and formatting them appropriately.

sub comma { \$^i.flip.comb(3).join(',').flip }

use Math::Primesieve;

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

my @primes = \$sieve.primes(10_000_000);

my %filter = @primes.Set;

my \$primes = @primes.classify: { %filter{(\$_ - 1)/2} ?? 'safe' !! 'unsafe' };

for 'safe', 35, 'unsafe', 40 -> \$type, \$quantity {
say "The first \$quantity \$type primes are:";

say \$primes{\$type}[^\$quantity]».&comma;

say "The number of \$type primes up to {comma \$_}: ",
comma \$primes{\$type}.first(* > \$_, :k) // +\$primes{\$type} for 1e6, 1e7;

say '';
}
Output:
The first 35 safe primes are:
(5 7 11 23 47 59 83 107 167 179 227 263 347 359 383 467 479 503 563 587 719 839 863 887 983 1,019 1,187 1,283 1,307 1,319 1,367 1,439 1,487 1,523 1,619)
The number of safe primes up to 1,000,000: 4,324
The number of safe primes up to 10,000,000: 30,657

The first 40 unsafe primes are:
(2 3 13 17 19 29 31 37 41 43 53 61 67 71 73 79 89 97 101 103 109 113 127 131 137 139 149 151 157 163 173 181 191 193 197 199 211 223 229 233)
The number of unsafe primes up to 1,000,000: 74,174
The number of unsafe primes up to 10,000,000: 633,922

Phix

while sieved<10_000_000 do
end while
sequence safe = {}, unsafe = {}
procedure filter_range(integer lo, hi)
for i=lo to hi do
integer p = primes[i]
if p>2 and is_prime((p-1)/2) then
safe &= p
else
unsafe &= p
end if
end for
end procedure
integer k = abs(binary_search(1_000_000,primes)),
l = abs(binary_search(10_000_000,primes))
filter_range(1,k-1)
integer ls = length(safe), lu = length(unsafe)
filter_range(k,l-1)
printf(1,"The first 35 safe primes: %s\n",{sprint(safe[1..35])})
printf(1,"Count of safe primes below 1,000,000: %,d\n",ls)
printf(1,"Count of safe primes below 10,000,000: %,d\n",length(safe))
printf(1,"The first 40 unsafe primes: %s\n",{sprint(unsafe[1..40])})
printf(1,"Count of unsafe primes below 1,000,000: %,d\n",lu)
printf(1,"Count of unsafe primes below 10,000,000: %,d\n",length(unsafe))
Output:
The first 35 safe primes: {5,7,11,23,47,59,83,107,167,179,227,263,347,359,383,467,479,503,563,587,719,839,863,887,983,1019,1187,1283,1307,1319,1367,1439,1487,1523,1619}
Count of safe primes below 1,000,000: 4,324
Count of safe primes below 10,000,000: 30,657
The first 40 unsafe primes: {2,3,13,17,19,29,31,37,41,43,53,61,67,71,73,79,89,97,101,103,109,113,127,131,137,139,149,151,157,163,173,181,191,193,197,199,211,223,229,233}
Count of unsafe primes below 1,000,000: 74,174
Count of unsafe primes below 10,000,000: 633,922

Python

primes =[]
sp =[]
usp=[]
n = 10000000
if 2<n:
primes.append(2)
for i in range(3,n+1,2):
for j in primes:
if(j>i/2) or (j==primes[-1]):
primes.append(i)
if((i-1)/2) in primes:
sp.append(i)
break
else:
usp.append(i)
break
if (i%j==0):
break

print('First 35 safe primes are:\n' , sp[:35])
print('There are '+str(len(sp[:1000000]))+' safe primes below 1,000,000')
print('There are '+str(len(sp))+' safe primes below 10,000,000')
print('First 40 unsafe primes:\n',usp[:40])
print('There are '+str(len(usp[:1000000]))+' unsafe primes below 1,000,000')
print('There are '+str(len(usp))+' safe primes below 10,000,000')

Output:
First 35 safe primes:
[5,7,11,23,47,59,83,107,167,179,227,263,347,359,383,467,479,503,563,587,719,839,863,887,983,1019,1187,1283,1307,1319,1367,1439,1487,1523,1619]
There are 4,234 safe primes below 1,000,000
There are 30,657 safe primes below 10,000,000
First 40 unsafe primes:
[2,3,13,17,19,29,31,37,41,43,53,61,67,71,73,79,89,97,101,103,109,113,127,131,137,139,149,151,157,163,173,181,191,193,197,199,211,223,229,233]
There are 74,174 unsafe primes below 1,000,000
There are 633,922 unsafe primes below 10,000,000

REXX

/*REXX program lists a sequence  (or a count)  of  ──safe──   or   ──unsafe──   primes. */
parse arg N kind _ . 1 . okind; upper kind /*obtain optional arguments from the CL*/
if N=='' | N=="," then N= 35 /*Not specified? Then assume default.*/
if kind=='' | kind=="," then kind= 'SAFE' /* " " " " " */
if _\=='' then call ser 'too many arguments specified.'
if kind\=='SAFE' & kind\=='UNSAFE' then call ser 'invalid 2nd argument: ' okind
if kind =='UNSAFE' then safe= 0; else safe= 1 /*SAFE is a binary value for function.*/
w = linesize() - 1 /*obtain the usable width of the term. */
tell= (N>0); @.=; N= abs(N) /*N is negative? Then don't display. */
!.=0;  !.1=2;  !.2=3;  !.3=5;  !.4=7;  !.5=11;  !.6=13;  !.7=17;  !.8=19; #= 8
@.=''; @.2=1; @.3=1; @.5=1; @.7=1; @.11=1; @.13=1; @.17=1; @.19=1; start= # + 1
m= 0; lim=0 /*# is the number of low primes so far*/
\$=; do i=1 for # while lim<=N; j= !.i /* [↓] find primes, and maybe show 'em*/
call safeUnsafe; \$= strip(\$) /*go see if other part of a KIND prime.*/
end /*i*/ /* [↑] allows faster loop (below). */
/* [↓] N: default lists up to 35 #'s.*/
do j=!.#+2 by 2 while lim<N /*continue on with the next odd prime. */
if j // 3 == 0 then iterate /*is this integer a multiple of three? */
parse var j '' -1 _ /*obtain the last decimal digit of J */
if _ == 5 then iterate /*is this integer a multiple of five? */
if j // 7 == 0 then iterate /* " " " " " " seven? */
if j //11 == 0 then iterate /* " " " " " " eleven?*/
if j //13 == 0 then iterate /* " " " " " " 13 ? */
if j //17 == 0 then iterate /* " " " " " " 17 ? */
if j //19 == 0 then iterate /* " " " " " " 19 ? */
/* [↓] divide by the primes. ___ */
do k=start to # while !.k * !.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 */
#= # + 1 /*bump the count of number of primes. */
!.#= j; @.j= 1 /*define a prime and its index value.*/
call safeUnsafe /*go see if other part of a KIND prime.*/
end /*j*/
/* [↓] display number of primes found.*/
if \$\=='' then say \$ /*display any residual primes in \$ list*/
say
if tell then say commas(m)' ' kind "primes found."
else say commas(m)' ' kind "primes found below or equal to " commas(N).
exit /*stick a fork in it, we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
add: m= m+1; lim= m; if \tell & j>N then do; lim= j; m= m-1; end; else call app; return 1
app: if tell then if length(\$ j)>w then do; say \$; \$ =j; end; else \$= \$ j; return 1
ser: say; say; say '***error***' arg(1); say; say; exit 13 /*tell error message. */
commas: parse arg _; do jc=length(_)-3 to 1 by -3; _=insert(',', _, jc); end; return _
/*──────────────────────────────────────────────────────────────────────────────────────*/
safeUnsafe: ?= (j-1) % 2 /*obtain the other part of KIND prime. */
if safe then if @.? == '' then return 0 /*not a safe prime.*/
else return add() /*is " " " */
else if @.? == '' then return add() /*is an unsafe prime.*/
else return 0 /*not " " " */

This REXX program makes use of   LINESIZE   REXX program (or BIF) which is used to determine the screen width (or linesize) of the terminal (console).   Some REXXes don't have this BIF.

The   LINESIZE.REX   REXX program is included here   ───►   LINESIZE.REX.

output   when using the default input of:     35

Shown at   5/6   size.)

5 7 11 23 47 59 83 107 167 179 227 263 347 359 383 467 479 503 563 587 719 839 863 887 983 1019 1187 1283 1307 1319 1367 1439 1487 1523 1619

35  SAFE primes found.
output   when using the input:     -1000000
4,324  SAFE primes found below or equal to  1,000,000.
output   when using the input:     -10000000
30,657  SAFE primes found below or equal to  10,000,000.
output   when using the input:     40   unsafe

(Shown at   5/6   size.)

2 3 13 17 19 29 31 37 41 43 53 61 67 71 73 79 89 97 101 103 109 113 127 131 137 139 149 151 157 163 173 181 191 193 197 199 211 223 229 233

40  UNSAFE primes found.
output   when using the input:     -1000000   unsafe
74,174  UNSAFE primes found below or equal to  1,000,000.
output   when using the input:     -10000000
633,922  UNSAFE primes found below or equal to  10,000,000.

Ruby

require "prime"
class Integer
def safe_prime? #assumes prime
((self-1)/2).prime?
end
end

def format_parts(n)
partitions = Prime.each(n).partition(&:safe_prime?).map(&:count)
"There are %d safes and %d unsafes below #{n}."% partitions
end

puts "First 35 safe-primes:"
p Prime.each.lazy.select(&:safe_prime?).take(35).to_a
puts format_parts(1_000_000), "\n"

puts "First 40 unsafe-primes:"
p Prime.each.lazy.reject(&:safe_prime?).take(40).to_a
puts format_parts(10_000_000)

Output:
First 35 safe-primes:
[5, 7, 11, 23, 47, 59, 83, 107, 167, 179, 227, 263, 347, 359, 383, 467, 479, 503, 563, 587, 719, 839, 863, 887, 983, 1019, 1187, 1283, 1307, 1319, 1367, 1439, 1487, 1523, 1619]
There are 4324 safes and 74174 unsafes below 1000000.

First 40 unsafe-primes:
[2, 3, 13, 17, 19, 29, 31, 37, 41, 43, 53, 61, 67, 71, 73, 79, 89, 97, 101, 103, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 173, 181, 191, 193, 197, 199, 211, 223, 229, 233]
There are 30657 safes and 633922 unsafes below 10000000.

Sidef

func is_safeprime(p) {
is_prime(p) && is_prime((p-1)/2)
}

func is_unsafeprime(p) {
is_prime(p) && !is_prime((p-1)/2)
}

func safeprime_count(from, to) {
from..to -> count_by(is_safeprime)
}

func unsafeprime_count(from, to) {
from..to -> count_by(is_unsafeprime)
}

say "First 35 safe-primes:"
say (1..Inf -> lazy.grep(is_safeprime).first(35).join(' '))
say ''
say "First 40 unsafe-primes:"
say (1..Inf -> lazy.grep(is_unsafeprime).first(40).join(' '))
say ''
say "There are #{safeprime_count(1, 1e6)} safe-primes bellow 10^6"
say "There are #{unsafeprime_count(1, 1e6)} unsafe-primes bellow 10^6"
say ''
say "There are #{safeprime_count(1, 1e7)} safe-primes bellow 10^7"
say "There are #{unsafeprime_count(1, 1e7)} unsafe-primes bellow 10^7"
Output:
First 35 safe-primes:
5 7 11 23 47 59 83 107 167 179 227 263 347 359 383 467 479 503 563 587 719 839 863 887 983 1019 1187 1283 1307 1319 1367 1439 1487 1523 1619

First 40 unsafe-primes:
2 3 13 17 19 29 31 37 41 43 53 61 67 71 73 79 89 97 101 103 109 113 127 131 137 139 149 151 157 163 173 181 191 193 197 199 211 223 229 233

There are 4324 safe-primes bellow 10^6
There are 74174 unsafe-primes bellow 10^6

There are 30657 safe-primes bellow 10^7
There are 633922 unsafe-primes bellow 10^7

Simula

BEGIN

CLASS BOOLARRAY(N); INTEGER N;
BEGIN
BOOLEAN ARRAY DATA(0:N-1);
END BOOLARRAY;

CLASS INTARRAY(N); INTEGER N;
BEGIN
INTEGER ARRAY DATA(0:N-1);
END INTARRAY;

REF(BOOLARRAY) PROCEDURE SIEVE(LIMIT);
INTEGER LIMIT;
BEGIN
REF(BOOLARRAY) C;
INTEGER P, P2;
LIMIT := LIMIT+1;
COMMENT TRUE DENOTES COMPOSITE, FALSE DENOTES PRIME. ;
C :- NEW BOOLARRAY(LIMIT); COMMENT ALL FALSE BY DEFAULT ;
C.DATA(0) := TRUE;
C.DATA(1) := TRUE;
COMMENT APART FROM 2 ALL EVEN NUMBERS ARE OF COURSE COMPOSITE ;
FOR I := 4 STEP 2 UNTIL LIMIT-1 DO
C.DATA(I) := TRUE;
COMMENT START FROM 3. ;
P := 3;
WHILE TRUE DO BEGIN
P2 := P * P;
IF P2 >= LIMIT THEN BEGIN
GO TO OUTER_BREAK;
END;
I := P2;
WHILE I < LIMIT DO BEGIN
C.DATA(I) := TRUE;
I := I + 2 * P;
END;
WHILE TRUE DO BEGIN
P := P + 2;
IF NOT C.DATA(P) THEN BEGIN
GO TO INNER_BREAK;
END;
END;
INNER_BREAK:
END;
OUTER_BREAK:
SIEVE :- C;
END SIEVE;

COMMENT MAIN BLOCK ;

REF(BOOLARRAY) SIEVED;
REF(INTARRAY) UNSAFE, SAFE;
INTEGER I, COUNT;

COMMENT SIEVE UP TO 10 MILLION ;
SIEVED :- SIEVE(10000000);

SAFE :- NEW INTARRAY(35);
COUNT := 0;
I := 3;
WHILE COUNT < 35 DO BEGIN
IF NOT SIEVED.DATA(I) AND NOT SIEVED.DATA((I-1)//2) THEN BEGIN
SAFE.DATA(COUNT) := I;
COUNT := COUNT+1;
END;
I := I+2;
END;
OUTTEXT("THE FIRST 35 SAFE PRIMES ARE:");
OUTIMAGE;
OUTCHAR('[');
FOR I := 0 STEP 1 UNTIL 35-1 DO BEGIN
IF I>0 THEN OUTCHAR(' ');
OUTINT(SAFE.DATA(I), 0);
END;
OUTCHAR(']');
OUTIMAGE;
OUTIMAGE;

COUNT := 0;
FOR I := 3 STEP 2 UNTIL 1000000 DO BEGIN
IF NOT SIEVED.DATA(I) AND NOT SIEVED.DATA((I-1)//2) THEN BEGIN
COUNT := COUNT+1;
END;
END;
OUTTEXT("THE NUMBER OF SAFE PRIMES BELOW 1,000,000 IS ");
OUTINT(COUNT, 0);
OUTIMAGE;
OUTIMAGE;

FOR I := 1000001 STEP 2 UNTIL 10000000 DO BEGIN
IF NOT SIEVED.DATA(I) AND NOT SIEVED.DATA((I-1)//2) THEN
COUNT := COUNT+1;
END;
OUTTEXT("THE NUMBER OF SAFE PRIMES BELOW 10,000,000 IS ");
OUTINT(COUNT, 0);
OUTIMAGE;
OUTIMAGE;

UNSAFE :- NEW INTARRAY(40);
UNSAFE.DATA(0) := 2; COMMENT SINCE (2 - 1)/2 IS NOT PRIME ;
COUNT := 1;
I := 3;
WHILE COUNT < 40 DO BEGIN
IF NOT SIEVED.DATA(I) AND SIEVED.DATA((I-1)//2) THEN BEGIN
UNSAFE.DATA(COUNT) := I;
COUNT := COUNT+1;
END;
I := I+2;
END;
OUTTEXT("THE FIRST 40 UNSAFE PRIMES ARE:");
OUTIMAGE;
OUTCHAR('[');
FOR I := 0 STEP 1 UNTIL 40-1 DO BEGIN
IF I>0 THEN OUTCHAR(' ');
OUTINT(UNSAFE.DATA(I), 0);
END;
OUTCHAR(']');
OUTIMAGE;
OUTIMAGE;

COUNT := 1;
FOR I := 3 STEP 2 UNTIL 1000000 DO BEGIN
IF NOT SIEVED.DATA(I) AND SIEVED.DATA((I-1)//2) THEN
COUNT := COUNT+1;
END;
OUTTEXT("THE NUMBER OF UNSAFE PRIMES BELOW 1,000,000 IS ");
OUTINT(COUNT, 0);
OUTIMAGE;
OUTIMAGE;

FOR I := 1000001 STEP 2 UNTIL 10000000 DO BEGIN
IF NOT SIEVED.DATA(I) AND SIEVED.DATA((I-1)//2) THEN
COUNT := COUNT+1;
END;
OUTTEXT("THE NUMBER OF UNSAFE PRIMES BELOW 10,000,000 IS ");
OUTINT(COUNT, 0);
OUTIMAGE;

END

Output:
THE FIRST 35 SAFE PRIMES ARE:
[5 7 11 23 47 59 83 107 167 179 227 263 347 359 383 467 479 503 563 587 719 839
863 887 983 1019 1187 1283 1307 1319 1367 1439 1487 1523 1619]

THE NUMBER OF SAFE PRIMES BELOW 1,000,000 IS 4324

THE NUMBER OF SAFE PRIMES BELOW 10,000,000 IS 30657

THE FIRST 40 UNSAFE PRIMES ARE:
[2 3 13 17 19 29 31 37 41 43 53 61 67 71 73 79 89 97 101 103 109 113 127 131 137
139 149 151 157 163 173 181 191 193 197 199 211 223 229 233]

THE NUMBER OF UNSAFE PRIMES BELOW 1,000,000 IS 74174

THE NUMBER OF UNSAFE PRIMES BELOW 10,000,000 IS 633922

Visual Basic .NET

Translation of: C#
Dependent on using .NET Core 2.1 or 2.0, or .NET Framework 4.7.2
Imports System.Console

Namespace safety
Module SafePrimes
Dim pri_HS As HashSet(Of Integer) = Primes(10_000_000).ToHashSet()

Sub Main()
For Each UnSafe In {False, True} : Dim n As Integer = If(UnSafe, 40, 35)
WriteLine(\$"The first {n} {If(UnSafe, "un", "")}safe primes are:")
WriteLine(String.Join(" ", pri_HS.Where(Function(p) UnSafe Xor
pri_HS.Contains(p \ 2)).Take(n)))
Next : Dim limit As Integer = 1_000_000 : Do
Dim part = pri_HS.TakeWhile(Function(l) l < limit),
sc As Integer = part.Count(Function(p) pri_HS.Contains(p \ 2))
WriteLine(\$"Of the primes below {limit:n0}: {sc:n0} are safe, and {part.Count() -
sc:n0} are unsafe."
) : If limit = 1_000_000 Then limit *= 10 Else Exit Do
Loop
End Sub

Private Iterator Function Primes(ByVal bound As Integer) As IEnumerable(Of Integer)
If bound < 2 Then Return
Yield 2
Dim composite As BitArray = New BitArray((bound - 1) \ 2)
Dim limit As Integer = (CInt((Math.Sqrt(bound))) - 1) \ 2
For i As Integer = 0 To limit - 1 : If composite(i) Then Continue For
Dim prime As Integer = 2 * i + 3 : Yield prime
Dim j As Integer = (prime * prime - 2) \ 2
While j < composite.Count : composite(j) = True : j += prime : End While
Next
For i As integer = limit To composite.Count - 1 : If Not composite(i) Then Yield 2 * i + 3
Next
End Function
End Module
End Namespace
If not using the latest version of the System.Linq namespace, you can implement the Enumerable.ToHashSet() method by adding
Imports System.Runtime.CompilerServices
to the top and this module inside the safety namespace:
Module Extensions
<Extension()>
Function ToHashSet(Of T)(ByVal src As IEnumerable(Of T), ByVal Optional _
IECmp As IEqualityComparer(Of T) = Nothing) As HashSet(Of T)
Return New HashSet(Of T)(src, IECmp)
End Function
End Module
Output:
The first 35 safe primes are:
5 7 11 23 47 59 83 107 167 179 227 263 347 359 383 467 479 503 563 587 719 839 863 887 983 1019 1187 1283 1307 1319 1367 1439 1487 1523 1619
The first 40 unsafe primes are:
2 3 13 17 19 29 31 37 41 43 53 61 67 71 73 79 89 97 101 103 109 113 127 131 137 139 149 151 157 163 173 181 191 193 197 199 211 223 229 233
Of the primes below 1,000,000: 4,324 are safe, and 74,174 are unsafe.
Of the primes below 10,000,000: 30,657 are safe, and 633,922 are unsafe.

zkl

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

Extensible prime generator#zkl could be used instead.

var [const] BI=Import("zklBigNum");  // libGMP
// saving 664,578 primes (vs generating them on the fly) seems a bit overkill

fcn safePrime(p){ ((p-1)/2).probablyPrime() } // p is a BigInt prime

fcn safetyList(sN,nsN){
p,safe,notSafe := BI(2),List(),List();
do{
if(safePrime(p)) safe.append(p.toInt()) else notSafe.append(p.toInt());
p.nextPrime();
}while(safe.len()<sN or notSafe.len()<nsN);
println("The first %d safe primes are: %s".fmt(sN,safe[0,sN].concat(",")));
println("The first %d unsafe primes are: %s".fmt(nsN,notSafe[0,nsN].concat(",")));
}(35,40);
Output:
The first 35   safe primes are: 5,7,11,23,47,59,83,107,167,179,227,263,347,359,383,467,479,503,563,587,719,839,863,887,983,1019,1187,1283,1307,1319,1367,1439,1487,1523,1619
The first 40 unsafe primes are: 2,3,13,17,19,29,31,37,41,43,53,61,67,71,73,79,89,97,101,103,109,113,127,131,137,139,149,151,157,163,173,181,191,193,197,199,211,223,229,233

safetyList could also be written as:

println("The first %d  safe primes are: %s".fmt(N:=35,
Walker(BI(1).nextPrime) // gyrate (vs Walker.filter) because p mutates
.pump(N,String,safePrime,Void.Filter,String.fp1(","))));
println("The first %d unsafe primes are: %s".fmt(N=40,
Walker(BI(1).nextPrime) // or save as List
.pump(N,List,safePrime,'==(False),Void.Filter,"toInt").concat(",")));

Time to count:

fcn safetyCount(N,s=0,ns=0,p=BI(2)){
do{
if(safePrime(p)) s+=1; else ns+=1;
p.nextPrime()
}while(p<N);
println("The number of safe primes below %10,d is %7,d".fmt(N,s));
println("The number of unsafe primes below %10,d is %7,d".fmt(N,ns));
return(s,ns,p);
}

s,ns,p := safetyCount(1_000_000);
println();
safetyCount(10_000_000,s,ns,p);
Output:
The number of   safe primes below  1,000,000 is   4,324
The number of unsafe primes below  1,000,000 is  74,174

The number of   safe primes below 10,000,000 is  30,657
The number of unsafe primes below 10,000,000 is 633,922