Task: Safe primes and unsafe primes
C: #include <stdbool.h>
#include <stdio.h>

int primes[] = {
    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, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199,
    211, 223, 227, 229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281, 283, 293, 307, 311, 313, 317, 331
};
#define PCOUNT (sizeof(primes) / sizeof(int))

bool isPrime(int n) {
    int i;

    if (n < 2) {
        return false;
    }

    for (i = 0; i < PCOUNT; i++) {
        if (n == primes[i]) {
            return true;
        }
        if (n % primes[i] == 0) {
            return false;
        }
        if (n < primes[i] * primes[i]) {
            return true;
        }
    }

    for (i = primes[PCOUNT - 1] + 2; i * i <= n; i += 2) {
        if (n % i == 0) {
            return false;
        }
    }

    return true;
}

int main() {
    int beg, end;
    int i, count;

    
    
    beg = 2;
    end = 1000000;
    count = 0;
    printf("First 35 safe primes:\n");
    for (i = beg; i < end; i++) {
        if (isPrime(i) && isPrime((i - 1) / 2)) {
            if (count < 35) {
                printf("%d ", i);
            }
            count++;
        }
    }
    printf("\nThere are  %d safe primes below  %d\n", count, end);

    beg = end;
    end = end * 10;
    for (i = beg; i < end; i++) {
        if (isPrime(i) && isPrime((i - 1) / 2)) {
            count++;
        }
    }
    printf("There are %d safe primes below %d\n", count, end);

    
    
    beg = 2;
    end = 1000000;
    count = 0;
    printf("\nFirst 40 unsafe primes:\n");
    for (i = beg; i < end; i++) {
        if (isPrime(i) && !isPrime((i - 1) / 2)) {
            if (count < 40) {
                printf("%d ", i);
            }
            count++;
        }
    }
    printf("\nThere are  %d unsafe primes below  %d\n", count, end);

    beg = end;
    end = end * 10;
    for (i = beg; i < end; i++) {
        if (isPrime(i) && !isPrime((i - 1) / 2)) {
            count++;
        }
    }
    printf("There are %d unsafe primes below %d\n", count, end);

    return 0;
}

VB: 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