primec.csharp

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

public class PrimeNumberCalculator
{
    /**
     * Returns a list of prime numbers up to n using the Sieve of Eratosthenes algorithm.
     */
    public static List<int> SieveOfEratosthenes(int n)
    {
        if (n < 2)
        {
            return new List<int>();
        }
        bool[] isPrime = new bool[n + 1];
        for (int i = 0; i <= n; i++)
        {
            isPrime[i] = true;
        }
        isPrime[0] = isPrime[1] = false;  // 0 and 1 are not primes
        for (int p = 2; p * p <= n; p++)
        {
            if (isPrime[p])
            {
                for (int multiple = p * p; multiple <= n; multiple += p)
                {
                    isPrime[multiple] = false;
                }
            }
        }
        List<int> primes = new List<int>();
        for (int p = 2; p <= n; p++)
        {
            if (isPrime[p])
            {
                primes.Add(p);
            }
        }
        return primes;
    }

    /**
     * Finds all prime numbers in the range [start, end].
     */
    public static List<int> FindPrimesInRange(int start, int end)
    {
        if (start > end || start < 0 || end < 0)
        {
            return new List<int>();
        }
        List<int> primesUpToEnd = SieveOfEratosthenes(end);
        List<int> primesInRange = new List<int>();
        foreach (int p in primesUpToEnd)
        {
            if (p >= start)
            {
                primesInRange.Add(p);
            }
        }
        return primesInRange;
    }

    /**
     * Provides the last three prime numbers and the count of prime numbers in the range [start, end].
     */
    public static (List<int>, int) PrimeStatistics(int start, int end)
    {
        List<int> primesInRange = FindPrimesInRange(start, end);
        int countOfPrimes = primesInRange.Count;
        List<int> lastThreePrimes = new List<int>();
        if (primesInRange.Count >= 3)
        {
            lastThreePrimes = primesInRange.GetRange(primesInRange.Count - 3, 3);
        }
        else
        {
            lastThreePrimes = primesInRange;
        }
        return (lastThreePrimes, countOfPrimes);
    }

    /**
     * Prompts the user to enter the start and end of the range, with validation.
     */
    public static (int, int) GetUserInput()
    {
        int start, end;
        while (true)
        {
            try
            {
                Console.Write("Please enter the start of the range (non-negative integer): ");
                start = int.Parse(Console.ReadLine());
                Console.Write("Please enter the end of the range (non-negative integer): ");
                end = int.Parse(Console.ReadLine());
                if (start > end)
                {
                    Console.WriteLine("Error: Start of the range must be less than or equal to the end of the range.");
                }
                else if (start < 0 || end < 0)
                {
                    Console.WriteLine("Error: Range values must be non-negative.");
                }
                else
                {
                    return (start, end);
                }
            }
            catch (FormatException)
            {
                Console.WriteLine("Error: Invalid input. Please enter integer values.");
            }
        }
    }

    /**
     * Displays the results of the prime number calculations.
     */
    public static void DisplayResults(int start, int end, List<int> lastThreePrimes, int countOfPrimes)
    {
        if (lastThreePrimes.Count > 0)
        {
            Console.Write("The last prime numbers in the range [" + start + ", " + end + "] are: ");
            for (int i = 0; i < lastThreePrimes.Count; i++)
            {
                Console.Write(lastThreePrimes[i]);
                if (i < lastThreePrimes.Count - 1)
                {
                    Console.Write(", ");
                }
            }
            Console.WriteLine();
        }
        else
        {
            Console.WriteLine("No prime numbers found in the range [" + start + ", " + end + "]");
        }
        Console.WriteLine("The number of prime numbers in this range is: " + countOfPrimes);
    }

    public static void Main(string[] args)
    {
        // Get user input for the range
        var (start, end) = GetUserInput();
        // Calculate and display the results
        var (lastThreePrimes, countOfPrimes) = PrimeStatistics(start, end