Program to find prime numbers

class CheckIfPrime
   {
    static void Main()
      {
          while (true)
        {
            Console.Write("Enter a number: ");
            decimal a = decimal.Parse(Console.ReadLine());
            decimal[] k = new decimal[int.Parse(a.ToString())];
            decimal p = 0;
            for (int i = 2; i < a; i++)
            {
                if (a % i != 0)
                {
                    p += i;
                    k[i] = i;
                }
                else
                    p += i;
            }
            if (p == k.Sum())
               { Console.WriteLine ("{0} is prime!", a);}
            else
               {Console.WriteLine("{0} is NOT prime", a);}

        }
    }

}

There are some very optimal ways to implement the algorithm. But if you don't know much about maths and you simply follow the definition of prime as the requirement: a number that is only divisible by 1 and by itself (and nothing else), here's a simple to understand code for positive numbers.

public bool IsPrime(int candidateNumber)
{
    int fromNumber = 2;
    int toNumber = candidateNumber - 1;

    while(fromNumber <= toNumber)
    {
        bool isDivisible = candidateNumber % fromNumber == 0;
        if (isDivisible)
        {
            return false;
        }

        fromNumber++;
    }
    return true;
}

Since every number is divisible by 1 and by itself, we start checking from 2 onwards until the number immediately before itself. That's the basic reasoning.

You can do also this:

class Program
  {
    static void Main(string[] args)
    {
      long numberToTest = 350124;
      bool isPrime = NumberIsPrime(numberToTest);
      Console.WriteLine(string.Format("Number {0} is prime? {1}", numberToTest, isPrime));
      Console.ReadLine();
    }

    private static bool NumberIsPrime(long n)
    {
      bool retVal = true;

      if (n <= 3)
      {
        retVal = n > 1;
      } else if (n % 2 == 0 || n % 3 == 0)
      {
        retVal = false;
      }

      int i = 5;

      while (i * i <= n)
      {
        if (n % i == 0 || n % (i + 2) == 0)
        {
          retVal = false;
        }
        i += 6;
      }

      return retVal;
    }
  }

An easier approach , what i did is check if a number have exactly two division factors which is the essence of prime numbers .

    List<int> factorList = new List<int>();
    int[] numArray = new int[] { 1, 0, 6, 9, 7, 5, 3, 6, 0, 8, 1 };

    foreach (int item in numArray)
    {

        for (int x = 1; x <= item; x++)
        {
          //check for the remainder after dividing for each number less that number
            if (item % x == 0)
            {
                factorList.Add(x);
            }
        }
        if (factorList.Count == 2) // has only 2 division factors ; prime number
        {
            Console.WriteLine(item + " is a prime number ");
        }
        else
        {Console.WriteLine(item + " is not a prime number ");}

        factorList = new List<int>(); // reinitialize list
    }

Here is a solution with unit test:

The solution:

public class PrimeNumbersKata
    {
        public int CountPrimeNumbers(int n)
        {
            if (n < 0) throw new ArgumentException("Not valide numbre");
            if (n == 0 || n == 1) return 0;
            int cpt = 0;
            for (int i = 2; i <= n; i++)
            {
                if (IsPrimaire(i)) cpt++;
            }
            return cpt;
        }

        private bool IsPrimaire(int number)
        {

            for (int i = 2; i <= number / 2; i++)
            {
                if (number % i == 0) return false;
            }
            return true;
        }
    }

The tests:

[TestFixture]
    class PrimeNumbersKataTest
    {
        private PrimeNumbersKata primeNumbersKata;
        [SetUp]
        public void Init()
        {
            primeNumbersKata = new PrimeNumbersKata();
        }
        [TestCase(1,0)]
        [TestCase(0,0)]
        [TestCase(2,1)]
        [TestCase(3,2)]
        [TestCase(5,3)]
        [TestCase(7,4)]
        [TestCase(9,4)]
        [TestCase(11,5)]
        [TestCase(13,6)]
        public void CountPrimeNumbers_N_AsArgument_returnCountPrimes(int n, int expected)
        {
            //arrange
            //act
            var actual = primeNumbersKata.CountPrimeNumbers(n);
            //assert
            Assert.AreEqual(expected,actual);
        }

        [Test]
        public void CountPrimairs_N_IsNegative_RaiseAnException()
        {
            var ex = Assert.Throws<ArgumentException>(()=> { primeNumbersKata.CountPrimeNumbers(-1); });
            //Assert.That(ex.Message == "Not valide numbre");
             Assert.That(ex.Message, Is.EqualTo("Not valide numbre"));

        }
    }

in the university it was necessary to count prime numbers up to 10,000 did so, the teacher was a little surprised, but I passed the test. Lang c#

void Main()
{
    int number=1;
    for(long i=2;i<10000;i++)
    {
        if(PrimeTest(i))
        {
            Console.WriteLine(number+++" " +i);
        }
    }
}

List<long> KnownPrime = new List<long>();
private bool PrimeTest(long i)
{
    if (i == 1) return false;
    if (i == 2)
    {
        KnownPrime.Add(i);
        return true;
    }
    foreach(int k in KnownPrime)
    {
        if(i%k==0)
            return false;
    }
    KnownPrime.Add(i);
    return true;
}

for (int i = 2; i < 100; i++)
{
    bool isPrimeNumber = true;
    for (int j = 2; j <= i && j <= 100; j++)
    {
        if (i != j && i % j == 0)
        {
            isPrimeNumber = false; break;
        }
    }
    if (isPrimeNumber)
    {
        Console.WriteLine(i);
    }
}