I'm trying to display last prime number for a given interval. For example:

1. if n is 10 last prime nubmer is 7

2. if n is 11 last prime nubmer is 11

3. if n is 14 last prime number is 13

4. etc...

    public static uint LastPrimeNumberInInterval(uint n)
    {
        uint result = 0;
        uint i = 2;
   
        while (i <= n)
        {
            bool b = false;
   
            while (i <= n / 2)
            {
                if (n % i == 0)
                {
                    b = true;
                    break;
                }
   
                i  ;
            }
   
            if (!b)
            {
                result = i;
            }
   
            i  ;
        }
   
        return result;
    }
   

but I'm stuck on displaying only correct answer when n is prime number. Can someone point out where is my fault?

CodePudding user response：

You need to use a different variable in your nested loop and reset it for each number being checked.

public static uint LastPrimeNumberInInterval(uint n)
 {
     uint result = 0;
     uint i = 2;

 while (i <= n)
 {
     bool b = false;
     uint j = 2;

     while (j <= n / 2)
     {
         if (n % j == 0)
         {
             b = true;
             break;
         }

         j  ;
     }

     if (!b)
     {
         result = i;
     }

     i  ;
 }

 return result;


}

CodePudding user response：

Based on your method:

public static uint LastPrimeNumberInInterval(uint n)
 {
     uint result = 0;
     uint i = 2;
     
     if (n < 2) return result;

     while (i <= n)
     {
         bool b = false;

         uint j = 2;
         while (j <= n / 2)
         {
             if (i % j == 0)
             {
                 b = true;
                 break;
             }

             j  ;
         }

         if (!b)
         {
             result = i;
         }

         i  ;
     }

     return result;
 }