Finding Adjacent Prime Numbers using Linq


Suppose you have a range of numbers defined by a lower bound L and an upper bound U, the problem is to find 2 pairs of prime numbers, one of which will be the closest primes and the second pair will have the farthest distance in between. For example, given a range of integers from 1 to 20, the two most adjacent primes are 2 and 3 having a distance 1 while the least adjacent primes are 7 and 11 having a distance of 4.

Given a list of integers from 1 to 10,000 can you tell the least adjacent prime numbers? (the answer is 9551 and 9587) 🙂

Writing an algorithm that find adjacent primes can be tricky but is also short and straightforward. below is a naive vb.net implementation for this algorithm. I will not explain code in detail but basically what it does is to loop over the given range from L up to square root of U and flags all prime numbers within the range. then we compute the distance using linq and another query orders by distance in asc and desc in order to select the top 1 from each query.

    Sub Main()
While True

Dim PrimeNumbers As New List(Of Integer)

Console.WriteLine(“———-Prime Distance———-“)
Console.Write(“Enter L: “)
Dim lowerLimit As String = Console.ReadLine().Trim
Console.Write(“Enter U: “)
Dim upperLimit As Integer = Console.ReadLine().Trim
Dim prime(upperLimit) As Integer
Dim c1, c2, c3 As Integer
For c1 = 2 To upperLimit
prime(c1) = 0
Next
prime(0) = 1
prime(1) = 1

For c2 = 2 To Math.Sqrt(upperLimit) + 1
If prime(c2) = 0 Then
c1 = c2
c3 = 2 * c1
While c3 < upperLimit + 1
prime(c3) = 1
c3 = c3 + c1
End While
End If
Next
For c1 = 0 To upperLimit
If prime(c1) = 0 Then
PrimeNumbers.Add(c1)
End If
Next
PrimeNumbers = PrimeNumbers.Where(Function(x) x >= lowerLimit).ToList
Dim ordered = From a In PrimeNumbers Order By a Ascending Select a
Dim query = From a In ordered, b In ordered Where b > a Select P1 = a, P2 = b, Distance = Math.Abs(a – b)
query = query.GroupBy(Function(x) x.P1).Select(Function(x) x.First) ‘distinct
Dim minDistance = (From a In query Order By a.Distance Ascending Select a).Take(1).SingleOrDefault
Dim maxDistance = (From a In query Order By a.Distance Descending Select a).Take(1).SingleOrDefault
Console.WriteLine(“———————————“)
Console.WriteLine(“{0},{1} are the closest primes having distance {2}”, minDistance.P1, minDistance.P2, minDistance.Distance)
Console.WriteLine(“———————————“)
Console.WriteLine(“{0},{1} are the most distant primes having distance {2}”, maxDistance.P1, maxDistance.P2, maxDistance.Distance)
Console.WriteLine(“———————————“)
Console.ReadKey()
End While
End Sub

Adjacent Primes Simulation

Adjacent Primes Simulation

 

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