Imports System
Imports System.Collections.Generic
' A happy number is a number that eventually reaches 1
' when repeatedly replaced by the sum of the squares of its digits.
'
' 19 = 1^2 + 9^2 = 82
' 19 -> 82 -> 68 ->100 -> 1 eventually reaches 1
' 19 = happy numbe
Module HappyNumbers
' Compute the sum of squares of digits of n
Function SumOfDigitSquares(n As Integer) As Integer
Dim sum As Integer = 0
While n > 0
Dim d As Integer = n Mod 10
sum += d * d
n \= 10
End While
Return sum
End Function
' Determine whether n is a happy number
Function IsHappy(n As Integer) As Boolean
Dim seen As New HashSet(Of Integer)()
While n <> 1 AndAlso Not seen.Contains(n)
seen.Add(n)
n = SumOfDigitSquares(n)
End While
Return n = 1
End Function
Sub Main()
Dim a As Integer = 1
Dim b As Integer = 100
If a > b Then
Dim temp = a
a = b
b = temp
End If
Dim happyNumbers As New List(Of Integer)()
For i As Integer = a To b
If IsHappy(i) Then
happyNumbers.Add(i)
End If
Next
Console.WriteLine($"Happy numbers in range [{a}, {b}]:")
For Each n In happyNumbers
Console.Write(n & " ")
Next
End Sub
End Module
'
' run:
'
' Happy numbers in range [1, 100]:
' 1 7 10 13 19 23 28 31 32 44 49 68 70 79 82 86 91 94 97 100
'
