Listing Prime numbers within a group of numbers

[sosarya]

Hello!

How can I re-write this program with using arrays this time?
Thank you

 Dim lowerLimit, upperLimit, primeNumber As Integer

        Dim isPrime As Boolean = False



        lowerLimit = CType(TextBox1.Text, Integer)

        upperLimit = CType(TextBox2.Text, Integer)



        Do While lowerLimit < upperLimit



            primeNumber = lowerLimit



            If primeNumber = 2 Or primeNumber = 3 Then

                isPrime = True

            ElseIf primeNumber = 1 Then

                isPrime = False

            ElseIf primeNumber = upperLimit Then

                Exit Do

            Else

                For i As Integer = 2 To (primeNumber - 1)

                    If primeNumber Mod i = 0 Then

                        isPrime = False

                        Exit For

                    Else

                        isPrime = True

                    End If

                Next

            End If



            If isPrime = True Then

                TextBox3.Text += primeNumber & vbCrLf

            End If



            lowerLimit += 1

        Loop

[dunfiddlin]

No idea. There does not appear to be any sane reason for using an array at any point in this process, not least because there is no fixed size collection anywhere to be recorded. If you were using a sieve method I suppose there might be an argument for using a 2D array but you're not, so there isn't.

As a tip, bear in mind that no even numbers (after 2) can be prime so you might as well set your loop up to be from the first odd number in the range and then step through 2 at a time rather than the normal 1. It will speed things up more than a little.

[GBeats]

what you could do with an array is set up a table that lists all prime numbers up to a certain number, for example if number your going to check never going to be more than 100 then setup a table with all prime number between 0 and 100

dim primearray(99) as boolean

set all prime numbers to true using a text file for example when the program starts and then say you have number 50 to check

if primearray(Yournumber - 1) = true then
'''''''''its a prime number
end if

this method is much faster than a loop, andwith booleans you could have a massive table without a memory problem

or

have an array that simple lists all prime numbers

dim primearray(99) as long ' put all prime number in here with text file or pre-calculate at startup

then loop through the array instead of looping through and calculating each number in between

[Joacim Andersson]

I'm unsure what you mean by using arrays. Do you mean that you want to get an array of all prime numbers between a lower and upper bound?

There is a better way to calculate if a number is a prime than what you're doing. A number is a prime if it can't be evenly divided by any number up to (including) the square root of that number. So a better algorithm to check if is a number is a prime is this:

  Public Function IsPrime(number As Integer) As Boolean
    If number = 2 Then
      Return True
    End If
    If number Mod 2 = 0 OrElse number < 2 Then
      'no even numbers except 2 is a prime and 1 is by definition not a prime
      Return False
    End If
    Dim sqr As Integer = CInt(Math.Ceiling(Math.Sqrt(number)))
    'We can skip all even numbers in the loop since only even numbers are evenly divisible with
    'another even number and those aren't prime numbers
    For i = 3 To sqr Step 2
      If number Mod i = 0 Then
        'Have a divisor, not a prime
        Return False
      End If
    Next
    Return True
  End Function

Now, if you want to write a function that returns an array of all primes between two given number then it could look something like this:

  Public Function GetPrimeNumbers(lowerLimit As Integer, upperLimit As Integer) As Integer()
    If lowerLimit > upperLimit Then
      Throw New ArgumentException("lowerLimit cannot be higher than upperLimit")
    End If
    Dim list As New List(Of Integer)
    For i As Integer = lowerLimit To upperLimit
      If IsPrime(i) Then
        list.Add(i)
      End If
    Next
    Return list.ToArray()
  End Function

[sosarya]

What I need to do is to write a program that gets the lower and upper bounds from the user and list the prime numbers between those bounds. I want get the prime values and load them into the arrays and then show those values in textbox

[Joacim Andersson]

That's exactly what the code I posted above does. Well, it get's the array of all prime numbers between a lower and upper bound. Now all you have to do is to list them the way you want.

[sosarya]

I can't understand how do I get the numbers out to textbox3 ?

Public Function IsPrime(number As Integer) As Boolean
If number = 2 Then
Return True
End If
If number Mod 2 = 0 OrElse number < 2 Then
'no even numbers except 2 is a prime and 1 is by definition not a prime
Return False
End If
Dim sqr As Integer = CInt(Math.Ceiling(Math.Sqrt(number)))
'We can skip all even numbers in the loop since only even numbers are evenly divisible with
'another even number and those aren't prime numbers
For i = 3 To sqr Step 2
If number Mod i = 0 Then
'Have a divisor, not a prime
Return False
End If
Next
Return True
End Function
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim lowerLimit, upperLimit, primeNumber, i As Integer
Dim isPrime As Boolean = False
Integer.TryParse(TextBox1.Text, lowerLimit)
Integer.TryParse(TextBox2.Text, upperLimit)
Do While lowerLimit < upperLimit
primeNumber = lowerLimit


TextBox3.Text += ?????????????????????? & vbCrLf

lowerLimit += 1
Loop
End Sub

[Joacim Andersson]

First of all you only pasted one of the two functions I posted, you need both of them. Secondly you actually also need to call on them.
Is your TextBox a multiline TextBox? Wouldn't a ListBox be better to show the prime numbers?
I will assume that TextBox3 is a multiline textbox and that you want to show one prime number on each line.

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
  Dim lower, upper As Integer
  If Integer.TryParse(TextBox1.Text, lower) AndAlso Integer.TryParse(TextBox2.Text, upper) Then
    Dim primeNumbers() As Integer = GetPrimeNumbers(lower, upper)
    Dim text As String = ""
    For Each prime As Integer In primeNumbers
      text &= prime.ToString() & vbCrLf
    Next
    TextBox3.Text = text
  End If
End Sub

[sosarya]

First of all you only pasted one of the two functions I posted, you need both of them. Secondly you actually also need to call on them.
Is your TextBox a multiline TextBox? Wouldn't a ListBox be better to show the prime numbers?
I will assume that TextBox3 is a multiline textbox and that you want to show one prime number on each line.

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
  Dim lower, upper As Integer
  If Integer.TryParse(TextBox1.Text, lower) AndAlso Integer.TryParse(TextBox2.Text, upper) Then
    Dim primeNumbers() As Integer = GetPrimeNumbers(lower, upper)
    Dim text As String = ""
    For Each prime As Integer In primeNumbers
      text &= prime.ToString() & vbCrLf
    Next
    TextBox3.Text = text
  End If
End Sub

Thank you very much

This is exactly what I wanted

Have a really good day !