[RESOLVED] Prime number code

Hey guys, I needed to make a program for finding prime numbers. The logic is that you have to put limits n & m and then LOOP and odd-integer P between n & m inclusive. I have a button to click on my form that displays 2 message boxes, one saying " enter upper limit" and the other say "enter lower limit". I need now to get the results of the odd prime numbers between the limits into a list box! How do I do that? Am I doing something wrong? I am new at the visual basic... I would appreciate your help. Thanks, S :o

Code:

---

Public Class Form1

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
       
        Dim Isprime As Integer
        Dim str As String
        Dim n As Integer
        Dim TestLimit As String
        Dim TestPrime As String
        Dim TestNum As String


        str = ""
        n = InputBox("Enter Upper limit")
        n = InputBox("Enter lower limit")
        If (n > 50) Then
            MsgBox("outside range")
            Exit Sub

        End If

        ' 
        ' Eliminate even numbers
        If TestPrime Mod 2 = 0 Then Exit Sub
        '  Loop through ODD numbers starting with 3
        TestNum = 3
        TestLimit = TestPrime
        Do While TestLimit > TestNum

            If TestPrime Mod TestNum = 0 Then
                '  Uncomment this if you really want to know
                '  MsgBox "Divisible by " & TestNum 

                Exit Sub
            End If

            '  There's logic to this.  Think about it.
            TestLimit = TestPrime \ TestNum

            '  Remember, we only bother to check odd numbers
            TestNum = TestNum + 2

        Loop

        '  If we made it through the loop, the number is a prime.

        Isprime = True


    End Sub

---

the returns from the input boxes have the same name "N"
one of them should be "M"

numbers should not be strings

and odd numbers are every second number starting from an odd number so

for x=<an odd number> to <an odd number> step 2
will set x to odd numbers
do your testing and ...
if prime then list.additem x
next x

oh your right.. thanks

how do i incorporate what u said in the code?

"for x=<an odd number> to <an odd number> step 2"

do i plug in random values where u wrote odd number?

im sorry im really new at this

oh your right.. thanks

how do i incorporate what u said in the code?

"for x=<an odd number> to <an odd number> step 2"

do i plug in random values where u wrote odd number?

im sorry im really new at this

Quote:

---

Originally Posted by incidentals

the returns from the input boxes have the same name "N"
one of them should be "M"

numbers should not be strings

and odd numbers are every second number starting from an odd number so

for x=<an odd number> to <an odd number> step 2
will set x to odd numbers
do your testing and ...
if prime then list.additem x
next x

---

Try this:

vb Code:

Public Class Form1
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        
        Dim Isprime As Integer
        Dim str As String
        Dim m As Integer
        Dim n As Integer
        Dim TestLimit As String
        Dim TestPrime As String
        Dim TestNum As String
 
 
        str = ""
        m = InputBox("Enter Upper limit")
        n = InputBox("Enter lower limit")
        If (n > 50) Then
            MsgBox("outside range")
            Exit Sub
 
        End If
 
        '   
        ' Eliminate even numbers
        If TestPrime Mod 2 = 0 Then Exit Sub
        '   Loop through ODD numbers starting with 3
        TestNum = 3
        TestLimit = TestPrime
        Do While TestLimit > TestNum
          for x= TestNum to  TestLimit step 2
            If TestPrime Mod TestNum = 0 Then
                '   Uncomment this if you really want to know
                '   MsgBox "Divisible by " & TestNum   
 
                Exit Sub
            End If
 
            '   There's logic to this.  Think about it.
            TestLimit = TestPrime \ TestNum
 
            '   Remember, we only bother to check odd numbers
            TestNum = TestNum + 2
           if prime then list.additem x
             next x 
        Loop
 
        '   If we made it through the loop, the number is a prime.
 
        Isprime = True
 
 
    End Sub

I have requested the thread moved since you are using vb.net not vb6. Something such as that although, I haven't tested it so I can't be sure.

Thank you for the code!!
you know where you put

"if prime then list.additem x
next x
Loop"

the prime is underlined saying prime is not declared. should i declare it as an integer? and "list.additemx" is underlined as well. do i have to declare x as an integer as well? and it also says that additem is not part of the application. how do i make it part of the application? So many questions sorry lol u have beena great help..:afrog:

Quote:

---

Originally Posted by Nightwalker83

Try this:

vb Code:

Public Class Form1
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        
        Dim Isprime As Integer
        Dim str As String
        Dim m As Integer
        Dim n As Integer
        Dim TestLimit As String
        Dim TestPrime As String
        Dim TestNum As String
 
 
        str = ""
        m = InputBox("Enter Upper limit")
        n = InputBox("Enter lower limit")
        If (n > 50) Then
            MsgBox("outside range")
            Exit Sub
 
        End If
 
        '   
        ' Eliminate even numbers
        If TestPrime Mod 2 = 0 Then Exit Sub
        '   Loop through ODD numbers starting with 3
        TestNum = 3
        TestLimit = TestPrime
        Do While TestLimit > TestNum
          for x= TestNum to  TestLimit step 2
            If TestPrime Mod TestNum = 0 Then
                '   Uncomment this if you really want to know
                '   MsgBox "Divisible by " & TestNum   
 
                Exit Sub
            End If
 
            '   There's logic to this.  Think about it.
            TestLimit = TestPrime \ TestNum
 
            '   Remember, we only bother to check odd numbers
            TestNum = TestNum + 2
           if prime then list.additem x
             next x 
        Loop
 
        '   If we made it through the loop, the number is a prime.
 
        Isprime = True
 
 
    End Sub

I have requested the thread moved since you are using vb.net not vb6. Something such as that although, I haven't tested it so I can't be sure.

---

Should it be Isprime?

oh ya your right. thanks
now for the x variable, it gives me this error
"Error 1 Type of 'x' is ambiguous because the loop bounds and the step clause do not convert to the same type. C:\Users\Sharda\AppData\Local\Temporary Projects\WindowsApplication1\Form1.vb 35 17 WindowsApplication1
"

what should i do?
(:p you are so kind for answering these questions, i will give u the biggest thank you after this!!)

Quote:

---

Originally Posted by Nightwalker83

Should it be Isprime?

---

You need to add the following to your code:

vb Code:

Dim x As Integer

Also, what is testprime? You haven't specified it anywhere.

ah ok

test prime is if someone enters a random number, it is to test if the random number is a prime or not. does that make sense? its giving me a problem in the program also. one problem is popping up after the next. ive been working on this all day i cant get it right. do you have a better way to right the program for what i want? using loops of course. I would really appreciate it.

(i

Quote:

---

Originally Posted by Nightwalker83

You need to add the following to your code:

vb Code:

Dim x As Integer

Also, what is testprime? You haven't specified it anywhere.

---

Is testprime suppose to eqaul m or n? At the moment irt doesn't equal anything.

Try this:

vb Code:

Public Class Form1
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim lb As Integer = 2
        Dim ub As Integer = 50
 
        Dim isPrime As Boolean
 
        '~~~ read the numbers and convert it into integer numbers. Because InputBox() will always return Strings
        lb = Integer.Parse(InputBox("Lower Limit ?"))
        ub = Integer.Parse(InputBox("Upper Limit ?"))
 
        '~~~ Check whether it is in valid range
        If lb < 2 OrElse ub > 50 Then
            MessageBox.Show("Not a valid range !")
            Exit Sub  '~~~ exit from the rest of the code
        End If
 
        '~~~ loop through the numbers between Lower and Upper limits
        For i As Integer = lb To ub
            If Not i Mod 2 = 0 Then     '~~~ if it is even, skip that number
                isPrime = True          '~~~ assume it is true
 
                '~~~ find whether it is divisible by any other number. If so, it is not prime
                For j As Integer = 3 To i - 1
                    If i Mod j = 0 Then
                        isPrime = False     '~~~ if divisible, it is not prime
                        Exit For            '~~~ exit this inner for loop
                    End If
                Next
 
                '~~~ if prime, add it to listbox
                If isPrime = True Then ListBox1.Items.Add(i.ToString)
            End If
            
        Next
 
    End Sub
End Class

I hope the comments included in the code will help you to understand it's working.

BTW, this is a VB.net question. And you have posted in VB6 section. I'll request a mod to move this thread to the appropriate section.

:wave:

Moved to the VB.Net Forum. Thanks for the reports Nightwalker83 and akhileshbc :thumb:

Gary

Quote:

---

Originally Posted by shardooni

oh your right.. thanks

how do i incorporate what u said in the code?

"for x=<an odd number> to <an odd number> step 2"

do i plug in random values where u wrote odd number?

im sorry im really new at this

---

sorry for delay...
just had a huge power cut

the example above is about using n and m

but it expects the the limits ( the things being tested ) to be odd values

so take the the next or previous values from n and m and use them in the loop

if the loop goes from n to m then you only need

Code:

---

'alter n
if n mod 2 = 0 then n=n+1

---

you loop would be

Code:

---

for x=n to m step2
    'test x here to see if prime
    'if x is prime with your function isprime() then add to list
    if isprime(x) then list.additem x
next

---

as simple as that

sorry for confusion

I normally write psuedo code so that you do the thinking and final coding yourself

here to talk

No problem, thanks for replying.
I put what you said in my code, but im getting this error where i wrote " If isprime(x) Then List.additem(x)". it says that the expression "isprime" is not part of the array or method and cannot have an argument list? what did i do wrong now? lol

Code:

---


        Dim str As String
        Dim m As Integer
        Dim n As Integer
        Dim TestLimit As String
        Dim TestPrime As String
        Dim TestNum As String
        Dim x As Integer
        Dim isprime As Integer


        str = ""
        m = InputBox("Enter Upper limit")
        n = InputBox("Enter lower limit")
        If (n > 50) Then
            MsgBox("outside range")
            Exit Sub

        End If

        ' 
        ' Eliminate even numbers
        If n Mod 2 = 0 Then n = n + 1

        '  Loop through ODD numbers starting with 3
        TestNum = 3
        TestLimit = TestPrime
        Do While TestLimit > TestNum
            For x = TestNum To TestLimit Step 2

            Next
            If TestPrime Mod TestNum = 0 Then
                '  Uncomment this if you really want to know
                '  MsgBox "Divisible by " & TestNum 

                Exit Sub
            End If

            '  There's logic to this.  Think about it.
            TestLimit = TestPrime \ TestNum

            '  Remember, we only bother to check odd numbers
            For x = n To m Step 2
                'test x here to see if prime
                'if x is prime with your function isprime() then add to list

                If isprime(x) Then List.additem(x)
            Next
        Loop

        '  If we made it through the loop, the number is a prime.

        Isprime = True

    End Sub

   

End Class

---

Quote:

---

Originally Posted by incidentals

sorry for delay...
just had a huge power cut

the example above is about using n and m

but it expects the the limits ( the things being tested ) to be odd values

so take the the next or previous values from n and m and use them in the loop

if the loop goes from n to m then you only need

Code:

---

'alter n
if n mod 2 = 0 then n=n+1

---

you loop would be

Code:

---

for x=n to m step2
    'test x here to see if prime
    'if x is prime with your function isprime() then add to list
    if isprime(x) then list.additem x
next

---

as simple as that

sorry for confusion

I normally write psuedo code so that you do the thinking and final coding yourself

here to talk

---

your code is getting messed up

you still have not grasped the concepts and are plugging stuff into your code in all the wrong places..

I will have a look at the code in full... and get back to you

Quote:

---

Originally Posted by shardooni

I put what you said in my code, but im getting this error where i wrote " If isprime(x) Then List.additem(x)". it says that the expression "isprime" is not part of the array or method and cannot have an argument list? what did i do wrong now? lol

---

If you put something inside a bracket, it should either be an array or be a function. So, in your code, you tried to use:

Code:

---

If isprime(x) Then List.additem(x)

---

At that line, the compiler can't find an array or function named "isprime". So it pops that error.

In "incidentals"'s last post, he stated this psuedocode:

Code:

---

for x=n to m step2
    'test x here to see if prime
    'if x is prime with your function isprime() then add to list
    if isprime(x) then list.additem x
next

---

That means, he was expecting isprime as a function. So, create a function that will check whether a number passed to it as argument is prime or not. Then loop through the lower and upper limit, and call this function passing the number as argument. So, in the loop you will call the function and if it returns true(ie. the passed number is prime), add it to the ListBox.

:wave:

sorry but my teacher hasnt taught us how to use any of this stuff! its all trial and error for me. he isnt responding to my emails either, so i need to ask people for help. i honestly am really bad at programming, im taking it as a complimentary and i will never do programming again in my life :blush:

Quote:

---

Originally Posted by incidentals

your code is getting messed up

you still have not grasped the concepts and are plugging stuff into your code in all the wrong places..

I will have a look at the code in full... and get back to you

---

Code:

---

       
Public Class Form1

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
       
   
        Dim str As String ' dont know why you want this
        Dim n As Integer 'variable for the lower value
        Dim m As Integer 'variable for the upper value

        Dim TestLimit As String 'type all messed up
        Dim TestPrime As String 'type all messed up
        Dim TestNum As String  'type all messed

        Dim x As Integer 'unnecessary as x is a local counter value but good practice

        Dim isprime As boolean ' no its the result of a function and it will be a boolean


        str = "" 'why do you need this
        n = InputBox("Enter lower limit") ' ok lets getvthe first value
        m = InputBox("Enter Upper limit") ' ok lets get the next value

                                          ' good thats 2 value done, but no tests yet
                                          ' the actual output of the inboxes will be strings but the typing of n and m will cast them to integers

        If (n > 50) Then                  ' this test is inadequate you said n>2 and m<50, why only test >50
            MsgBox("outside range")      'message box should say n too big , still the wrong test though!
            Exit Sub
        End If

'***********************************************************
' the 2 tests your spec setout are

if n<2 then
msgbox("the lower limit is too small - start again")
exit sub
end if

if m>50 then
msgbox("the upperlimit is too big - start again")
exit sub
end if

'***********************************************************

        ' 
        ' Eliminate even numbers
        If n Mod 2 = 0 Then n = n + 1 'this ensures your lowerlimit is odd to start from

        '  Loop through ODD numbers starting with 3  ' what is the lowerlimit was 23

'***********************************************************
'junk the loop

        TestNum = 3
        TestLimit = TestPrime
        Do While TestLimit > TestNum
            For x = TestNum To TestLimit Step 2

            Next
            If TestPrime Mod TestNum = 0 Then
                '  Uncomment this if you really want to know
                '  MsgBox "Divisible by " & TestNum 

                Exit Sub
            End If

            '  There's logic to this.  Think about it.
            TestLimit = TestPrime \ TestNum

            '  Remember, we only bother to check odd numbers
            For x = n To m Step 2
                'test x here to see if prime
                'if x is prime with your function isprime() then add to list

                If isprime(x) Then List.additem(x)
            Next
        Loop

        '  If we made it through the loop, the number is a prime.

        Isprime = True

'end of junk
'*****************************************************************************

'*****************************************************************************
'replacement loop

'  Remember, we only bother to check odd numbers

            For x = n To m Step 2                ' this for does from an odd lowerlimit then +2,+4,+... up to the last odd number before the upperlimit

                'test x here to see if prime
                'if x is prime with your function isprime() then add to list

                If isprime(x) Then List.additem(x) 'if the result of a new function called isprime is true then store the x that was tested in the listbox
            Next                                  ' loop some more


  End Sub


'now you need to write the prime testing function

I give you a start

public function isprime(arg)as boolean

isprime=false

'do your test loop here
'return true if arg is indeed prime with isprime=true

end function

End Class

---

now its your turn read this remove extraneous stuff

understand the way it works and then write the isprime function
and then post it back and we will see where you have got to!

here to talk

Quote:

---

Originally Posted by shardooni

sorry but my teacher hasnt taught us how to use any of this stuff! its all trial and error for me. he isnt responding to my emails either, so i need to ask people for help. i honestly am really bad at programming, im taking it as a complimentary and i will never do programming again in my life :blush:

---

No need to be ashamed of anything. Just follow some some good tutorials, understand it, try it out(in VB)... errors may pop out in some cases.. try to figure out what it says... try to solve it... if not successful, post it here... we'll help you..

:wave:

agreed

he is trying and thats what counts

some people come exspecting to be given the answers

apparently having done nothing but asked

here to talk

Okay I did the last part for the "isprime". Is it right? :confused: this is all so confusing

Code:

---

Public Class Form1

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

        Dim str As String ' dont know why you want this
        Dim n As Integer 'variable for the lower value
        Dim m As Integer 'variable for the upper value

        Dim TestLimit As String 'type all messed up
        Dim TestPrime As String 'type all messed up
        Dim TestNum As String  'type all messed

        Dim x As Integer 'unnecessary as x is a local counter value but good practice

        Dim isprime As Boolean ' no its the result of a function and it will be a boolean


        str = "" 'why do you need this
        n = InputBox("Enter lower limit") ' ok lets getvthe first value
        m = InputBox("Enter Upper limit") ' ok lets get the next value

        ' good thats 2 value done, but no tests yet
        ' the actual output of the inboxes will be strings but the typing of n and m will cast them to integers


        '***********************************************************
        ' the 2 tests your spec setout are

        If n < 2 Then
            msgbox("the lower limit is too small - start again")
            Exit Sub
        End If

        If m > 50 Then
            msgbox("the upperlimit is too big - start again")
            Exit Sub
        End If

        '***********************************************************

        ' 
        ' Eliminate even numbers
        If n Mod 2 = 0 Then n = n + 1 'this ensures your lowerlimit is odd to start from

        '  Loop through ODD numbers starting with 3  ' what is the lowerlimit was 23

        '***********************************************************
        'junk the loop

        TestNum = 3
        TestLimit = TestPrime
        Do While TestLimit > TestNum
            For x = TestNum To TestLimit Step 2

            Next
            If TestPrime Mod TestNum = 0 Then
                '  Uncomment this if you really want to know
                '  MsgBox "Divisible by " & TestNum 

                Exit Sub
            End If

            '  There's logic to this.  Think about it.
            TestLimit = TestPrime \ TestNum

            '  Remember, we only bother to check odd numbers
            For x = n To m Step 2
                'test x here to see if prime
                'if x is prime with your function isprime() then add to list

                If isprime(x) Then List.additem(x)
            Next
        Loop

        '  If we made it through the loop, the number is a prime.

        Isprime = True

        'end of junk
        '*****************************************************************************

        '*****************************************************************************
        'replacement loop

        '  Remember, we only bother to check odd numbers

        For x = n To m Step 2                ' this for does from an odd lowerlimit then +2,+4,+... up to the last odd number before the upperlimit

            'test x here to see if prime
            'if x is prime with your function isprime() then add to list
       
        Next
            If isprime(x) Then List.additem(x) 'if the result of a new function called isprime is true then store the x that was tested in the listbox
        ' loop some more
       

    End Sub


    'now you need to write the prime testing function

    'I give you a start

    Public Function isprime(ByVal arg) As Boolean

        Dim n As Integer

              isprime = True            ' testing as long as true
        Dim F As Integer
        For F = 2 To n - 1
            If n Mod F = 0 Then
                isprime = False  ' divisible
                Exit For
            End If
        Next
        If isprime = True Then    ' Prime found

        End If

       

    End Function

End Class

---

Quote:

---

Originally Posted by incidentals

Code:

---

       
Public Class Form1

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
       
   
        Dim str As String ' dont know why you want this
        Dim n As Integer 'variable for the lower value
        Dim m As Integer 'variable for the upper value

        Dim TestLimit As String 'type all messed up
        Dim TestPrime As String 'type all messed up
        Dim TestNum As String  'type all messed

        Dim x As Integer 'unnecessary as x is a local counter value but good practice

        Dim isprime As boolean ' no its the result of a function and it will be a boolean


        str = "" 'why do you need this
        n = InputBox("Enter lower limit") ' ok lets getvthe first value
        m = InputBox("Enter Upper limit") ' ok lets get the next value

                                          ' good thats 2 value done, but no tests yet
                                          ' the actual output of the inboxes will be strings but the typing of n and m will cast them to integers

        If (n > 50) Then                  ' this test is inadequate you said n>2 and m<50, why only test >50
            MsgBox("outside range")      'message box should say n too big , still the wrong test though!
            Exit Sub
        End If

'***********************************************************
' the 2 tests your spec setout are

if n<2 then
msgbox("the lower limit is too small - start again")
exit sub
end if

if m>50 then
msgbox("the upperlimit is too big - start again")
exit sub
end if

'***********************************************************

        ' 
        ' Eliminate even numbers
        If n Mod 2 = 0 Then n = n + 1 'this ensures your lowerlimit is odd to start from

        '  Loop through ODD numbers starting with 3  ' what is the lowerlimit was 23

'***********************************************************
'junk the loop

        TestNum = 3
        TestLimit = TestPrime
        Do While TestLimit > TestNum
            For x = TestNum To TestLimit Step 2

            Next
            If TestPrime Mod TestNum = 0 Then
                '  Uncomment this if you really want to know
                '  MsgBox "Divisible by " & TestNum 

                Exit Sub
            End If

            '  There's logic to this.  Think about it.
            TestLimit = TestPrime \ TestNum

            '  Remember, we only bother to check odd numbers
            For x = n To m Step 2
                'test x here to see if prime
                'if x is prime with your function isprime() then add to list

                If isprime(x) Then List.additem(x)
            Next
        Loop

        '  If we made it through the loop, the number is a prime.

        Isprime = True

'end of junk
'*****************************************************************************

'*****************************************************************************
'replacement loop

'  Remember, we only bother to check odd numbers

            For x = n To m Step 2                ' this for does from an odd lowerlimit then +2,+4,+... up to the last odd number before the upperlimit

                'test x here to see if prime
                'if x is prime with your function isprime() then add to list

                If isprime(x) Then List.additem(x) 'if the result of a new function called isprime is true then store the x that was tested in the listbox
            Next                                  ' loop some more


  End Sub


'now you need to write the prime testing function

I give you a start

public function isprime(arg)as boolean

isprime=false

'do your test loop here
'return true if arg is indeed prime with isprime=true

end function

End Class

---

now its your turn read this remove extraneous stuff

understand the way it works and then write the isprime function
and then post it back and we will see where you have got to!

here to talk

---

I don't see you assigning a value to "n" in the IsPrime function.

Should "n = arg?"

Or do you want to use "arg - 1" instead of "n - 1?"

ahh I have no idea what arg means !! SEriously im really new at this, its like learning a new language and right now Im a newbie :o

Quote:

---

Originally Posted by vbfbryce

I don't see you assigning a value to "n" in the IsPrime function.

Should "n = arg?"

Or do you want to use "arg - 1" instead of "n - 1?"

---

thanks you guys. oh and by the way I am a girl :p not a guy

ok so i think i finally got the "isprime" part correct.. now im still getting errors with the code for adding the primes to the list box. I don't understand what I am suppose to put? ;/ its driving me insane! :mad:

Code:

---

Public Class Form1

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click


        Dim n As Integer 'variable for the lower value
        Dim m As Integer 'variable for the upper value
        Dim TestLimit As Integer 'type all messed up
        Dim TestPrime As Integer 'type all messed up
        Dim TestNum As Integer  'type all messed
        Dim isprime As Boolean ' no its the result of a function and it will be a boolean


        n = InputBox("Enter lower limit") ' ok lets getvthe first value
        m = InputBox("Enter Upper limit") ' ok lets get the next value

        ' good thats 2 value done, but no tests yet
        ' the actual output of the inboxes will be strings but the typing of n and m will cast them to integers




        If n < 2 Then
            msgbox("the lower limit is too small - start again")
            Exit Sub
        End If

        If m > 50 Then
            msgbox("the upperlimit is too big - start again")
            Exit Sub
        End If



     
        If n Mod 2 = 0 Then n = n + 1 'this ensures your lowerlimit is odd to start from

        '  Loop through ODD numbers starting with 3  ' what is the lowerlimit was 23

        '***********************************************************
        'junk the loop

        TestNum = 3
        TestLimit = TestPrime
        Do While TestLimit > TestNum
            For x = TestNum To TestLimit Step 2

            Next
            If TestPrime Mod TestNum = 0 Then
                Exit Sub
            End If


            TestLimit = TestPrime \ TestNum

            '' to check odd numbers
            For x = n To m Step 2
                'test x here to see if prime
                'if x is prime with your function isprime() then add to list

                If isprime(x) Then List.additem(x)
            Next
        Loop

        '  If we made it through the loop, the number is a prime.

        Isprime = True

        'end of junk
        '*****************************************************************************

        '*****************************************************************************
        'replacement loop

        '  Remember, we only bother to check odd numbers

        For x = n To m Step 2                ' this for does from an odd lowerlimit then +2,+4,+... up to the last odd number before the upperlimit

       
       
        Next
          If isprime(x) Then List.additem(x) 'if the result of a new function called isprime is true then store the x that was tested in the listbox
        ' loop some more
       

    End Sub

    Public Function isprime(ByVal arg) As Boolean
        Dim n As Integer

        Dim ReturnValue As Boolean = True
        For F = 2 To n
            If n Mod F = 0 Then
                ReturnValue = False
                Exit For
            End If
        Next
        Return ReturnValue
    End Function



End Class

---

Quote:

---

Originally Posted by akhileshbc

No need to be ashamed of anything. Just follow some some good tutorials, understand it, try it out(in VB)... errors may pop out in some cases.. try to figure out what it says... try to solve it... if not successful, post it here... we'll help you..

:wave:

---

Quote:

---

Originally Posted by incidentals

agreed

he is trying and thats what counts

some people come exspecting to be given the answers

apparently having done nothing but asked

here to talk

---

Hello, I am posting a totally new solution. I did not use previous post's code. This has some advantages I will describe

vb.net Code:

'Find Prime Numbers
 
    Private Sub btnGo_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnGo.Click
        Dim N As Integer
        Dim M As Integer
        Dim isPrime As Boolean
 
        N = (CInt(txtLower.Text) \ 2) * 2 + 1
        M = CInt(txtUpper.Text)
        If N < 2 Then N = 3
 
        lstPrimes.Items.Clear()
 
        For i = N To M Step 2
            isPrime = True
            For j = 0 To lstPrimes.Items.Count - 1
                If i Mod lstPrimes.Items(j) = 0 Then
                    isPrime = False
                    Exit For
                End If
            Next
            If isPrime Then lstPrimes.Items.Add(i)
        Next
 
    End Sub

First. Instead of inputboxes I used a couple of textBoxes named txtLower and txtUpper, and a button (btnGo) that starts the process.

the formula (CInt(txtLower.Text) \ 2) * 2 + 1 makes sure your number is an odd integer. That is not necesary for the upper limit.

I did not put a limit on the upper value as you can find primes for any range, but can't start before 2 as 1 may mess things up. The first prime is 2 so there is no need for the message when the lower is less than 2, just start at 2

The interesting thing in this loop is that it does not test against all available numbers but just against found primes, as there is no need to test for other numbers because all of them are multiples of primes. This makes the loop more efficient. Also, if an exact division is found, the for stops and continues with next value.

i Mod lstPrimes.Items(j) is enaugh to see if it can be divided exactly.

This code does not test against Alpha input so it can mess the process, but as a case study it does what it needs to do.

you still have not fixed the main sub routine

and the test function is being called outwide the loop

we will enevtually get rid of the junk
and end up with a cleaner piece of code

the problem her that too many people are making odd ( unfortunately not helpful comments)

the word arg is in the brackets of the declaration of the function isprime()

so it is a value passed to the function and used within it to do what ever task you have specified!

when we call the function isprime(arg) we use the variable that we want to test
in your case the variable is the X thing from the loop

we want to test x to see if it is a prime
the test is exhaustive as the definition of a prime is a value that can be devided by itsself and 1 only!

so lets look at you atempt at ISPRIME()

Code:

---

Public Function isprime(ByVal arg) As Boolean
        Dim n As Integer ' why have you introduced n here - do you think it is the upperlimit
                              ' if we needed it I would have suggested passing more than one argument

        Dim ReturnValue As Boolean = True ' what is the purpose of this declaration - is it to allow us to  pass the answer back?
                                                      ' answers (returned values) are returned in the name of the function
                                                      ' so when we are ready the result will be isprime=<something>

        For F = 2 To n  ' ok so now your cooking you are about to test from 2 to n the upper limit
                            ' whoops this function was never passed the upper limit
                            ' why would you pass 50 to test and see if lets say 9 was a prime number?
           
        If n Mod F = 0 Then ' right a test , good thinking, but what are you testing?
                                    ' i see its that fictitious upperlimit again ( aargh!)

                ReturnValue = False 'yup thats the right output, but in the wrong place
                                            'we don't have a function called returnvalue
                                            'its called ISPRIME
                Exit For                  'and yes cancel any more test once devisable it cannot be prime
            End If
        Next
        Return ReturnValue 'this is not need as you will have already filled the variable with the result
                                  'the function is waiting to end
    End Function

---

my comments are true for the vast number of languages that support function, true some of them use return <something> to send back the result, but vb does not ( although i'm not sure about .net)

anyway...

good attempt now lets fix it

Code:

---

Public Function isprime(ByVal arg) As Boolean

        isprime = True        ' the default state, all numbers are prime unless they are not
                                  ' no need for "if else"
        For F = 2 To arg-1  ' arg is the value you want to test we know it devides itself
                                  ' can anything smaller devide it

            If arg Mod F = 0 Then ' if a lesser value can devide it
                isprime = False  'set the return value to false (this arg is not a prime)
                Exit for ' you might be able to "exit function" here as well i cant test iam on wrong machine!
            End If
        Next            ' get the next smaller value to test arg with
 
    ' the isprime return value was set true to start and false when arg was devisable
    End Function

---

hope that makes more sense
now for the final bit cleaning up the main sub

Code:

---

Public Class Form1

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click


        Dim n As Integer 'variable for the lower value
        Dim m As Integer 'variable for the upper value
   

        n = InputBox("Enter lower limit") ' ok lets getvthe first value
        m = InputBox("Enter Upper limit") ' ok lets get the next value

        ' good thats 2 value done, but no tests yet

        ' the actual output of the inboxes will be strings but the typing of n and m will cast them to integers

        If n < 2 Then
            msgbox("the lower limit is too small - start again")
            Exit Sub
        End If

        If m > 50 Then
            msgbox("the upperlimit is too big - start again")
            Exit Sub
        End If

        If n Mod 2 = 0 Then n = n + 1 'this ensures your lowerlimit is odd to start from

        'replacement loop

        '  Remember, we only bother to check odd numbers

        For x = n To m Step 2                ' this for does from an odd lowerlimit then +2,+4,+... up to the last odd number before the upperlimit

       
                  If isprime(x) Then List.additem(x) 'if the result of a new function called isprime is true then store the x that was tested in the listbox
 
        Next
        ' loop some more
       

    End Sub

'put isprime() function definition here

End Class

---

now is that crystal clear now

you may have to fix syntax
i am writting this in text only
dont have a mcchine here to try anything on

sorry

hope this has helped

please rate this and my other posts if the have helped

ask even if there is the tiniest bit you don't understand

nice to see your a girly programmer

sorry if the he hurt but its english for you dont know the gender

here to talk

kaliman

she is having enough problems with her code in terms of understanding

and while you code is fine, it does not address her absolute criterior

and you introduce even more complexity ( looks simpler but is more difficult to full appreciate at her level

maybe she can come back to it with a little more understanding

here to talk

My bad. The code I posted fails if you put a lower limit higher than 2 because it does not check for the first primes. Working on that, will post in a few minutes. Will try to make it again without advanced logic.

once all of the coments are removed it looks way simpler

but with out the comments it is just and solution with no

knowledge transfer

here to talk

Here is a working, simpler code (simpler than my previous one). As Insidentals says, this may be a little bit more advanced, but I think it is clear. Look into it, you may find it helpfull

vb.net Code:

'Find primes
 
    Private Sub btnGo_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnGo.Click
        Dim N As Integer 'Lower Limit
        Dim M As Integer 'Upper Limit
        Dim isPrime As Boolean 'Flag to check if it is prime
 
        'Clear the list for a new set
        lstPrimes.Items.Clear()
 
        'Input both limits
        N = InputBox("Enter Lower limit")
        M = InputBox("Enter Upper Limit")
 
        'The first prime is 2, the only pair, so if the user inputs 2 or less 
        'on the lower limit then add the 2 to the list and set the limit to 3 
        'to star testing from there
        If N < 3 Then
            lstPrimes.Items.Add(2)
            N = 3
        End If
 
        'Make sure N is an odd number. 
        If N Mod 2 = 0 Then N += 1
 
        'i would be the tested number, only odd (step 2)
        For i = N To M Step 2
            'Start with the assumption that the number is prime
            isPrime = True
            'j will be the divisor. There is never a need to test against a number 
            'higher than half the value (i/2), and only odd numbers (step 2)
            For j = 3 To i / 2 Step 2
                'If the module (residual) is 0, then it can be exactly divided
                If i Mod j = 0 Then
                    'If it can be divided then it is not prime
                    isPrime = False
                    'If you know it is not a prime, no need to test the rest of divisors
                    Exit For
                End If
            Next j
            'If its a prime add it to the list
            If isPrime Then lstPrimes.Items.Add(i)
        Next i
 
    End Sub

Kaliman makes and observation that i had missed

he is correctly states that the test does not need to go right upto the value being tested

but instead only half way

so function isprime can work much faster ( potentially twice as fast ) if we change the limits of the test loop

and the divisible by 2 doenot need to be tested as the candidates are all odd

so

For F = 2 To arg-1 ' arg is the value you want to test we know it devides itself
' can anything smaller devide it

'becomes

For F = 3 To arg/2 ' arg is the value you want to test we know it devides itself
' can anything half its size or less devide it

this is looking better all the time

here to talk

Quote:

---

Originally Posted by incidentals

Kaliman makes and observation that i had missed

he is correctly states that the test does not need to go right upto the value being tested

but instead only half way

so function isprime can work much faster ( potentially twice as fast ) if we change the limits of the test loop

so

For F = 2 To arg-1 ' arg is the value you want to test we know it devides itself
' can anything smaller devide it

'becomes

For F = 2 To arg/2 ' arg is the value you want to test we know it devides itself
' can anything half its size or less devide it

this is looking better all the time

here to talk

---

Since you are not testing even numbers it is not necesary to test VS 2 either. So your loop should be

For F = 3 to arg/2

there is nothing wrong with putting the whole problem into a single sub routine as kaliman does

but it is good practice to create functions that you can use again in other solutions

and for the newbies it is good to understand a little more about the programming world they have entered

here to talk

Quote:

---

Originally Posted by incidentals

there is nothing wrong with putting the whole problem into a single sub routine as kaliman does

but it is good practice to create functions that you can use again in other solutions

and for the newbies it is good to understand a little more about the programming world they have entered

here to talk

---

I totally agree, that is the principle of a good structured program. This may be a simple algorithm, but you should try to structure procedures and functions as Incidentals suggests.

Thank you sooo much for helping me out.
NOW A NEW ERROR POPS UP! when i start debugging it gives me this and says that the method or operation is not implemented. what is that suppose to mean? :


Class List

Shared Sub additem(ByVal x As Object)
Throw New NotImplementedException
End Sub

End Class

Quote:

---

Originally Posted by incidentals

you still have not fixed the main sub routine

and the test function is being called outwide the loop

we will enevtually get rid of the junk
and end up with a cleaner piece of code

the problem her that too many people are making odd ( unfortunately not helpful comments)

the word arg is in the brackets of the declaration of the function isprime()

so it is a value passed to the function and used within it to do what ever task you have specified!

when we call the function isprime(arg) we use the variable that we want to test
in your case the variable is the X thing from the loop

we want to test x to see if it is a prime
the test is exhaustive as the definition of a prime is a value that can be devided by itsself and 1 only!

so lets look at you atempt at ISPRIME()

Code:

---

Public Function isprime(ByVal arg) As Boolean
        Dim n As Integer ' why have you introduced n here - do you think it is the upperlimit
                              ' if we needed it I would have suggested passing more than one argument

        Dim ReturnValue As Boolean = True ' what is the purpose of this declaration - is it to allow us to  pass the answer back?
                                                      ' answers (returned values) are returned in the name of the function
                                                      ' so when we are ready the result will be isprime=<something>

        For F = 2 To n  ' ok so now your cooking you are about to test from 2 to n the upper limit
                            ' whoops this function was never passed the upper limit
                            ' why would you pass 50 to test and see if lets say 9 was a prime number?
           
        If n Mod F = 0 Then ' right a test , good thinking, but what are you testing?
                                    ' i see its that fictitious upperlimit again ( aargh!)

                ReturnValue = False 'yup thats the right output, but in the wrong place
                                            'we don't have a function called returnvalue
                                            'its called ISPRIME
                Exit For                  'and yes cancel any more test once devisable it cannot be prime
            End If
        Next
        Return ReturnValue 'this is not need as you will have already filled the variable with the result
                                  'the function is waiting to end
    End Function

---

my comments are true for the vast number of languages that support function, true some of them use return <something> to send back the result, but vb does not ( although i'm not sure about .net)

anyway...

good attempt now lets fix it

Code:

---

Public Function isprime(ByVal arg) As Boolean

        isprime = True        ' the default state, all numbers are prime unless they are not
                                  ' no need for "if else"
        For F = 2 To arg-1  ' arg is the value you want to test we know it devides itself
                                  ' can anything smaller devide it

            If arg Mod F = 0 Then ' if a lesser value can devide it
                isprime = False  'set the return value to false (this arg is not a prime)
                Exit for ' you might be able to "exit function" here as well i cant test iam on wrong machine!
            End If
        Next            ' get the next smaller value to test arg with
 
    ' the isprime return value was set true to start and false when arg was devisable
    End Function

---

hope that makes more sense
now for the final bit cleaning up the main sub

Code:

---

Public Class Form1

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click


        Dim n As Integer 'variable for the lower value
        Dim m As Integer 'variable for the upper value
   

        n = InputBox("Enter lower limit") ' ok lets getvthe first value
        m = InputBox("Enter Upper limit") ' ok lets get the next value

        ' good thats 2 value done, but no tests yet

        ' the actual output of the inboxes will be strings but the typing of n and m will cast them to integers

        If n < 2 Then
            msgbox("the lower limit is too small - start again")
            Exit Sub
        End If

        If m > 50 Then
            msgbox("the upperlimit is too big - start again")
            Exit Sub
        End If

        If n Mod 2 = 0 Then n = n + 1 'this ensures your lowerlimit is odd to start from

        'replacement loop

        '  Remember, we only bother to check odd numbers

        For x = n To m Step 2                ' this for does from an odd lowerlimit then +2,+4,+... up to the last odd number before the upperlimit

       
                  If isprime(x) Then List.additem(x) 'if the result of a new function called isprime is true then store the x that was tested in the listbox
 
        Next
        ' loop some more
       

    End Sub

'put isprime() function definition here

End Class

---

now is that crystal clear now

you may have to fix syntax
i am writting this in text only
dont have a mcchine here to try anything on

sorry

hope this has helped

please rate this and my other posts if the have helped

ask even if there is the tiniest bit you don't understand

nice to see your a girly programmer

sorry if the he hurt but its english for you dont know the gender

here to talk

---

This works!! and I actually understand what you were doing in each step. THANK YOU! thank you for the explanation, you guys are so helpful and patient.. what a great forum :)

Quote:

---

Originally Posted by kaliman79912

Here is a working, simpler code (simpler than my previous one). As Insidentals says, this may be a little bit more advanced, but I think it is clear. Look into it, you may find it helpfull

vb.net Code:

'Find primes
 
    Private Sub btnGo_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnGo.Click
        Dim N As Integer 'Lower Limit
        Dim M As Integer 'Upper Limit
        Dim isPrime As Boolean 'Flag to check if it is prime
 
        'Clear the list for a new set
        lstPrimes.Items.Clear()
 
        'Input both limits
        N = InputBox("Enter Lower limit")
        M = InputBox("Enter Upper Limit")
 
        'The first prime is 2, the only pair, so if the user inputs 2 or less 
        'on the lower limit then add the 2 to the list and set the limit to 3 
        'to star testing from there
        If N < 3 Then
            lstPrimes.Items.Add(2)
            N = 3
        End If
 
        'Make sure N is an odd number. 
        If N Mod 2 = 0 Then N += 1
 
        'i would be the tested number, only odd (step 2)
        For i = N To M Step 2
            'Start with the assumption that the number is prime
            isPrime = True
            'j will be the divisor. There is never a need to test against a number 
            'higher than half the value (i/2), and only odd numbers (step 2)
            For j = 3 To i / 2 Step 2
                'If the module (residual) is 0, then it can be exactly divided
                If i Mod j = 0 Then
                    'If it can be divided then it is not prime
                    isPrime = False
                    'If you know it is not a prime, no need to test the rest of divisors
                    Exit For
                End If
            Next j
            'If its a prime add it to the list
            If isPrime Then lstPrimes.Items.Add(i)
        Next i
 
    End Sub

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I will keep this in mind. I am so pleased with your help. Thanks a bunch. :p

Quote:

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Originally Posted by incidentals

there is nothing wrong with putting the whole problem into a single sub routine as kaliman does

but it is good practice to create functions that you can use again in other solutions

and for the newbies it is good to understand a little more about the programming world they have entered

here to talk

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glad she has got it

didn't we do well

the problem with my code is its for vb6 not net

so
listbox.additem x
is now
listbox.items.add x

oh well

btw kaliman the mod operator is the modulus (the remainder due to whole number division) not the module

I have this need to teach... sorry

now if she would just remember to rate out posts

here to talk

hahaha yes thanks so much for sticking with me through this journey of mine incidentals :p i rated all your guys posts, with good ratings of course :) i have even harder programs to make starting tomorrow. I know I will need more help.

Quote:

---

Originally Posted by incidentals

glad she has got it

didn't we do well

the problem with my code is its for vb6 not net

so
listbox.additem x
is now
listbox.items.add x

oh well

btw kaliman the mod operator is the modulus (the remainder due to whole number division) not the module

I have this need to teach... sorry

now if she would just remember to rate out posts

here to talk

---

Quote:

---

Originally Posted by incidentals

Kaliman makes and observation that i had missed

he is correctly states that the test does not need to go right upto the value being tested

but instead only half way

---

Nice catch Khali... :thumb:

Me too missed that one. :)

@shardooni: Glad to hear that you got the solution. It would be a good thing if you have marked this thread as RESOLVED. You could use the "Thread Tools" found at the top of this page and choose "Mark as RESOLVED" or you could edit the very first post and add "[RESOLVED]" in the beginning of the title. The reason is that, when others view your thread, they could easily understand that you have got it solved.

:wave:

here's my solution:

vb Code:

Dim uBound As Integer = 999
Dim primes() As Integer = Enumerable.Range(2, uBound - 1).Where(Function(i1) i1 = 2 OrElse Not Enumerable.Range(2, i1 - 3).Any(Function(i2) i1 Mod i2 = 0)).ToArray
ListBox1.Items.AddRange(Array.ConvertAll(primes, Function(x) x.ToString))

best thing to do then is

set out you requirements

and make a little plan of action and then we can help you build it from the
start

outline plan and fill in bit by bit

here to talk

here's an improved Prime number finder:

http://www.vbforums.com/showthread.p...45#post4108845

Quote:

---

Originally Posted by incidentals

best thing to do then is

set out you requirements

and make a little plan of action and then we can help you build it from the
start

outline plan and fill in bit by bit

here to talk

---

soundss great. should I post the new part for this assignment in another thread or can I just talk to you guys through this one? The next part of the assignment involves embedding another loop of primes into the loop the first "prime number" loop.

okay so the flow chart for the next part is called " simple prime sifter". i have to add this part INSIDE the loop of the first prime number loop you guys helped me on. I am going to attach a picture of the loop to this message. I am going to start working on it meanwhile, but hopefully you guys can help me out with how to start off. flow chart

http://tinypic.com/r/23ux2mh/5

heres the pic. sorry i dont know why those others pic i posted arent working
flow chart

i have a feeling that flow chart is the code we just did yesterday !! is it?

Quote:

---

Originally Posted by incidentals

Kaliman makes and observation that i had missed

he is correctly states that the test does not need to go right upto the value being tested

but instead only half way

so function isprime can work much faster ( potentially twice as fast ) if we change the limits of the test loop

and the divisible by 2 doenot need to be tested as the candidates are all odd

---

I realize I'm late to the party, but in case this thread ever gets read by anyone else, I wanted to point out a couple items.

First, the method described for primality testing in this thread is a variant of what is called the Sieve of Eratosthenes. This technique is fine for determining if small numbers are prime, but is horrible for large numbers (by which I mean the kinds of prime numbers used in modern encryption algorithms). For those kinds of numbers you should use other methods.

That said, when using the Sieve, you only need to count up to the square root of the number being tested, not half the number, which can make a significant difference. Thus, line 25 in akhileshbc's last example of code should be:

Code:

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For j As Integer = 3 To Math.Floor(Math.Sqrt(i))

---

Thanks for sharing it with us Lenggries :wave:

Quote:

---

Originally Posted by incidentals

glad she has got it

didn't we do well

the problem with my code is its for vb6 not net

so
listbox.additem x
is now
listbox.items.add x

oh well

btw kaliman the mod operator is the modulus (the remainder due to whole number division) not the module

I have this need to teach... sorry

now if she would just remember to rate out posts

here to talk

---

Thank you, its kind of embarassing, but keep in mind English is not my native language, in Spanish is "Modulo", which translates both to "Modulus" and "Module".