Programming Forums > Application Development > Visual Basic

Johnny_English · Sep 29th, 2004, 9:24 AM

Public Function primechk(max As Integer)
I'm still writing the prime number algorithm and so far I have this code for my function:

(Toggle Plain Text)

Dim primes() As Integer
ReDim primes(0)
primes(0) = 2
For x = 3 To max
  bound = UBound(primes)
  For index = 0 To bound
    If x Mod primes(index) = 0 Then
      Exit For
    End If
    indices = index
  Next index
  If indices = bound Then
    ReDim Preserve primes(bound + 1)
    primes(bound + 1) = x
  End If
  
Next x
  Print primes(bound)
End Function

It works fine, but I'm still optimizing, before I was using this:

(Toggle Plain Text)

For index = 0 To bound
    If x Mod primes(index) = 0 Then
      Exit For
    End If
    If index = bound Then
       ReDim Preserve primes(bound + 1)
       primes(bound + 1) = x
    End If
Next index

But I realized that checking the index each iteration was very inefficient, so I decided to put the If then statement outside the embeded for loop:

(Toggle Plain Text)

Next index
  If index = bound Then
    ReDim Preserve primes(bound + 1)
    primes(bound + 1) = x
  End If
  
Next x

Unfortunately, the program was not working... After a quick print test with the variable index, I came to the realization that it was destroyed as soon as the loop exited. So I decided to use the sligtly more efficient

(Toggle Plain Text)

For index = 0 To bound
    If x Mod primes(index) = 0 Then
      Exit For
    End If
    indices = index
  Next index

It's deffinetly better than repeating the if then and comparisons. But I still feel that the assignment is taking up too much time. In order to make this program truly optimized I want to reuse index.

How do I keep the variable index from being destroyed after the loop is exited so I can reuse its value later?

Thanks!

Berto · Sep 29th, 2004, 10:09 AM

If ou declare a variable inside a loop it only remains for the scope of the
loop, so i would decalre it at the start of the method or make it a gloabal variabel <--- not a good idea to do that one,.

Sep 29th, 2004, 9:24 AM	#1
Johnny_English Newbie Join Date: Sep 2004 Posts: 7 Rep Power: 0	Public Function primechk(max As Integer) I'm still writing the prime number algorithm and so far I have this code for my function: (Toggle Plain Text) Dim primes() As Integer ReDim primes(0) primes(0) = 2 For x = 3 To max bound = UBound(primes) For index = 0 To bound If x Mod primes(index) = 0 Then Exit For End If indices = index Next index If indices = bound Then ReDim Preserve primes(bound + 1) primes(bound + 1) = x End If Next x Print primes(bound) End Function Dim primes() As Integer ReDim primes(0) primes(0) = 2 For x = 3 To max bound = UBound(primes) For index = 0 To bound If x Mod primes(index) = 0 Then Exit For End If indices = index Next index If indices = bound Then ReDim Preserve primes(bound + 1) primes(bound + 1) = x End If Next x Print primes(bound) End Function It works fine, but I'm still optimizing, before I was using this: (Toggle Plain Text) For index = 0 To bound If x Mod primes(index) = 0 Then Exit For End If If index = bound Then ReDim Preserve primes(bound + 1) primes(bound + 1) = x End If Next index For index = 0 To bound If x Mod primes(index) = 0 Then Exit For End If If index = bound Then ReDim Preserve primes(bound + 1) primes(bound + 1) = x End If Next index But I realized that checking the index each iteration was very inefficient, so I decided to put the If then statement outside the embeded for loop: (Toggle Plain Text) Next index If index = bound Then ReDim Preserve primes(bound + 1) primes(bound + 1) = x End If Next x Next index If index = bound Then ReDim Preserve primes(bound + 1) primes(bound + 1) = x End If Next x Unfortunately, the program was not working... After a quick print test with the variable index, I came to the realization that it was destroyed as soon as the loop exited. So I decided to use the sligtly more efficient (Toggle Plain Text) For index = 0 To bound If x Mod primes(index) = 0 Then Exit For End If indices = index Next index For index = 0 To bound If x Mod primes(index) = 0 Then Exit For End If indices = index Next index It's deffinetly better than repeating the if then and comparisons. But I still feel that the assignment is taking up too much time. In order to make this program truly optimized I want to reuse index. How do I keep the variable index from being destroyed after the loop is exited so I can reuse its value later? Thanks!

Sep 29th, 2004, 10:09 AM	#2
Berto Programming Guru Join Date: Aug 2004 Posts: 1,022 Rep Power: 6	If ou declare a variable inside a loop it only remains for the scope of the loop, so i would decalre it at the start of the method or make it a gloabal variabel <--- not a good idea to do that one,. __________________ "Put your hand on a hot stove for a minute, and it seems like an hour. Sit with a pretty girl for an hour, and it seems like a minute. THAT'S relativity." - Albert Einstein

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Programming Forums > Application Development > Visual Basic

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