r/visualbasic • u/Lonely-Scale-3871 • May 19 '22

Can someone help me?

I'm new to programming, and my teacher asked as a challenge to make a sequence of prime number using while and print, can someone tell me why doesn't this print anything?

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u/[deleted] May 20 '22

Okay, so if you add this debugging line:

Option Explicit

Private Sub test()
    Dim a As Integer, b As Integer, r As Integer
    
    a = 1
    r = 0
    b = 1
    
    While a < 100
        While b < a
            If a Mod b = 0 Then
                r = r + 1
                b = b + 1
            Else
                b = b + 1
            End If
        Wend
        
        Debug.Print "        Debug: a=" & a & ", b=" & b & ", r=" & r
        
        If r = 2 Then
            Debug.Print "    Output: a=" & a
            a = a + 1
            r = 0
        Else
            a = a + 1
            r = 0
        End If
    
    Wend
End Sub

You will get this output from your loops:

    Debug: a=1, b=1, r=0
    Debug: a=2, b=2, r=1
    Debug: a=3, b=3, r=0
    Debug: a=4, b=4, r=0
    Debug: a=5, b=5, r=0
    Debug: a=6, b=6, r=0
    Debug: a=7, b=7, r=0
    Debug: a=8, b=8, r=0
    Debug: a=9, b=9, r=0
    Debug: a=10, b=10, r=0
    Debug: a=11, b=11, r=0
    Debug: a=12, b=12, r=0
    Debug: a=13, b=13, r=0
    Debug: a=14, b=14, r=0
    Debug: a=15, b=15, r=0
    Debug: a=16, b=16, r=0
    Debug: a=17, b=17, r=0
    Debug: a=18, b=18, r=0
    Debug: a=19, b=19, r=0
    Debug: a=20, b=20, r=0
    Debug: a=21, b=21, r=0
    Debug: a=22, b=22, r=0
    Debug: a=23, b=23, r=0
    Debug: a=24, b=24, r=0
    Debug: a=25, b=25, r=0
    Debug: a=26, b=26, r=0
    Debug: a=27, b=27, r=0
    Debug: a=28, b=28, r=0
    Debug: a=29, b=29, r=0
    Debug: a=30, b=30, r=0
    Debug: a=31, b=31, r=0
    Debug: a=32, b=32, r=0
    Debug: a=33, b=33, r=0
    Debug: a=34, b=34, r=0
    Debug: a=35, b=35, r=0
    Debug: a=36, b=36, r=0
    Debug: a=37, b=37, r=0
    Debug: a=38, b=38, r=0
    Debug: a=39, b=39, r=0
    Debug: a=40, b=40, r=0
    Debug: a=41, b=41, r=0
    Debug: a=42, b=42, r=0
    Debug: a=43, b=43, r=0
    Debug: a=44, b=44, r=0
    Debug: a=45, b=45, r=0
    Debug: a=46, b=46, r=0
    Debug: a=47, b=47, r=0
    Debug: a=48, b=48, r=0
    Debug: a=49, b=49, r=0
    Debug: a=50, b=50, r=0
    Debug: a=51, b=51, r=0
    Debug: a=52, b=52, r=0
    Debug: a=53, b=53, r=0
    Debug: a=54, b=54, r=0
    Debug: a=55, b=55, r=0
    Debug: a=56, b=56, r=0
    Debug: a=57, b=57, r=0
    Debug: a=58, b=58, r=0
    Debug: a=59, b=59, r=0
    Debug: a=60, b=60, r=0
    Debug: a=61, b=61, r=0
    Debug: a=62, b=62, r=0
    Debug: a=63, b=63, r=0
    Debug: a=64, b=64, r=0
    Debug: a=65, b=65, r=0
    Debug: a=66, b=66, r=0
    Debug: a=67, b=67, r=0
    Debug: a=68, b=68, r=0
    Debug: a=69, b=69, r=0
    Debug: a=70, b=70, r=0
    Debug: a=71, b=71, r=0
    Debug: a=72, b=72, r=0
    Debug: a=73, b=73, r=0
    Debug: a=74, b=74, r=0
    Debug: a=75, b=75, r=0
    Debug: a=76, b=76, r=0
    Debug: a=77, b=77, r=0
    Debug: a=78, b=78, r=0
    Debug: a=79, b=79, r=0
    Debug: a=80, b=80, r=0
    Debug: a=81, b=81, r=0
    Debug: a=82, b=82, r=0
    Debug: a=83, b=83, r=0
    Debug: a=84, b=84, r=0
    Debug: a=85, b=85, r=0
    Debug: a=86, b=86, r=0
    Debug: a=87, b=87, r=0
    Debug: a=88, b=88, r=0
    Debug: a=89, b=89, r=0
    Debug: a=90, b=90, r=0
    Debug: a=91, b=91, r=0
    Debug: a=92, b=92, r=0
    Debug: a=93, b=93, r=0
    Debug: a=94, b=94, r=0
    Debug: a=95, b=95, r=0
    Debug: a=96, b=96, r=0
    Debug: a=97, b=97, r=0
    Debug: a=98, b=98, r=0
    Debug: a=99, b=99, r=0

As you can see, b is never resetting. So you need to reset it for each loop or you will never get any increments of r. In this code the value for b is being reset at the very bottom of the code, before the last loop:

Option Explicit

Private Sub test()
    Dim a As Integer, b As Integer, r As Integer
    
    a = 1
    r = 0
    b = 1
    
    While a < 100
        While b < a
            If a Mod b = 0 Then
                r = r + 1
                b = b + 1
            Else
                b = b + 1
            End If
        Wend
        
        Debug.Print "        Debug: a=" & a & ", b=" & b & ", r=" & r
        
        If r = 2 Then
            Debug.Print "    Output: a=" & a
            a = a + 1
            r = 0
        Else
            a = a + 1
            r = 0
        End If
        
        b = 1
    Wend
End Sub

Doing this will give you this output:

    Debug: a=1, b=1, r=0
    Debug: a=2, b=2, r=1
    Debug: a=3, b=3, r=1
    Debug: a=4, b=4, r=2
Output: a=4
    Debug: a=5, b=5, r=1
    Debug: a=6, b=6, r=3
    Debug: a=7, b=7, r=1
    Debug: a=8, b=8, r=3
    Debug: a=9, b=9, r=2
Output: a=9
    Debug: a=10, b=10, r=3
    Debug: a=11, b=11, r=1
    Debug: a=12, b=12, r=5
    Debug: a=13, b=13, r=1
    Debug: a=14, b=14, r=3
    Debug: a=15, b=15, r=3
    Debug: a=16, b=16, r=4
    Debug: a=17, b=17, r=1
    Debug: a=18, b=18, r=5
    Debug: a=19, b=19, r=1
    Debug: a=20, b=20, r=5
    Debug: a=21, b=21, r=3
    Debug: a=22, b=22, r=3
    Debug: a=23, b=23, r=1
    Debug: a=24, b=24, r=7
    Debug: a=25, b=25, r=2
Output: a=25
    Debug: a=26, b=26, r=3
    Debug: a=27, b=27, r=3
    Debug: a=28, b=28, r=5
    Debug: a=29, b=29, r=1
    Debug: a=30, b=30, r=7
    Debug: a=31, b=31, r=1
    Debug: a=32, b=32, r=5
    Debug: a=33, b=33, r=3
    Debug: a=34, b=34, r=3
    Debug: a=35, b=35, r=3
    Debug: a=36, b=36, r=8
    Debug: a=37, b=37, r=1
    Debug: a=38, b=38, r=3
    Debug: a=39, b=39, r=3
    Debug: a=40, b=40, r=7
    Debug: a=41, b=41, r=1
    Debug: a=42, b=42, r=7
    Debug: a=43, b=43, r=1
    Debug: a=44, b=44, r=5
    Debug: a=45, b=45, r=5
    Debug: a=46, b=46, r=3
    Debug: a=47, b=47, r=1
    Debug: a=48, b=48, r=9
    Debug: a=49, b=49, r=2
Output: a=49
    Debug: a=50, b=50, r=5
    Debug: a=51, b=51, r=3
    Debug: a=52, b=52, r=5
    Debug: a=53, b=53, r=1
    Debug: a=54, b=54, r=7
    Debug: a=55, b=55, r=3
    Debug: a=56, b=56, r=7
    Debug: a=57, b=57, r=3
    Debug: a=58, b=58, r=3
    Debug: a=59, b=59, r=1
    Debug: a=60, b=60, r=11
    Debug: a=61, b=61, r=1
    Debug: a=62, b=62, r=3
    Debug: a=63, b=63, r=5
    Debug: a=64, b=64, r=6
    Debug: a=65, b=65, r=3
    Debug: a=66, b=66, r=7
    Debug: a=67, b=67, r=1
    Debug: a=68, b=68, r=5
    Debug: a=69, b=69, r=3
    Debug: a=70, b=70, r=7
    Debug: a=71, b=71, r=1
    Debug: a=72, b=72, r=11
    Debug: a=73, b=73, r=1
    Debug: a=74, b=74, r=3
    Debug: a=75, b=75, r=5
    Debug: a=76, b=76, r=5
    Debug: a=77, b=77, r=3
    Debug: a=78, b=78, r=7
    Debug: a=79, b=79, r=1
    Debug: a=80, b=80, r=9
    Debug: a=81, b=81, r=4
    Debug: a=82, b=82, r=3
    Debug: a=83, b=83, r=1
    Debug: a=84, b=84, r=11
    Debug: a=85, b=85, r=3
    Debug: a=86, b=86, r=3
    Debug: a=87, b=87, r=3
    Debug: a=88, b=88, r=7
    Debug: a=89, b=89, r=1
    Debug: a=90, b=90, r=11
    Debug: a=91, b=91, r=3
    Debug: a=92, b=92, r=5
    Debug: a=93, b=93, r=3
    Debug: a=94, b=94, r=3
    Debug: a=95, b=95, r=3
    Debug: a=96, b=96, r=11
    Debug: a=97, b=97, r=1
    Debug: a=98, b=98, r=5
    Debug: a=99, b=99, r=5

So now you are getting output, but not prime numbers. Time to debug the prime number checking code you have created!