Question

vb编程，找出介于A B之间的所有只有素数因子（1和自身除外）的数。哪点错了，为什么没有输出

Option Explicit
Private Sub Command1_Click()
Dim i As Integer, j As Integer, n As Integer, a As Integer, b As Integer, k As Integer, c() As Integer
a = Text1: b = Text2
For n = a To b
k = 0
If prime(n) Then Exit Sub
For i = 2 To n - 1
If n Mod i = 0 Then
If prime(i) Then
k = k & i & ","
Else
Exit Sub
End If
End If
Next i
List1.AddItem n & "的素数因子是：" & k
Next n
End Sub

Private Function prime(n As Integer) As Boolean
Dim i As Integer
For i = 2 To Sqr(n)
If n Mod i = 0 Then Exit Function
Next i
prime = True
End Function

Private Sub Command2_Click()
Text1 = "": Text2 = "": List1.Clear
Text1.SetFocus
End Sub

Private Sub Command3_Click()
End
End Sub

Answer 1

楼主，这段代码有 4 处错误。

1、将K声明为Integer（k As Integer）。而后的运算中k是当作字符串的，参加字符串运算

    k = k & i & ","
这个运算，当然结果是错误的。

2、 If prime(n) Then Exit Sub，如果n是素数，就退出整个过程。n本身是素数，还需要进行n之后数据的处理呀！

3、这个If块中的是错误的 Exit Sub是错误的。也就是说某一个数有非素数因子，就退出整个过程，以后合法的数据也不在理会了。这当然是错误的。应该是Exit For。

            If prime(i) Then
                k = k & i & ","
            Else
                Exit Sub
            End If
4、其实这里还隐含了以个错误。3中将Exit Sub改为Exit For，我们就看出了这个问题：发现了非素数以后，是不是还要向ListBox中加入k呢？回答是：当然不加！

那么就需要加以判断。

 

上述四个问题修改了以后，程序运行正常了。正确代码如下：

 

Option Explicit
Private Sub Command1_Click()
Dim i As Integer, j As Integer, n As Integer, a As Integer, b As Integer, c() As Integer
Dim k As String
Dim bln As Boolean
a = Text1: b = Text2
For n = a To b
     k = 0
     If Not prime(n) Then
        bln = True
         For i = 2 To n - 1
            If n Mod i = 0 Then
                If prime(i) Then
                    k = k & i & ","
                Else
                    bln = False
                    Exit For
                End If
            End If
        Next i
        If bln Then List1.AddItem n & "的素数因子是：" & k              '只有素数因子
    End If
Next n
End Sub

'判断一个数是不是素数。是，返回True；不是，返回False
Private Function prime(n As Integer) As Boolean
Dim i As Integer
For i = 2 To Sqr(n)
    If n Mod i = 0 Then Exit Function
Next i
prime = True
End Function

Private Sub Command2_Click()
Text1 = "": Text2 = "": List1.Clear
Text1.SetFocus
End Sub

Private Sub Command3_Click()
End
End Sub

Answer 2

楼主，可以把 step 设置为2 ,因为偶数不可能是素数（如果A是奇数的话）
for n =A to B step 2
k=int(sqr(n))
i=2
while i<=k

if n mod i = 0 then
print i **就是素数
else
i=i+1
end if
wend
next n
这样简单些。。。没必要去循环每个数。。。

突然脑子短路了。。。。。楼主私信我。。我们私聊。。一下想不起来了

Answer 3

Private Sub Command1_Click()

Dim i As Integer, j As Integer, n As Integer, a As Integer, b As Integer, k As Integer, c() As Integer

a = Val(Text1.Text)

b = Val(Text2.Text)

For n = a To b

  i = prime(n)

  If i <> n Then

    k = n / i

    If prime(k) = k Then

      If i < k Then

        List1.AddItem n & "的素数因子是：" & i & "和" & k

      Else

        List1.AddItem n & "的素数因子是：" & i

      End If

    End If

  End If

Next n

End Sub

Private Function prime(n As Integer) As Integer   '返回n除1以外的最小因子,若n是素数,则prime(n) = n

Dim i As Integer

For i = 2 To Sqr(n)

    If n Mod i = 0 Then

      prime = i

      Exit Function

    End If

Next i

prime = n

End Function

Answer 4

错误在这里：
If prime(i) Then
k = k & i & "," '符合条件时
Else
Exit Sub ‘ 当有不符合条件的情况，直接退出子程序了，不进行下一条判断，因此问题出在该句。
End If

改为：
If prime(i) Then
k = k & i & ","
Else
Exit For
End If

就可以了。