【加试题】编写VB程序，实现如下功能：单击“生成密文”按钮Command2，在文本框Text3中产生与每个小写字母对应的密文（每个明文字母对应1个不重复的密文字母）。在文本框Text1中输入单词字符串，单击“加密”按钮Command1后，进行逐个字符加密，加密方法：在“明文”中找到相应字符，再得到明文对应位置的密文,并在文本框Text2中显示对应密文（运行效果如图17所示）。实现上述功能的VB代码如下： 
Dim m2 As String
Private Sub Command1Click()
Dim flag(1 To 26) As Boolean
Dim num As Integer,t As Integer
Randomize
num = 1
Text3.Text = " "
For i = 1 To 26
flag(i) = False
Next i
Do While num <= 26
x = Int(Rnd * 26) + 97
①
If flag(t) = False Then
m2 = m2 + Chr(x)
num = num + 1
②
End If
Loop
Text3.Text = m2
End Sub
Private Sub Command2Click()
Dim s As String， m As String， result As String
Dim k As String， c As Integer
s = Text1.Text
m = "abcdefghijklmnopqrstuvwxyz"
For i = 1 To Len(s)
k = Mid(s, i, 1)
If k >= "A" And k <= "Z" Then
k = Chr(Asc(k) + 32)
End If
c = 1: continue = True
Do While ③
If k = Mid(m, c, 1) Then
continue = False
End If
c = c + 1
Loop
④
Next i
Text2.Text = result
End Sub
程序①处的代码是{#blank#}1{#/blank#}
程序②处的代码是{#blank#}2{#/blank#}
程序③处的代码是{#blank#}3{#/blank#}
程序④处的代码是{#blank#}4{#/blank#}
