VB中最经典的东西是什么？没错就是三种基本结构，顺序结构选择结构、、循环结构。
 
1、顺序结构
 
程序代码按照重上到下的顺序一行一行的执行，执行过程找那个没有分支，没有重复。
 

 
 
'输入a b
'交换a b 的值
Dim a As Integer
Dim b As Integer
a = Val(InputBox("a"))
b = Val(InputBox("b"))
c = a
a = b
b = c
MsgBox ("交换后a=" & a & "b=" & b)



2、选择结构
根据不同的条件判断来决定程序执行走向。

a)单分支结构 
'当a大于1时  显示a
If a > 1 Then
   Print a
End If
b)双分支结构
'比较X,Y两个值中较大的一个

If x > Y Then
    Print x
Else
    Print Y
End If
c)多分支结构
'按成绩等级分类
If x >= 90 Then
        Print "优秀"
    ElseIf x >= 80 Then
            Print "良好"
        ElseIf x >= 70 Then
                Print ""
            ElseIf x >= 60 Then
                    Print ""
                Else
                    Print ""
End If
d)情况语句



3、循环结构
根据条件判断某些语句可能需要执行0次1次或多次。
1)当型循环



 '循环次数
 Dim I As Long, S As Long
 For I = 1 To 100
   S = S + I
 Next
 MsgBox "For...Next：1+2+3+...+100 = " & S


Dim I As Long, S As Long
   I = 0: S = 0
   Do While I < 100
      I = I + 1
      S = S + I
   Loop
   MsgBox "Do While：1+2+3+...+100 = " & S


Dim I As Long, S As Long
   I = 0: S = 0
   Do Until I > 100
      S = S + I
      I = I + 1
   Loop
   MsgBox "Do Until：1+2+3+...+100 = " & S


Dim I As Long, S As Long
   I = 0: S = 0
   While I < 100
     I = I + 1
     S = S + I
   Wend
   MsgBox "While...Wend：1+2+3+...+100 = " & S





2)直到型循环

Dim I As Long, S As Long
   I = 0: S = 0
   Do
      I = I + 1
      If I > 100 Then Exit Do
      S = S + I
   Loop
   MsgBox "Do...Loop：1+2+3+...+100 = " & S


Dim I As Long, S As Long
 I = 0: S = 0
   Do
      I = I + 1
      S = S + I
   Loop While I < 100
   MsgBox "Loop While：1+2+3+...+100 = " & S


Dim I As Long, S As Long
   I = 0: S = 0
   Do
      S = S + I
      I = I + 1
   Loop Until I > 100
   MsgBox "Loop Until：1+2+3+...+100 = " & S

VB的基本语法之循环结构
 
VB班含有两种循环结构，一种是计数循环，一种是条件循环。其中，条件循环包括有：直到型循环和当型循环。
 （一）计数循环
 For-Next循环是计数循环，常用于循环次数已知的情况。
 
'For-Next循环结构
For 循环变量=初值 To 终值 （step 步长）
	语句块1
	（Exit for）
	语句块2
Next 循环变量
 
（二）条件循环之直到型循环
 Do-Loop循环具有两种语法结构，分别是前测型和后测型循环。引用本人对于直到型循环的总结的一句话:直到型循环就是程序不断执行，直到不满足限定条件为止。
 
'前测型循环结构
Do 条件
	语句块1
	(Exit Do)
	语句块2
Loop
 
'后测型循环结构
Do
	语句块1
	(Exit Do)
	语句块2
Loop 条件
 
（三）条件循环之当型循环
 While-Wend循环具有两种语法结构，分别是前测型和后测型循环。引用本人对于直到型循环的总结的一句话:当型循环就是当满足条件程序不断执行，直到不满足限定条件为止。
 
'前测型循环结构
whlie 条件
	语句块1
	(Exit Do)
	语句块2
wend
 
'后测型循环结构
whlie
	语句块1
	(Exit Do)
	语句块2
wend 条件

 Private Sub Command1_Click()
 
 Cls
 
 Dim n As Integer
 
 Dim s As Integer
 
 Dim a(10) As Integer
 

 
 
 

 n = 1
 
 While n <= 10
 
 a(n) = Int(Rnd * 1000)
 
 n = n + 1
 
 Wend
 
 n = 1
 
 While n <= 10
 
 Print a(n)
 
 n = n + 1
 
 Wend
 

 
 
 

 s = 0
 
 n = 1
 
 While n <= 10
 
 s = s + a(n)
 
 n = n + 1
 
 Wend
 

 
 
 

 Print s
 

 
 
 

 
 
 

 
 
 

 End Sub
 
 

 
 

 
 
2.
 

 Private Sub Command1_Click()
 
 Dim i As Integer
 
 Dim sum As Integer
 
 Dim a As Integer
 

 
 
 

 Do
 
 Randomize
 
 a = Int(Rnd * 1000)
 
 sum = sum + a
 
 i = i + 1
 
 Loop Until i > 10
 
 MsgBox "10个随机数得和:" & sum
 

 
 
 

 
 End Sub
 
 
 
 

 
 

 
 

 
 
3.
 

 Private Sub Command1_Click()
 
 Dim i As Integer
 
 Dim sum As Integer
 
 Dim a As Integer
 

 
 
 

 Do
 
 Randomize
 
 a = Int(Rnd * 1000)
 
 sum = sum + a
 
 i = i + 1
 
 Loop while i <= 10
 
 MsgBox "10个随机数得和:" & sum
 

 
 
 

 
 End Sub
 
 
 
 

 
 

 
 

 
 
4.
 

 Private Sub Command1_Click()
 
 Dim i As Integer
 
 Dim sum As Integer
 
 Dim a As Integer
 

 
 
 

 i = 1
 
 Do While i <= 10
 
 Randomize
 
 a = Int(Rnd * 1000)
 
 sum = sum + a
 
 i = i + 1
 
 Loop
 
 MsgBox "10个随机数得和:" & sum
 

 
 
 

 
 End Sub
 
 
 
 

 
 

 
 
5.
 

 Private Sub Command1_Click()
 
 Dim i As Integer
 
 Dim sum As Integer
 
 Dim a As Integer
 

 
 
 

 i = 1
 
 Do Until i > 10
 
 Randomize
 
 a = Int(Rnd * 1000)
 
 sum = sum + a
 
 i = i + 1
 
 Loop
 
 MsgBox "10个随机数得和:" & sum
 

 
 
 

 
 End Sub
 
 
 
 

 
 

 
 

 
 
6.
 

 Private Sub Command1_Click()
 
 Cls
 
 Print f(10)
 

 
 
 

 
 End Sub
 
 Private Function f(x As Integer) As Long
 
 If x = 1 Then
 
 f = x
 
 Else
 
 f = x + f(x - 1)
 
 End If
 

 
 
 

 End Function
 
 
 
 

 
 

 
 
7.
 

 Private Sub Command1_Click()
 
 Dim sum As Integer
 
 Dim i As Integer
 
 Dim a As Integer
 
 sum = 0
 
 For i = 1 To 10
 
 Randomize
 
 a = Int(1000 * Rnd)
 
 sum = sum + a
 
 Next
 
 MsgBox "10个随机数得和:" & sum
 

 
 
 

 
 End Sub
 
 
 
 

 
 

 
 
8.
 

 Private Sub Command1_Click()
 
 Cls
 

 
 
 

 Dim n As Integer
 
 Dim s As Integer
 
 Dim a(10) As Integer
 

 
 
 

 n = 1
 
 While n <= 10
 
 a(n) = Int(Rnd * 1000)
 
 n = n + 1
 
 Wend
 
 n = 1
 
 While n <= 10
 
 Print a(n)
 
 n = n + 1
 
 Wend
 

 
 
 

 s = 0
 
 n = 1
 

 
 
 

 For Each t In a
 
 s = s + t
 
 Next
 

 
 
 

 
 
 

 
 
 

 Print s
 

 
 
 

 End Sub
 
 
 
 

 
 

 
 

 

for循环
 
1.语法结构
 
2.执行顺序
 
3.注意说明
 
（一）语法结构：
 For 循环变量=初值 to 终值【step 步长】
 【循环体】
 【Exit for】
 Next 【循环变量】【，循环变量】
 
计算1累加到100的和

Dim i as integer         '声明变量i
Dim sum as long          '声明变量sum
i=1:sum=0                '将1赋值给变量i，将0赋值给变量sum
For  i=1 to 100 step 1   '循环1-100之间的数，循环完一次就将i+1
	sum=i+sum        '将sum+i的值赋值给新的sum
Next i
print sum                '循环结束后输出结果sum

 
执行顺序
 
①将1赋值给i，然后判断i是否在1-100区间内
 ②如果在，执行循环体内的sum=sum+i
 ③由于step 1，所以相当于变量i=i+1
 ④再继续判断是否在1-100区间内
 ⑤一直判断到 i>100 跳出循环，输出sum
 
说明
 1、步长：默认为1时可以省略，不可为0
 2、循环次数：Int（终值-初值)/步长+1
 3、For-Next循环次数可以使用嵌套循环
 
for循环的嵌套使用
 
Dim i as integer       
Dim j as integer         
i=1:j=1               
For  i=1 to 2 step 1  
print "*"
	For j=1 to 2 step 1
	print "#"
	Next j     
Next i
print sum       
 
for嵌套的例子
 九九乘法表
 
Dim i as Integer
Dim j as Integer 
i=1:j=1
For i=1 to 9
	For j=1 to i  '注意一行里面输出的列的个数不能超过行的个数；所以终值是i所以第一个循环跟第二个的循环次数相同
	Print i & "*" & j & "=" & i * j;space(2);   ’在输出语句后面加分号（;）代表着 连着输出不换行
	Next j
print   '把第一行循环的结果全部输出在一行上 ，再换行
Next i