Question

vb 循环结构的试题看不懂，请高手指点

Dim x As Integer
x = 1
n = 0
Do While x < 28
x = x * 3
n = n + 1
Loop
Print n, x
答案是 4和81。为什么n到3就停止循环了？最后n=4是怎么的出来的？

Answer 1

Dim x As Integer
x = 1
n = 0
Do While x < 28 条件成立时下面代码运行,条件不成立时跳出循环，
x = x * 3 '(x=1*3/ x=3*3 / x=9*3 / x=27*3=81 循环至3次后x=27条件仍成立，第四次检查 时X为第3次循环结束时的结果x=27＜28，条件成立继续执行一次x=27*3，
在第五次的条件判断中x=27*3>28，跳出循环，此时X＝81）
n = n + 1 '（n=0+1 / n=1+1 / n= 2+1 / n= 3+1 同为4次运算，故循环结束时n=4）
Loop
Print n, x

Answer 2

完成了，在窗体上添加4个命令按钮，分别对应你的四个题，在窗体上直接输出结果，测试没问题，有疑问消息我
Private Sub Command1_Click()
Dim i As Integer
Cls
For i = 0 To 500
If isprime(i) Then Print i
Next

End Sub

Function isprime(n As Integer) As Boolean '求质数
isprime = True '赋初值
For i = 2 To Int(Sqr(n))
If n Mod i = 0 Then '如果能除尽
isprime = False '证明是素数
Exit For
End If
Next i
End Function

Private Sub Command2_Click()
Dim x(1 To 10) As Integer
Randomize
Cls
For i = 1 To 10
x(i) = Int(Rnd * 100)
Print x(i)
Next
Print "最大："; max(x())
Print "最小："; Min(x())
End Sub

Function Min(a() As Integer) As Integer '求最小
Min = a(1)
For i = 2 To 10
If a(i) < Min Then Min = a(i)
Next
End Function

Function max(a() As Integer) As Integer '求最大
max = a(1)
For i = 2 To 10
If a(i) > max Then max = a(i)
Next
End Function

Private Sub Command3_Click()
Cls
For m = 1 To 4
For i = 1 To 5
Print i;
Next
Print
Print Space(m);
Next
Print

s = "abcde"
For i = 1 To 5
Print s
s = Right(s, 1) & Left(s, 4)
Next
Print

For i = 1 To 7 Step 2
Print Space((8 - i - 1) / 2);
Print String(i, "*")
Next

End Sub

Private Sub Command4_Click()
Cls
For x = 0 To 9
For y = 0 To 9
For z = 0 To 9
If x * 100 + y * 10 + z + y * 100 + z * 10 + z = 532 Then Print x, y, z
Next
Next
Next
End Sub

Answer 3

第一次循环 X =1 ，n=0 循环后 X=3，n=1
第二次循环后x=9，n=2
第三次循环后x=27，n=3 此时X<28仍可以继续循环
第四次循环后x=81，n=4此时x>28不能再循环，结束循环。
只要X<28，就可以执行循环。