中缀表达式转换成后缀表达式的实现(VB)

在对表达式求值过程中用到了后缀表达式:
比如: 2+3*(1.5-6*(3-2+4)/3)
其后缀表达式为:2 3 1.5 6 3 2 – 4 + * 3 / - * +
下面直接贴出VB代码,


Option Base 1 ' 让数组的下标从 1 开始




Dim ptr As String ' 遍历源字符串的指针


Dim str As String ' 无空格字符串


Dim strIndex As Integer ' str迭代量


Dim dstr() As String ' 存放后缀表达式各字符串


Dim dIndex As Integer ' dstr() 迭代量


Dim Stack() As String ' 一个栈,在后缀表达式转换过程可以用于符号栈,在求值可以存放值


Dim Top As Integer ' 指向栈顶,初始值为1


Dim InV As Boolean ' 判断是否在一个超过两位数字的数字内


Dim strlen As Integer ' dstr, stack的数组维数






'主要程序,用于遍历源字符串str1




Public Sub mainPro()Sub mainPro(str1 As String)


' 迭代量初始化


strIndex = 1


dIndex = 0


Top = 1


InV = False


str = "" '切记要将str初始化




str1 = str1 + "#" ' 添加结束符"#"




str1 = Trim(str1) ' 清除str1两端的空格


ptr = Mid(str1, strIndex, 1)




' 消除str1里面的空格 ,其实这个初始化过程可以考虑另写一个函数


Do While ptr <> "#"


If ptr <> " " Then


str = str + ptr


End If




strIndex = strIndex + 1


ptr = Mid(str1, strIndex, 1)


Loop




str = str + "#"




strlen = Len(str) + 2 ' 取长度,然后重新定义动态数组


ReDim dstr(strlen)


ReDim Stack(strlen)


Call push("#")




strIndex = 1


ptr = Mid(str, strIndex, 1)


' 判断str第一个字符是否"+" or "-"


If ptr = "+" Or ptr = "-" Then


push (ptr)


dIndex = dIndex + 1


dstr(dIndex) = "0"


strIndex = strIndex + 1


End If




' 开始遍历str,遇到"#"停止


Do While ptr <> "#"


Select Case ptr


Case "0" To "9", "."


If InV Then


dstr(dIndex) = dstr(dIndex) + ptr


Else


dIndex = dIndex + 1


dstr(dIndex) = ptr


InV = True


End If


Case "+", "-", "*", "/"


InV = False


dealop (ptr)


Case "("


Call push(ptr)


Case ")"


Call dealclose




Case Else


MsgBox ("表达式出现未知字符")


End Select




strIndex = strIndex + 1


ptr = Mid(str, strIndex, 1)


Loop




If Top > 1 Then


Dim tmp As String


tmp = pop()


Do While tmp <> "#"


dIndex = dIndex + 1


dstr(dIndex) = tmp


tmp = pop()


Loop


End If




dIndex = dIndex + 1


dstr(dIndex) = "#"




End Sub






Private Sub push()Sub push(ptr As String)


If Top > strlen Then


MsgBox ("符号栈上溢出 in push()")


End


End If




Stack(Top) = ptr


Top = Top + 1


End Sub






Private Function pop()Function pop() As String


If Top <= 1 Then


MsgBox ("符号栈下溢 in pop()")


End


End If




Top = Top - 1


pop = Stack(Top)


End Function






Private Function getTop()Function getTop() As String


If Top <= 1 Then


MsgBox ("栈空 in getTop()")


End


End If




getTop = Stack(Top - 1)


End Function






Private Sub dealop()Sub dealop(ptr As String)


Dim stillIn As Boolean ' 判断是否正在处理当前操作符,即当前操作符优先级小于等于符号栈顶的


' 操作符时,应该将栈顶操作符出栈,放到dstr中


Dim tmp As String ' 用来存放栈顶元素


stillIn = True


Select Case ptr


Case "+"


If Mid(str, strIndex - 1, 1) = "(" Then


dIndex = dIndex + 1


dstr(dIndex) = "0"


Call push(ptr)


Else


Do While (stillIn)


tmp = getTop()


Select Case tmp


Case "*", "/", "-", "+"


dIndex = dIndex + 1


dstr(dIndex) = pop()


Case "(", "#"


Call push(ptr)


stillIn = False


Case Else


MsgBox ("运算符出错 in +")


End '结束程序


End Select


Loop


End If




Case "-"


If Mid(str, strIndex - 1, 1) = "(" Then


dIndex = dIndex + 1


dstr(dIndex) = "0"


push (ptr)


Else


Do While (stillIn)


tmp = getTop()


Select Case tmp


Case "*", "/", "+", "-"


dIndex = dIndex + 1


dstr(dIndex) = pop()


Case "(", "#"


Call push(ptr)


stillIn = False


Case Else


MsgBox ("运算符出错 in -")


End


End Select


Loop


End If




Case "*"


Do While (stillIn)


tmp = getTop()


Select Case tmp


Case "/", "*"


dIndex = dIndex + 1


dstr(dIndex) = pop()


Case "(", "#", "+", "-"


Call push(ptr)


stillIn = False


Case Else


MsgBox ("运算符出错 in *")


End


End Select


Loop






Case "/"


Do While (stillIn)


tmp = getTop()


Select Case tmp


Case "*", "/"


dIndex = dIndex + 1


dstr(dIndex) = pop()


Case "(", "#", "+", "-"


Call push(ptr)


stillIn = False


Case Else


MsgBox ("运算符出错 in /")


End


End Select


Loop




Case Else


MsgBox ("Error happen in dealop() case else")


End


End Select


End Sub






Private Sub dealclose()Sub dealclose()


Dim tmp As String ' 用来存放栈顶元素


tmp = pop()


Do While tmp <> "("


dIndex = dIndex + 1


dstr(dIndex) = tmp


tmp = pop()


Loop




End Sub






Public Function execute()Function execute() As String


Dim v1 As String, v2 As String, v3 As String ' 定义运算暂存的变量




dIndex = 1


ptr = dstr(dIndex)


Do While ptr <> "#"


Select Case ptr


Case "+"


v1 = pop()


v2 = pop()


v3 = Val(v1) + Val(v2) ' 这里会将val值隐式转换成string么?


push (v3)




Case "-"


v1 = pop()


v2 = pop()


v3 = Val(v2) - Val(v1)


push (v3)




Case "*"


v1 = pop()


v2 = pop()


v3 = Val(v1) * Val(v2)


push (v3)




Case "/"


v1 = pop()


v2 = pop()


If Val(v1) = 0 Then


MsgBox ("式子中除数为 0 ")


End


End If


v3 = Val(v2) / Val(v1)


push (v3)




Case Else


push (ptr)


End Select




dIndex = dIndex + 1


ptr = dstr(dIndex)


Loop




v1 = pop() '判断最终结果绝对值是否小于1,如果




If Abs(Val(v1)) < 1 Then


v1 = Format(Val(v1), "0.######")


End If


execute = v1


End Function