Use System. LINQ. expressions to implement four arithmetic calculators (2)

Exploitation System. LINQ. Expressions Implement a four-digit Calculator (2) )

How to convert a mathematical expression to an Expression Tree?

As mentioned above, several classes are used to encapsulate mathematical expressions.5*4/(3 + 2-1)"After conversion to the Expression Tree, such:

"Expression list" and"/(3 + 2-1)"Are all usedBwexpressionnodecollectionIndicates,"5","* 4","3","+ 2","-1"Both useBwconstantexpressionnode.
How can I convert a mathematical expression string to an Expression Tree? The answer is to use a regular expression.

The regular expression that matches a number is""D + (". "D + )?", The expression that matches the symbol is"("+ |-|" * | /)". The regular expression matching the brackets is""([^" (")] * (? 'Open' "() [^" (")] *) + ((? '-Open' ") [^" (")] *) + )*(? (Open )(?!)) ")". How to write and use regular expressions? Please search for them online!

In a mathematical expression, if the first digit is a negative number, you can directly write it as"-2 + 3". If the negative number is in the middle of the expression, enclose it in parentheses."2 * (-2) + 3". No matter where the negative number is, there is a rule. In the simple expression summarized above, the negative number is always the first. SoProgramDuring conversion, you only need to consider the case where the first number is a negative number (the negative number in the brackets can only be the first one ). The regular expression that matches a negative number is"-? "D + (". "D + )?".

To generate an Expression Tree, see the following flowchart:

All the work for generating the Expression Tree is handed overBwexpressionnodecollection, I'm designingBwexpressionnodecollectionClass, use the constructor to input a mathematical expression string:

///<Summary> ///Expression Node Set ///</Summary> ///<Param name = "input">Expression string</Param> ///<Param name = "type">Operator before this expression</Param> PublicBwexpressionnodecollection (StringInput,BwexpressionnodetypeType) { Base. Expressionnodetype = type; This. Expressionstring = input; This. Expressionnodelist =New Bwexpressionnodelist(); Double? Val = parsenegative (); If(Val. hasvalue) { This. Expressionnodelist. Add (New Bwconstantexpressionnode(Val. value,Bwexpressionnodetype. Start )); } Else { StringParenthesescontent = getparentheses (); If(!String. Isnullorempty (parenthesescontent )) { This. Expressionnodelist. Add (New Bwexpressionnodecollection(Parenthesescontent,Bwexpressionnodetype. Start )); } Else { Throw Error. Genericexception; } } While(!String. Isnullorempty (This. Expressionstring )) { Char? Sign = getsign (); If(Sign. hasvalue) { BwexpressionnodetypeNexttype; Switch(Sign) { Case '+': Nexttype =Bwexpressionnodetype. Addition; Break; Case '-': Nexttype =Bwexpressionnodetype. Subtration; Break; Case '*': Nexttype =Bwexpressionnodetype. Multiplication; Break; Case '/': Nexttype =Bwexpressionnodetype. Division; Break; Default: Throw Error. Genericexception; } Double? Operand = parsepositive (); If(Operand. hasvalue) { This. Expressionnodelist. Add (New Bwconstantexpressionnode(Operand. Value, nexttype )); } Else { StringParenthesescontent = getparentheses (); If(!String. Isnullorempty (parenthesescontent )) { This. Expressionnodelist. Add (New Bwexpressionnodecollection(Parenthesescontent, nexttype )); } Else { Throw Error. Genericexception; } } } Else { Throw Error. Genericexception; } } }

We can see that if a mathematical expression encounters a bracket, the mathematical expression in the bracket is generated into a childBwexpressionnodecollectionObject.

So far, the expression tree has been generated:

string input = @" 1 + 5*9-(1 + 2) * 3/4 + 6 * (-2) "; bwexpressionnodecollection nodes = New bwexpressionnodecollection (input, bwexpressionnodetype . start);

next we will introduce how to generate from the Expression Tree system. LINQ. expressions class. Please note!