executing the value of a variable as a expression

Question

let say i have a variable var1; var1= '2+3'; How can i execute it as a expression that returns 5. May be its a simple case but in some diffult case if i want to build a language interpreter like executing command how can i do that

bebelmcgrady · Answer

Thank you Werdna, I will keep reading the code and I hope you don't mind if I end up asking another question. Regards,

andrew · Answer

This is mostly used for debugging. So that you can print list of tokens (before they get to parser) if there is something wrong with the parser. "(" + (int)kind + ")" will convert kind to string and put it between ().

bebelmcgrady · Answer

Also Werdna, what is happening here: "(" + (int)kind + "):" Are you converting a string to int? or a TokenKind to int and then convert it all into string? Am very confused here, Thanx

bebelmcgrady · Answer

oh ok, Thank you Werdna, Now it makes more sense. I was not getting it at all. Your answer brings up another question: the (int)kind is used just in case the token kind is something like an number? and also, this override function does not have anything to do with the parser or your AST? it is sollely used for the prettyPrinter class? Sorry, for bugging you, I am just trying to read your code and understand your thinking process. As they say, it is by undertanding the great thinkers that one become a great thinker as well. Thanks a great deal,

andrew · Answer

I'm not sure to which version you're referring. If it's the one that has: return "[" + kind.ToString() + "(" + (int)kind + "):" + line + ":" + column + ":" + str + "]"; it's only used for printing info about a Token. Prints Token Kind and line and column number where the token is found and if it contains any string data.

bebelmcgrady · Answer

Hello Werdna, Nice job with the interpreter on your website. I have a question: Can you please explain to me the ToString function you have on your Token.cs? I am kind of confused with it. I am a fairly novice in this C# world. So, please, bear with me through this. Thanx, McGrady

andrew · Answer

Doing something like this cannot be easily explained in few paragraphs. There are whole books and full semester courses on how to do that. Basic idea is to take your source code (string or file) and convert it into tokens, then you take those tokens and form AST (abstract syntax tree). In this tree statements, expressions, etc. form easily manageable chunks that can be used to either do code generation (for compilers) or executing (for interpreters) For example, given expression: x = 5 + i; you'll have AST element called BinaryExpression. You then have AssignmentExpression and PlusExpression that inherits from BinaryExpression. Each binary expression has LHS (left side) and RHS (right side) and those are also expression, so with this you can nest those. The above example will look like this: AssignmentExpression |- LHS: VariableExpression (holds id: x) |- RHS: PlusExpression |- LHS: LiteralInteger (holds: 5) |- RHS: VariableExpression (holds id: i) for the interpreter you have to keep a table of all variables (string -> value mapping) When you execute your expressions, an easy way is to use Visitor pattern and each expression then returns Value (a class that holds a number, or if you only have integers in your program, you can just return int) For example to execute PlusExpression: int evaluatePlusExpression(PlusExpression e) { int lhsValue = evaluateExpression(e.LHS); int rhsValue = evaluateExpression(e.RHS); return lhsValue + rhsValue; }

Tanvir Huda · Answer

Hello Werdna, I am kind'a lost in ur code . Pls help me out . For a basic interpreter that just interprete a add method int i=0; int j=5; i=i+j; Print(i); Can u pls tell me what are the steps i have to write down in my code Obviously from then i have to build a proper interpreter . I mean that can handle all the functionality

Tanvir Huda · Answer

Hello Werdna, I have seen ur code before and sill now. Very good job --- i must compliment. But i need some help from u. i hope u will help me out. I download ur code and have a look at the code. In my case i am using XML document . As mentioned earlier. I can parse the XML document using the fuctionalality in .Net System.XMl namespace. So i think i dont need to implement the parser part (if i am wrong pls correct me) . Now the hard part is building the interpreter. So can u pls help me to build in that . Can u pls describe me how things are working in ur program. I hope u will describe me the interpreter part Thanks

andrew · Answer

I have written interpreter in C#. You can download it with all source code from http://www.adersoftware.com/?page=compilers

seema.adhav · Answer

This is a tricky problem. Hey, try reading the expression in an array and going from left to right evaluate the expression in a loop as you go. It would need one loop to read into the array. 2nd to put multiple digits together and next to evaluate. Would be easier if you know the operators you are expecting. Might work.

Tanvir Huda · Answer

I dont think Split will work; Split function split the string based on some character But how can i evaluates the string later on as i need proprer interpretation of the language Is there an Tokenizer function or expression function in C#

uzir · Answer

Hi As you say, you need an interpreter. You can substring/tokenize it (Split() method) and run it through a chain where each link inspects the format and can do actions on it or not, according to the format or whatever, and return the result accordingly hope this helps Uz

Tanvir Huda · Answer

AW example is useful though i dont understand the code list :) Solitaire thanks for ur comments . The whole purpose is i want to build an interpreter Say i have a line of code a=a+b*c/d; Now the value of the a,b,c,d will be input. so, a=1; b=2; c=3; d=1; a=a+b*c/d; I have given these line of code and i have to interprete these in C# . That means write a program in C# that shows a=1+2*3/4=7 . I mean display the output a=7 How can i do that . Anyone have any idea about building interpreter. Pls help me out .

joy.simpson · Answer

That's wrong. "2+3" enclosed within quotes is a string and should return 23, not 5. Maybe what you meant was you want to change the expression 2 + 3 into a string? First convert the 2 into a string. Then convert the 3 into a string. Then concatenate the "2" + "+" + "3" into a new string and save it to a variable. Maybe what you meant is that you want to turn the string into a mathematical expression? Use string manipulation with Substring() to extract the numerical segments of the string from either end. You may need to test each character with IsDigit() to determine how much of the string is numeric. Convert them to numbers and save them to variables. Separate the operator symbol and test it to see which operator to use with the numbers. Example: if(oper == "+") num1 + num2; Hope this helps.

Andrzej Wegierski · Answer

See http://www.codeproject.com/dotnet/evaluator.asp?msg=408583&Page=2&userid=56906&mode=all#xx408583xx

gjghkhj · Answer

Sorry, I can't understand the question, could you please try to re-phrase what you are trying to do?

executing the value of a variable as a expression

Answers (17)