Evaluating List as a mathematical expression

Question

I have a List<String> that is full of values and operators.

["123", "+", "(", "890", "-", "15.00", ")"]

I know that I can make an algorithm that will push these numbers and and operators onto a stack and pop them off and evaluate as I go. But, is there a better way to do with without using a external library?

Are these broken up from user input or fed to you by some other system? — user7116, Aug 16 '11 at 17:37
Are you trying to do it without either using an existing library OR writing your own implementation? Or are you looking for a library included in the standard libraries? — corsiKa, Aug 16 '11 at 17:38
I am looking for something that is included in the standard library or I will write my own algorithm. — J Lundberg, Aug 16 '11 at 17:39
Use can use any implementation of the standard infix (inorder) expression evaluator... [Sample Code to Evaluate an Infix Expression](http://scriptasylum.com/tutorials/infix_postfix/algorithms/infix-postfix/index.htm) — Devendra D. Chavan, Aug 16 '11 at 17:44
I have been very happy with ILCalc (http://ilcalc.codeplex.com/). It is an external library, but it is on codeplex, so you get source code if you need to see it. — Philipp Schmid, Aug 16 '11 at 17:48

score 4 · Accepted Answer · answered Aug 16 '11 at 17:39

Pushing the numbers and operators onto a stack would be an interpreter.

The obvious better way to do this (for some definition of "better") is to write a compiler!

You already have the input split into lexical tokens, so you can skip implementing a lexer and can dive right into building the AST. You can find suitable classes to transform your input to in the System.Linq.Expressions Namespace; have a look at the Expression Class. You can wrap the result in a lambda expression, compile it to IL and execute it on the CLR!

It seems kind of pointless to use a compiler to evaluate a constant, but if he had variables then it would be a good idea. — Gabe, Aug 16 '11 at 18:30

score 2 · Answer 2 · edited May 23 '17 at 12:04

2

You can join the List and then let the compiler evaluate it at runtime as Mehrdad stated.

Expression e = new Expression("5 * 2"); 
e.Evaluate();

I found a very similar question already asked here

update :

NCalc "NCalc - Mathematical Expressions Evaluator for .NET" even though this is an external library I think it is an open source project which means you can add the code directly to your project.

Update :

You can use the String.Join function to join the List.

string formula = String.Join("",listMathOperators);

edited May 23 '17 at 12:04

Community

1
1

answered Aug 16 '11 at 17:41

Jethro

5,896
3
23
24

Can you provide a link to the Expression class you're using in your code? – dtb Aug 16 '11 at 17:48

score 0 · Answer 3 · answered Aug 16 '11 at 17:34

0

You can always use the C# compiler classes in the .NET framework to evaluate it as a C# expression...

answered Aug 16 '11 at 17:34

user541686

205,094
128
528
886

Do you have a link or example of this? – J Lundberg Aug 16 '11 at 17:36
4

You'd need to validate the strings first, otherwise you could be executing a lot more than you expect. – wsanville Aug 16 '11 at 17:38
Do you mean CodeDOM or Expression trees? – Gabe Aug 16 '11 at 17:55

score 0 · Answer 4 · answered Aug 16 '11 at 17:41

I have a complete class that calculates any expression.

First, you need to extract all strings of your list to a single string, like:

string sExpression = "123+(890-15.00)";

Second, you need to evaluate this expression in a parser, I have a complete class that makes this for you, but I can't attach files here.

Evaluating List as a mathematical expression

4 Answers4