How to evaluate a mathematical expression in Unity's C#

Question

Is there an javascript-like Evaluate() function in C# or any other similar method?

I have a simple mathematical expression (no brackets, only -, +, / and * operands) stored in a string and I want to be able to evaluate that and get the result in a float.

Example: For the input string "2+3-5*7" the output should be -30.

I tried to implement a method myself, but I have troubles converting a character to an int. It seems like I get the ASCII code of the character in my int instead of the value I want.

Answer 1

Best Answer

Answer by ericksson · Nov 17, 2010 at 06:24 PM

I found what I was looking for here: http://www.logiclabz.com/c/evaluate-function-in-c-net-as-eval-function-in-javascript.aspx

The method that works in Unity is:

 public static double Evaluate(string expression)  
 {  
     return (double)new System.Xml.XPath.XPathDocument  
     (new System.IO.StringReader("< r />")).CreateNavigator().Evaluate  
     (string.Format("number({0})", new  
     System.Text.RegularExpressions.Regex(@"([\+\-\*])").Replace(expression, " ${1} ").Replace("/", " div ").Replace("%", " mod ")));  
 }

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a436t4ataf · Nov 04, 2014 at 05:32 PM 0

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That webpage has been deleted, is now just advertising.

Can you explain WHAT you did above?

Bunny83 · Nov 04, 2014 at 06:00 PM 0

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@a436t4ataf:
Well, i'm actually a regular expression noob but it looks like

 new System.Text.RegularExpressions.Regex(@"([\+\-\*])").Replace(expression, " ${1} ").Replace("/", " div ").Replace("%", " mod ")

Simply reformats your expression string to have a space before and after each operator. It also replaces "/" with "div" and "%" with "mod".

I'm also not very familiar with XPath, but

 new System.Xml.XPath.XPathDocument(new System.IO.StringReader("<r/>"))

will create an XPath document with one element < r />. On that object he calls GetNavigator and on the returned navigator Evaluate. To that method he basically passes the string: "number(" + your expression + ")" which most likely will be evaluated to a number by the Evaluate method. The result is casted to double and returned.

So if you input something like "5*6/2+5" will be reformatted as "number(5 * 6 div 2 + 5)" and evaluated by the Evaluate method of that XPathDocument's navigator ^^

So in a more sequential and more readable version it would look like this:

     public static double Evaluate(string expression)
     {
         var doc = new System.Xml.XPath.XPathDocument(new System.IO.StringReader("<r/>"));
         var nav = doc.CreateNavigator();
         var newString = expression;
         newString = (new System.Text.RegularExpressions.Regex(@"([\+\-\*])")).Replace(newString, " ${1} ");
         newString = newString.Replace("/", " div ").Replace("%", " mod ");
         return (double)nav.Evaluate("number(" + newString + ")");
     }

a436t4ataf · Nov 05, 2014 at 12:50 AM 0

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OP has a strange regexp. Replace "new Sytem.Text...*])") with the string you want to evaluate, and this works.

Bunny83 · Nov 05, 2014 at 04:16 AM 0

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@a436t4ataf: Well, it isn't that strange, he just has one thing wrong. "-" is a special character and need to be escaped. I'll fix my comment to reflect that ;)

edit
Correction, he actually has excaped all 3 characters, but since it's an old question it was formatted with < code >< pre > which seems to eat the backslash. When you edit the answer (which i did) you can see they are actually there. I fixed the code highlighting

AbdulRaman · Jun 04, 2018 at 09:26 AM 0

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it doesn't work with me it gives me this error XmlException: a name did not start with a legal character 32 ( ) Line 1, position 2. $$anonymous$$ono.Xml2.XmlTextReader.ReadName (System.String& prefix, System.String& localName) $$anonymous$$ono.Xml2.XmlTextReader.ReadStartTag () $$anonymous$$ono.Xml2.XmlTextReader.ReadContent () $$anonymous$$ono.Xml2.XmlTextReader.Read () System.Xml.XmlTextReader.Read () $$anonymous$$ono.Xml.EntityResolvingXmlReader.Read () $$anonymous$$ono.Xml.DTDValidatingReader.ReadContent () $$anonymous$$ono.Xml.DTDValidatingReader.Read () $$anonymous$$ono.Xml.Schema.XsdValidatingReader.Read () System.Xml.XmlValidatingReader.Read () $$anonymous$$ono.Xml.XPath.DT$$anonymous$$XPathDocumentBuilder2.Compile () $$anonymous$$ono.Xml.XPath.DT$$anonymous$$XPathDocumentBuilder2.Init (System.Xml.XmlReader reader, XmlSpace space, Int32 defaultCapacity) $$anonymous$$ono.Xml.XPath.DT$$anonymous$$XPathDocumentBuilder2..ctor (System.Xml.XmlReader reader, XmlSpace space, Int32 defaultCapacity) $$anonymous$$ono.Xml.XPath.DT$$anonymous$$XPathDocumentBuilder2..ctor (System.Xml.XmlReader reader, XmlSpace space) System.Xml.XPath.XPathDocument.Initialize (System.Xml.XmlReader reader, XmlSpace space) System.Xml.XPath.XPathDocument..ctor (System.IO.TextReader reader) Game$$anonymous$$anager.Evaluate (System.String expression) (at Assets/Scripts/Game$$anonymous$$anager.cs:110) Game$$anonymous$$anager.EquationGenerator () (at Assets/Scripts/Game$$anonymous$$anager.cs:102) Game$$anonymous$$anager.Update () (at Assets/Scripts/Game$$anonymous$$anager.cs:51)

Bunny83 AbdulRaman · Jun 04, 2018 at 12:05 PM 0

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Do you use the version in the answer or the one i posted in the comment? I guess it might be the spaces in "< r />". Try using "<r/>".

Though i would recommend to use a different solution. Using System.Xml will add quite a bit of overhead since the whole System.Xml.dll will be shipped with your game (if i remember correctly it was about 0.5 $$anonymous$$B). I've written a simple expression parser which i've linked in the comment below.

Answer 2

Answer by Herman-Tulleken · Nov 17, 2010 at 02:49 PM

Some more info here:

http://stackoverflow.com/questions/4629/c-eval-equivalent

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Answer 3

Answer by satan_6_6_6 · Nov 13, 2015 at 09:53 PM

https://github.com/akuukka/ExpressionSolver

This one does the trick.

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Bunny83 · Nov 14, 2015 at 05:13 AM 0

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I've written a similar parser a year ago. If you have a working Unity webplayer installed you can test it here. It evaluates the expression 2000 times each frame and draws a line between the results. "x" goes from -1 to 1

satan_6_6_6 Bunny83 · Nov 26, 2015 at 07:21 PM 0

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I've used that one and it works pretty well.

Noticed a few problems though: you check variable/function names with StartsWith and therefore if you have variables temp and temp2, temp2 gets treated as temp which results in very unexpected behaviour.

How to evaluate a mathematical expression in Unity's C#

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