how to convert a string to a mathematical function

[nta_3886]

i have a mathematical function,example x^2+sin(x),but it is String.i want it is mathematical function,it can return value float

leo

[David Crow]

nta_3886 wrote:

i want it is mathematical function,it can return value float

Ok.

---

"A good athlete is the result of a good and worthy opponent." - David Crow

"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne

[Dan Neely]

use a stack to convert the formula into Reverse Polish Notation (RPN). Then evaluate the RPN version. You can find many examples of how to do both steps of this via google.

-- If you view money as inherently evil, I view it as my duty to assist in making you more virtuous.

[El Corazon]

dan neely wrote:

use a stack to convert the formula into Reverse Polish Notation (RPN). Then evaluate the RPN version. You can find many examples of how to do both steps of this via google.

Semi-on-topic... I just found my code for this from Pascal class back in the 80's. I used it on my first job to build an RPN based spreadsheet program in RPG II. :)

_________________________ Asu no koto o ieba, tenjo de nezumi ga warau. Talk about things of tomorrow and the mice in the ceiling laugh. (Japanese Proverb)

[Russell]

a lot of work:doh:

---

Russell

[Urs Enzler]

If you are using the .net Framework then you can compile that into a method in a temporary assembly and execute it as normal code. I'm sure you can find a sample about that in the WWW.

-^-^-^-^-^- no risk no funk ................... please vote ------>

[Luc Pattyn]

Hi, if your expression uses a .NET language such as C# you can have .NET compile and run it for you, as in http://www.codeproject.com/csharp/evalcscode.asp[^] if you want to define your own langauge you will need to parse and evaluate everything yourself, as in http://www.codeproject.com/csharp/sota_expression_evaluator.asp[^] or http://www.codeproject.com/cpp/MathieuMathParser.asp[^] Did you notice all links are to existing CodeProject articles, which you can search using the Search box available on nearly every CP page? :)

Luc Pattyn [Forum Guidelines] [My Articles]

---

this weeks tips: - make Visual display line numbers: Tools/Options/TextEditor/... - show exceptions with ToString() to see all information - before you ask a question here, search CodeProject, then Google

---

[PICguy]

If you are using Rexx then interpret say "string" Except you first need to write sin(x) and other functions you use in Rexx. You also need to use Rexx rules for syntax. Rexx can compile and execute on the fly. With just a little more detail in dos one might type rexx math 1.2*4.7 5.64 prints The file math.cmd contains /* execute incomming statement */ numeric digits 40 arg cmd interpret say cmd It's been a while but saving x to a environment variable or some such is not that hard. My 40 digit ln() in ln.cmd is /* ln(x) */ numeric digits 40 arg x /* prescale to put x into reasonable range */ xp = 0 if x > 2 then do until x < 2 xp = xp + 1 x = x / 2 end /* now that x <=2 compute ln(x) method from Abramowitz etal section 4.1.39 ln(2) to 40 places takes 26 loops */ z = x-1 val = 0 do t = 26 to 1 by -1 t2z = t*t*z val = t2z / (t+t+val+1) val = t2z / (t+t+val) end val = z / (val+1) /* account for prescale */ if xp = 0 then return val return (const(ln2)*xp) + val