# Math parser? **URL:** \ **Category:** Rails\ **Created:** [May 18, 2008, 8:21pm UTC](https://www.ruby-forum.com/t/math-parser/137946 "2008-05-18T20:21:02Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![emuen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/96bed5/32.png) [@emuen](https://www.ruby-forum.com/u/emuen)\ **Post date:** [May 18, 2008, 8:21pm UTC](https://www.ruby-forum.com/t/math-parser/137946/1 "2008-05-18T20:21:02Z")
HI guys. I need to calculate a string as a math piece. E.i. “100\*500/2.5” Is there a build in parser in the Math class to do this? ---
**Author:** ![emuen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/96bed5/32.png) [@emuen](https://www.ruby-forum.com/u/emuen)\ **Post date:** [May 19, 2008, 1:20am UTC](https://www.ruby-forum.com/t/math-parser/137946/2 "2008-05-19T01:20:26Z")
> Is there a build in parser in the Math class to do this? I don’t think so, but we might roll our own. module Math def self.eval(expression) allowed\_characters = Regexp.escape('±_/.’) safe\_expression = expression.match(/[\d#{allowed\_characters}]_/).to\_s Kernel.eval(safe\_expression) end end I believe the above is safe, but don’t shoot me if it’s not. Tor Erik THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED … ---
**Author:** ![emuen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/96bed5/32.png) [@emuen](https://www.ruby-forum.com/u/emuen)\ **Post date:** [May 19, 2008, 7:17am UTC](https://www.ruby-forum.com/t/math-parser/137946/3 "2008-05-19T07:17:02Z")
Super. Ill try that. I would need to add this to the math class in the ruby root library? Tor Erik L. wrote: > > Is there a build in parser in the Math class to do this? > > I don’t think so, but we might roll our own. > > module Math > def self.eval(expression) > allowed\_characters = Regexp.escape('±_/.’) > safe\_expression = > expression.match(/[\d#{allowed\_characters}]_/).to\_s > Kernel.eval(safe\_expression) > end > end > > I believe the above is safe, but don’t shoot me if it’s not. > > Tor Erik > > THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS > OR IMPLIED, INCLUDING BUT NOT LIMITED … ---
**Author:** ![emuen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/96bed5/32.png) [@emuen](https://www.ruby-forum.com/u/emuen)\ **Post date:** [May 19, 2008, 7:28am UTC](https://www.ruby-forum.com/t/math-parser/137946/4 "2008-05-19T07:28:20Z")
You can put this file in the lib directory, and call it something simple like custom\_math.rb, then in your application.rb do include Math and it should include that. On Mon, May 19, 2008 at 2:47 PM, Emil K. \< removed\_email\_address@domain.invalid\> wrote: > > def self.eval(expression) > > > > THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS > > OR IMPLIED, INCLUDING BUT NOT LIMITED … > > – > Posted via [http://www.ruby-forum.com/](http://www.ruby-forum.com/). > > > – Appreciated my help? Reccommend me on Working With Rails [http://workingwithrails.com/person/11030-ryan-bigg](http://workingwithrails.com/person/11030-ryan-bigg) ---
**Author:** ![emuen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/96bed5/32.png) [@emuen](https://www.ruby-forum.com/u/emuen)\ **Post date:** [May 19, 2008, 5:44pm UTC](https://www.ruby-forum.com/t/math-parser/137946/5 "2008-05-19T17:44:55Z")
If you’re willing to wrap it with some error handling you can let ‘eval’ take care of it: eval(“100\*500/2.5”) =\> 20000.0 (It understands parens, too). > > eval(“100\*(500/2.5)”) > > =\> 20000.0 On May 19, 1:27 am, “Ryan B. (Radar)” [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) ---
**Author:** ![emuen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/96bed5/32.png) [@emuen](https://www.ruby-forum.com/u/emuen)\ **Post date:** [May 19, 2008, 9:38pm UTC](https://www.ruby-forum.com/t/math-parser/137946/6 "2008-05-19T21:38:51Z")
AndyV wrote: > If you’re willing to wrap it with some error handling you can let > ‘eval’ take care of it Just be carefull so you don’t unintentionally allow arbitrary code to be executed. (Depending on the use case this may or may not be an issue). If you change Regexp.escape(‘±\*/.’) to Regexp.escape('±\*/.() ') then you can use parentheses and spaces in my example too. Tor Erik ---
**Author:** ![emuen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/96bed5/32.png) [@emuen](https://www.ruby-forum.com/u/emuen)\ **Post date:** [November 24, 2008, 8:17pm UTC](https://www.ruby-forum.com/t/math-parser/137946/7 "2008-11-24T20:17:34Z")
> HI guys. > > I need to calculate a string as a math piece. E.i. “100\*500/2.5” Is > there a build in parser in the Math class to do this? Hi, I have just written a few blog posts about building a math parser. There is an introduction, some basic knowledge about grammars, sample implementation written in Ruby and a test suite. You can find these posts here: - [Math Parser, Part 1: Introduction - Lukasz Wrobel](http://lukaszwrobel.pl/blog/math-parser-part-1-introduction) - [Math Parser, Part 2: Grammar - Lukasz Wrobel](http://lukaszwrobel.pl/blog/math-parser-part-2-grammar) - [Math Parser, Part 3: Implementation - Lukasz Wrobel](http://lukaszwrobel.pl/blog/math-parser-part-3-implementation) - [Math Parser, Part 4: Tests - Lukasz Wrobel](http://lukaszwrobel.pl/blog/math-parser-part-4-tests) If you find the code useful, feel free to use it in your own program.