blocks|key|1534452|text|一个相当好的方法需要两个步骤。第一步涉及converting+the+expression+from+infix+to+postfix+(例如，通过Dijkstra's+shunting+yard)表示法。完成后，编写一个postfix+evaluator就非常简单了。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1534453|entityMap|0|LINK|mutability|MUTABLE|url|http://scriptasylum.com/tutorials/infix_postfix/algorithms/infix-postfix/index.htm|1|http://en.wikipedia.org/wiki/Shunting-yard_algorithm|2|http://scriptasylum.com/tutorials/infix_postfix/algorithms/postfix-evaluation/index.htm^0|K|1B|0|22|O|1|33|H|2|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@$A|Q|B|R|1|S]|$A|T|B|U|1|V]|$A|W|B|X|1|Y]]|C|$]]|$1|D|3|-4|5|6|7|Z|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]|L|$5|G|H|I|C|$J|M]]|N|$5|G|H|I|C|$J|O]]]]

A pretty good approach would involve two steps. The first step involves <a href="http://scriptasylum.com/tutorials/infix_postfix/algorithms/infix-postfix/index.htm" rel="noreferrer">converting the expression from infix to postfix</a> (e.g. via <a href="http://en.wikipedia.org/wiki/Shunting-yard_algorithm" rel="noreferrer">Dijkstra's shunting yard</a>) notation. Once that's done, it's pretty trivial to write a <a href="http://scriptasylum.com/tutorials/infix_postfix/algorithms/postfix-evaluation/index.htm" rel="noreferrer">postfix evaluator</a>.

blocks|key|1749659|text|我写了一些关于设计数学解析器的博客文章。这里有一个一般的introduction，关于grammars的基本知识，sample+implementation+written+in+Ruby和一个test+suite。也许你会发现这些材料很有用。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1749660|entityMap|0|LINK|mutability|MUTABLE|url|http://lukaszwrobel.pl/blog/math-parser-part-1-introduction|1|http://lukaszwrobel.pl/blog/math-parser-part-2-grammar|2|http://lukaszwrobel.pl/blog/math-parser-part-3-implementation|3|http://lukaszwrobel.pl/blog/math-parser-part-4-tests^0|S|C|0|17|8|1|1L|11|2|2P|A|3|0^^$0|@$1|2|3|4|5|6|7|R|8|@]|9|@$A|S|B|T|1|U]|$A|V|B|W|1|X]|$A|Y|B|Z|1|10]|$A|11|B|12|1|13]]|C|$]]|$1|D|3|-4|5|6|7|14|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]|L|$5|G|H|I|C|$J|M]]|N|$5|G|H|I|C|$J|O]]|P|$5|G|H|I|C|$J|Q]]]]

I wrote a few blog posts about designing a math parser. There is a general <a href="http://lukaszwrobel.pl/blog/math-parser-part-1-introduction" rel="noreferrer">introduction</a>, basic knowledge about <a href="http://lukaszwrobel.pl/blog/math-parser-part-2-grammar" rel="noreferrer">grammars</a>, <a href="http://lukaszwrobel.pl/blog/math-parser-part-3-implementation" rel="noreferrer">sample implementation written in Ruby</a> and a <a href="http://lukaszwrobel.pl/blog/math-parser-part-4-tests" rel="noreferrer">test suite</a>. Perhaps you will find these materials useful.

blocks|key|1534525|text|您有几种方法。您可以生成动态代码并执行它，以便在不需要编写大量代码的情况下获得答案。只需在.NET中搜索运行时生成的代码，周围有很多示例。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1534526|或者，您可以创建一个实际的解析器，并生成一个小的解析树，然后用它来计算表达式。同样，对于基本表达式来说，这是非常简单的。请查看codeplex，因为我相信他们有一个数学解析器。或者只需查找BNF，其中将包含示例。任何介绍编译器概念的网站都会将此作为基本示例。|1534527|Codeplex+Expression+Evaluator|offset|length|1534528|entityMap|0|LINK|mutability|MUTABLE|url|http://www.codeplex.com/Flee^0|0|0|0|T|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Q|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|R|8|@]|9|@$F|S|G|T|1|U]]|A|$]]|$1|H|3|-4|5|6|7|V|8|@]|9|@]|A|$]]]|I|$J|$5|K|L|M|A|$N|O]]]]

You have a couple of approaches. You could generate dynamic code and execute it in order to get the answer without needing to write much code. Just perform a search on runtime generated code in .NET and there are plenty of examples around.

Alternatively you could create an actual parser and generate a little parse tree that is then used to evaluate the expression. Again this is pretty simple for basic expressions. Check out codeplex as I believe they have a math parser on there. Or just look up BNF which will include examples. Any website introducing compiler concepts will include this as a basic example.

<a href="http://www.codeplex.com/Flee" rel="noreferrer">Codeplex Expression Evaluator</a>

blocks|key|1534646|text|如果你有一个"always+on“应用程序，只需将数学字符串发布到google并解析结果即可。简单的方式，但不确定这是否是你需要的--但我猜在某种程度上是聪明的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1534647|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

If you have an "always on" application, just post the math string to google and parse the result. Simple way but not sure if that's what you need - but smart in some way i guess.

blocks|key|1749765|text|我知道这是旧的，但我遇到了这一点，试图开发一个计算器作为更大的应用程序的一部分，并遇到一些问题，使用公认的答案。这些链接对理解和解决这个问题非常有帮助，不应该被忽视。我用Java语言编写了一个Android应用程序，对于表达式"string“中的每一项，当用户在键盘上键入时，我实际上在ArrayList中存储了一个字符串。对于中缀到后缀的转换，我迭代了ArrayList中的每个字符串，然后计算新排列的字符串的后缀ArrayList。对于少量的操作数/运算符来说，这是非常棒的，但更长的计算始终是错误的，特别是当表达式开始计算为非整数时。在提供的Infix+to+Postfix+conversion链接中，建议如果扫描的项目是运算符并且topStack项目具有更高的优先级，则弹出堆栈。我发现这几乎是正确的。如果topStack项的优先级高于或等于扫描的运算符，则弹出它，最终使我的计算结果正确。希望这能帮助任何致力于这个问题的人，并感谢Justin+Poliey+(和fas?)提供了一些无价的链接。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1749766|entityMap|0|LINK|mutability|MUTABLE|url|http://scriptasylum.com/tutorials/infix_postfix/algorithms/infix-postfix/index.htm^0|7M|R|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

I know this is old, but I came across this trying to develop a calculator as part of a larger app and ran across some issues using the accepted answer. The links were IMMENSELY helpful in understanding and solving this problem and should not be discounted. I was writing an Android app in Java and for each item in the expression "string," I actually stored a String in an ArrayList as the user types on the keypad. For the infix-to-postfix conversion, I iterated through each String in the ArrayList, then evaluated the newly arranged postfix ArrayList of Strings. This was fantastic for a small number of operands/operators, but longer calculations were consistently off, especially as the expressions started evaluating to non-integers. In the provided link for <a href="http://scriptasylum.com/tutorials/infix_postfix/algorithms/infix-postfix/index.htm" rel="nofollow">Infix to Postfix conversion</a>, it suggests popping the Stack if the scanned item is an operator and the topStack item has a higher precedence. I found that this is almost correct. Popping the topStack item if it's precedence is higher OR EQUAL to the scanned operator finally made my calculations come out correct. Hopefully this will help anyone working on this problem, and thanks to Justin Poliey (and fas?) for providing some invaluable links.

blocks|key|1749625|text|相关的问题Equation+(expression)+parser+with+precedence?也有一些关于如何开始使用它的很好的信息。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1749626|-Adam|1749627|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/28256/equation-expression-parser-with-precedence^0|5|19|0|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@$A|O|B|P|1|Q]]|C|$]]|$1|D|3|E|5|6|7|R|8|@]|9|@]|C|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|C|$]]]|G|$H|$5|I|J|K|C|$L|M]]]]

The related question <a href="https://stackoverflow.com/questions/28256/equation-expression-parser-with-precedence">Equation (expression) parser with precedence?</a> has some good information on how to get started with this as well.

-Adam

blocks|key|1749525|text|假设您的输入是字符串格式的中缀表达式，您可以将其转换为postfix，并使用两个堆栈:一个运算符堆栈和一个操作数堆栈，从那里开始求解。你可以在维基百科的链接上找到一般的算法信息。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1749526|entityMap|0|LINK|mutability|MUTABLE|url|http://en.wikipedia.org/wiki/Reverse_Polish_notation^0|R|7|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

Assuming your input is an infix expression in string format, you could convert it to <a href="http://en.wikipedia.org/wiki/Reverse_Polish_notation" rel="nofollow noreferrer">postfix</a> and, using a pair of stacks: an operator stack and an operand stack, work the solution from there. You can find general algorithm information at the Wikipedia link.

blocks|key|1534764|text|ANTLR是一个非常好的LL(*)解析器生成器。我强烈推荐它。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1534765|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

ANTLR is a very nice LL(*) parser generator. I recommend it highly.

blocks|key|1749841|text|开发人员总是希望有一种干净的方法，并尝试从头开始实现解析逻辑，通常以Dijkstra+Shunting-Yard+Algorithm结束。结果是看起来很整洁的代码，但可能充满了bug。我已经开发了这样一个应用程序接口，JMEP，它可以做所有的事情，但我花了几年时间才有了稳定的代码。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1749842|即使做了这么多工作，你也可以从那个项目页面上看到，我正在认真考虑改用JavaCC或ANTLR，即使所有的工作都已经完成了。|1749843|entityMap|0|LINK|mutability|MUTABLE|url|http://en.wikipedia.org/wiki/Shunting-yard_algorithm|1|https://code.google.com/p/jmep/^0|Y|W|0|31|4|1|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@$A|Q|B|R|1|S]|$A|T|B|U|1|V]]|C|$]]|$1|D|3|E|5|6|7|W|8|@]|9|@]|C|$]]|$1|F|3|-4|5|6|7|X|8|@]|9|@]|C|$]]]|G|$H|$5|I|J|K|C|$L|M]]|N|$5|I|J|K|C|$L|O]]]]

Developers always want to have a clean approach, and try to implement the parsing logic from ground up, usually ending up with the <a href="http://en.wikipedia.org/wiki/Shunting-yard_algorithm" rel="nofollow">Dijkstra Shunting-Yard Algorithm</a>. Result is neat looking code, but possibly ridden with bugs. I have developed such an API, <a href="https://code.google.com/p/jmep/" rel="nofollow">JMEP</a>, that does all that, but it took me years to have stable code.

Even with all that work, you can see even from that project page that I am seriously considering to switch over to using JavaCC or ANTLR, even after all that work already done.

What is the smartest way to design a math parser? What I mean is a function that takes a math string (like: "2 + 3 / 2 + (2 * 5)") and returns the calculated value? I did write one in VB6 ages ago but it ended up being way to bloated and not very portable (or smart for that matter...). General ideas, psuedo code or real code is appreciated.

Smart design of a math parser?

编译器

Android

Java

设计数学解析器最聪明的方法是什么？我指的是一个函数，它接受一个数学字符串(比如："2 +3/2+ (2 * 5)")并返回计算值？我在很久以前就用VB6写了一个，但它最终变得臃肿，而且不太便携(或者说它很聪明……)。一般想法，伪代码或真正的代码是赞赏的。

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