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POParse tree generation with Java CUP
 

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  1. POParse tree generation with Java CUP
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    <p>I am using CUP with JFlex to validate expression syntax. I have the basic functionality working: I can tell if an expression is valid or not.</p> <p>Next step is to implement simple arithmetic operations, such as "add 1". For example, if my expression is "1 + a", the result should be "2 + a". I need access to parse tree to do that, because simply identifying a numeric term won't do it: the result of adding 1 to "(1 + a) * b" should be "(1 + a) * b + 1", not "(2 + a) * b".</p> <p>Does anyone have a CUP example that generates a parse tree? I think I will be able to take it from there.</p> <p>As an added bonus, is there a way to get a list of all tokens in expression using JFlex? Seems like a typical use case, but I cannot figure out how to do it.</p> <p><strong>Edit:</strong> Found a promising clue on stack overflow: <a href="https://stackoverflow.com/questions/4650366/create-abstract-tree-problem-from-parser">Create abstract tree problem from parser</a></p> <p>Discussion of CUP and AST:</p> <p><a href="http://pages.cs.wisc.edu/~fischer/cs536.s08/lectures/Lecture16.4up.pdf" rel="nofollow noreferrer">http://pages.cs.wisc.edu/~fischer/cs536.s08/lectures/Lecture16.4up.pdf</a></p> <p>Specifically, this paragraph:</p> <blockquote> <p>The Symbol returned by the parser is associated with the grammar’s start symbol and contains the AST for the whole source program</p> </blockquote> <p>This does not help. How to traverse the tree given <strong>Symbol</strong> instance, if Symbol class does not have any navigation pointers to its children? In other words, it does not look or behave like a tree node:</p> <pre class="lang-java prettyprint-override"><code>package java_cup.runtime; /** * Defines the Symbol class, which is used to represent all terminals * and nonterminals while parsing. The lexer should pass CUP Symbols * and CUP returns a Symbol. * * @version last updated: 7/3/96 * @author Frank Flannery */ /* **************************************************************** Class Symbol what the parser expects to receive from the lexer. the token is identified as follows: sym: the symbol type parse_state: the parse state. value: is the lexical value of type Object left : is the left position in the original input file right: is the right position in the original input file ******************************************************************/ public class Symbol { /******************************* Constructor for l,r values *******************************/ public Symbol(int id, int l, int r, Object o) { this(id); left = l; right = r; value = o; } /******************************* Constructor for no l,r values ********************************/ public Symbol(int id, Object o) { this(id, -1, -1, o); } /***************************** Constructor for no value ***************************/ public Symbol(int id, int l, int r) { this(id, l, r, null); } /*********************************** Constructor for no value or l,r ***********************************/ public Symbol(int sym_num) { this(sym_num, -1); left = -1; right = -1; value = null; } /*********************************** Constructor to give a start state ***********************************/ Symbol(int sym_num, int state) { sym = sym_num; parse_state = state; } /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/ /** The symbol number of the terminal or non terminal being represented */ public int sym; /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/ /** The parse state to be recorded on the parse stack with this symbol. * This field is for the convenience of the parser and shouldn't be * modified except by the parser. */ public int parse_state; /** This allows us to catch some errors caused by scanners recycling * symbols. For the use of the parser only. [CSA, 23-Jul-1999] */ boolean used_by_parser = false; /******************************* The data passed to parser *******************************/ public int left, right; public Object value; /***************************** Printing this token out. (Override for pretty-print). ****************************/ public String toString() { return "#"+sym; } } </code></pre>
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