14-Jul-15 Parser Hints

The Stack To turn a “Recognizer” into a “Parser,” we need the use of a Stack All boolean Recognizer methods should continue to return boolean values, but in addition, if they succeed they should leave a Tree on the stack The stack should be an instance variable of the Parser Here’s one easy way to do it: Create a new Parser(String s) for each String you want to use, and call some parsing method ( term, command,...) for that particular String

Recognizer helper methods private boolean number() { return nextTokenMatches(Token.NUMBER); } private boolean name() { return nextTokenMatches(Token.NAME); } private boolean keyword(String expectedKeyword) { return nextTokenMatches(Token.KEYWORD, expectedKeyword); } private boolean symbol(String expectedSymbol) { return nextTokenMatches(Token.SYMBOL, expectedSymbol); }

nextTokenMatches private boolean nextTokenMatches(int type) { Token t = tokenizer.next(); if (t.getType() == type) { return true; } else tokenizer.pushBack(t); return false; } private boolean nextTokenMatches(int type, String value) { Token t = tokenizer.next(); if (type == t.getType() && value.equals(t.getValue())) { return true; } else tokenizer.pushBack(t); return false; }

Revised nextTokenMatches private boolean nextTokenMatches(int type) { Token t = tokenizer.next(); if (t.getType() == type) { stack.push(new Tree(t)); return true; } else tokenizer.pushBack(t); return false; } private boolean nextTokenMatches(int type, String value) { Token t = tokenizer.next(); if (type == t.getType() && value.equals(t.getValue())) { stack.push(new Tree(t)); return true; } else tokenizer.pushBack(t); return false; }

comparator ::= " “ public boolean comparator() { if (symbol(" ")) return true; return false; } Whatever a comparator is found, one Tree node (with the Token representing that comparator as its value) is left on the stack Since we recognized one thing, and we leave one thing on the stack, this method does not need to be changed in any way

while command public boolean whileCommand() { if (keyword("while")) { if (condition()) { if (block()) { return true; } } error("Error in \"while\" statement"); } return false; } while condition block We want to change this: command Into this: makeTree(3, 2, 1); The makeTree method will take three things off the stack and replace them with one thing

Counting I like to count this way: while condition block This way is easier to implement: while condition block I prefer the first way because, although it means trickier arithmetic in makeTree, it simplifies counting everywhere else while condition block

Accessing the Stack A stack, as a stack, has no methods for accessing the n th thing from the top, but... class Stack extends Vector...and Vector has an elementAt(int index) operator This represents a stack with five things in it The “ e ” is at the top of the stack This means that, with some annoying arithmetic, you can access any element of the stack you like, counting elementAt(vector.size() - 1) as the top abcde

Recursion in BNF ::= | “*” is bad, because of the left recursion Implemented in the obvious way, it would cause our program to go into an infinite recursion ::= | “*” has another problem Implemented in the obvious way, it will build a tree of the wrong shape ::= { “*” } works well Implemented in the obvious way—iteratively, not recursively—it builds a tree of the correct shape

term() public boolean term() { if (!factor()) return false; while (multiplyOperator()) { if (!factor()) error("No term after '*' or '/'"); makeTree(2, 3, 1); } return true; }

The End