Top Down or Bottom Up for Expression Evaluation ? whats the best custom essay website Last time I wrote about Top Down parsing, a technique for parsing that can be easilly implemented manually using function recursion. grade essays online Today it is time to talk about one of the other ways of parsing, called Bottom Up parsing. This technique try to find the most important units first and then, based on a language grammar and a set of rules, try to infer higher order structures starting from them. Bottom Up parsers are the common output of Parser Generators (you can find a good comparison of parser generators here) as they are easier to generate automatically then recursive parser because they can be implemented using sets of tables and simple state based machines.

go here mba dissertation sample Writing down manually a bottom up parser is quite a tedious task and require quite a lot of time; moreover, this kind of parsers are difficult to maintain and are usually not really expandable or portable. This is why for my tests I decided to port bYacc (a parser generator that usually generates C code) and edit it so it generates ActionScript code starting from Yacc-compatible input grammars. Having this kind of tool makes things a lot easier, because maintaining a grammar (that usually is composed by a few lines) is less time expensive than working with the generated code (that usually is many lines long). go I will not release today the port because actually I had not time to make sure it is bugfree and I’ve only a working version for Mac, but I plan to release it shortly if you will ever need it for your own tasks. My goal for today was to compare the speed of the parser I wrote with an automatically generated bottom up parser, to see which is the faster approach.

best resume writing services in mumbai writing and essay I created a bottom up parser which is able to execute the same expressions accepted by the expression evaluator I wrote last time. There are anyways some differences that – as you will probably and hopefully understand in the future – that make those parsers really different. Some will be disussed shortly here.

mba term paper To do that I created a Yacc grammar and some support classes. follow url The parser grammar is really simple and really readable:
%left '+' '-'
%left '*' '/'
%left NEG
: expr                { Vars.result = $1; }
: expr '+' expr       { $$ = $1 + $3; }
| expr '-' expr       { $$ = $1 - $3; }
| expr '*' expr       { $$ = $1 * $3; }
| expr '/' expr       { $$ = $1 / $3; }
| '-' expr %prec NEG  { $$ = -$2; }
| '(' expr ')'        { $$ = $2; }
| SYMBOL '(' expr ')' {
if( Vars.SYMBOL_TABLE[ $1 ] )
$$ = Vars.SYMBOL_TABLE[ $1 ]( $3 );
} else
trace( "can't find function" );
| SYMBOL              {
if( Vars.SYMBOL_TABLE[ $1 ] )
$$ = Vars.SYMBOL_TABLE[ $1 ];
} else
trace( "can't find symbol" );
| NUMBER              { $$ = yyval; }

already written essays paper about management Continue reading the extended entry to see the results. Continue reading


Runtime expression evaluation in ActionScript i need help with writing a research paper Finally I have a bit of free time to write down this post.
Some days ago a guy on the forum asked about how to how to write a 10 page paper evaluate at runtime simple mathematical expressions with support for symbols and function calls. does buying an essay work Now that the eval function have been removed from the language for reasons I’ve not enought time to talk about, we need to parse and execute those expressions manually. business plan pro free download I built a simple Actionscript library that can be used to parse mathematical expressions: it has support for function calls, for variables and it is able to convert the expression into a postfix rappresentation if you may need it. The expression parser is quite simple and have been built manually following some concepts related to programming language compilers; it includes a Scanner (or lexer – however you want to call it) to tokenize the expression, a Parser that convert the expression into an Abstract Syntax Tree and a simple routine that evaluates that AST using a Symbol Table as evaluation context.

source site phd thesis criminal law There are no comments inside the code right now; I hope to find a little bit of time to write an in depth discussion about this topic. In the mean time you can continue reading the entry for a general explanation about how does the code works and for some examples that may be useful.

page buy research papers no plagiarism Here is a simple example that shows how does the code works:

i need someone to do my term paper
import it.sephiroth.expr.CompiledExpression;
import it.sephiroth.expr.Parser;
import it.sephiroth.expr.Scanner;
public class Example
public static function run(): void
var expression: String = "sin( x / ( 8 / 2 + (-0.12 + 2.3) * x / x ) ) * 100";
var scanner: Scanner = new Scanner( expression );
var parser: Parser = new Parser( scanner );
var compiled: CompiledExpression = parser.parse();
var context: Object = {
x:    100,
sin:   Math.sin,
cos:  Math.cos
trace( 'Postfix:', compiled.toString() );
trace( 'Result:', compiled.execute( context ) );
} global business plan template Before I forgot, you can download the source code here with a simple example included.

professional resume writing services washington dc Continue reading

get link