Definition Of Parser

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What is a Parser? - Definition from Techopedia

What is a Parser? – Definition from Techopedia

What Does Parser Mean?
A parser is a compiler or interpreter component that breaks data into smaller elements for easy translation into another language. A parser takes input in the form of a sequence of tokens, interactive commands, or program instructions and breaks them up into parts that can be used by other components in programming. A parser usually checks all data provided to ensure it is sufficient to build a data structure in the form of a parse tree or an abstract syntax tree.
Techopedia Explains Parser
In order for the code written in human-readable form to be understood by a machine, it must be converted into machine language. This task is usually performed by a translator (interpreter or compiler). The parser is commonly used as a component of the translator that organizes linear text in a structure that can be easily manipulated (parse tree). To do so, it follows a set of defined rules called “grammar” overall process of parsing involves three stages:Lexical Analysis: A lexical analyzer is used to produce tokens from a stream of input string characters, which are broken into small components to form meaningful expressions. A token is the smallest unit in a programming language that possesses some meaning (such as +, -, *, “function”, or “new” in JavaScript). Syntactic Analysis: Checks whether the generated tokens form a meaningful expression. This makes use of a context-free grammar that defines algorithmic procedures for components. These work to form an expression and define the particular order in which tokens must be mantic Parsing: The final parsing stage in which the meaning and implications of the validated expression are determined and necessary actions are taken. A parser’s main purpose is to determine if input data may be derived from the start symbol of the grammar. If yes, then in what ways can this input data be derived? This is achieved as follows:Top-Down Parsing: Involves searching a parse tree to find the left-most derivations of an input stream by using a top-down expansion. Parsing begins with the start symbol which is transformed into the input symbol until all symbols are translated and a parse tree for an input string is constructed. Examples include LL parsers and recursive-descent parsers. Top-down parsing is also called predictive parsing or recursive Parsing: Involves rewriting the input back to the start symbol. It acts in reverse by tracing out the rightmost derivation of a string until the parse tree is constructed up to the start symbol This type of parsing is also known as shift-reduce parsing. One example is an LR rsers are widely used in the following technologies:Java and other programming and teractive data language and object definition base languages, such as deling languages, such as virtual reality modeling ripting otocols, such as HTTP and Internet remote function calls.
Parsing - Towards Data Science

Parsing – Towards Data Science

A Beginner’s Guide“shallow focus photography of spider web” by Robert Anasch on UnsplashLearn how you can parse, explore, modify and populate XML files with the Python ElementTree package, for loops and XPath expressions. As a data scientist, you’ll find that understanding XML is powerful for both web-scraping and general practice in parsing a structured documentExtensible Markup Language (XML) is a markup language which encodes documents by defining a set of rules in both machine-readable and human-readable format. Extended from SGML (Standard Generalized Markup Language), it lets us describe the structure of the document. In XML, we can define custom tags. …
Parsing | Set 1 (Introduction, Ambiguity and Parsers)

Parsing | Set 1 (Introduction, Ambiguity and Parsers)

In this article we will study about various types of parses. It is one of the most important topic in Compiler from GATE point of view. The working of various parsers will be explained from GATE question solving point of view. Prerequisite – basic knowledge of grammars, parse trees, ambiguity. Role of the parser:In the syntax analysis phase, a compiler verifies whether or not the tokens generated by the lexical analyzer are grouped according to the syntactic rules of the language. This is done by a parser. The parser obtains a string of tokens from the lexical analyzer and verifies that the string can be the grammar for the source language. It detects and reports any syntax errors and produces a parse tree from which intermediate code can be generated. Attention reader! Don’t stop learning now. Get hold of all the important CS Theory concepts for SDE interviews with the CS Theory Course at a student-friendly price and become industry going to types of parsers we will discuss on some ideas about the some important things required for understanding parsing. Context Free Grammars: The syntax of a programming language is described by a context free grammar (CFG). CFG consists of set of terminals, set of non terminals, a start symbol and set of productions. Notation –??? where? is a is a single variable [V]?? (V+T)* Ambiguity A grammar that produces more than one parse tree for some sentence is said to be ambiguous. Eg- consider a grammar S -> aS | Sa | a Now for string aaa we will have 4 parse trees, hence ambiguous For more information refer Removing Left Recursion: A grammar is left recursive if it has a non terminal (variable) S such that their is a derivation S -> Sα | β where α? (V+T)* and β? (V+T)* (sequence of terminals and non terminals that do not start with S) Due to the presence of left recursion some top down parsers enter into infinite loop so we have to eliminate left recursion. Let the productions is of the form A -> Aα1 | Aα2 | Aα3 | ….. | Aαm | β1 | β2 | …. | βn Where no βi begins with an A. then we replace the A-productions by A -> β1 A’ | β2 A’ | ….. | βn A’ A’ -> α1A’ | α2A’ | α3A’| ….. | αmA’ | ε The nonterminal A generates the same strings as before but is no longer left recursive. Let’s look at some example to understand better Removing Left Factoring: A grammar is said to be left factored when it is of the form – A -> αβ1 | αβ2 | αβ3 | …… | αβn | γ i. e the productions start with the same terminal (or set of terminals). On seeing the input α we cannot immediately tell which production to choose to expand A. Left factoring is a grammar transformation that is useful for producing a grammar suitable for predictive or top down parsing. When the choice between two alternative A-productions is not clear, we may be able to rewrite the productions to defer the decision until enough of the input has been seen to make the right choice. For the grammar A -> αβ1 | αβ2 | αβ3 | …… | αβn | γ The equivalent left factored grammar will be – A -> αA’ | γ A’ -> β1 | β2 | β3 | …… | βn The process of deriving the string from the given grammar is known as derivation (parsing). Depending upon how derivation is done we have two kinds of parsers:- Top Down ParserBottom Up ParserWe will be studying the parsers from GATE point of view. Top Down Parser Top down parsing attempts to build the parse tree from root to leaf. Top down parser will start from start symbol and proceeds to string. It follows leftmost derivation. In leftmost derivation, the leftmost non-terminal in each sentential is always chosen. Recursive Descent Parsing S()
{ Choose any S production, S ->X1X2…. ;
for (i = 1 to k)
{
If ( Xi is a non-terminal)
Call procedure Xi();
else if ( Xi equals the current input, increment input)
Else /* error has occurred, backtrack and try another possibility */}}Lets understand it better with an example A recursive descent parsing program consist of a set of procedures, one for each nonterminal. Execution begins with the procedure for the start symbol which halts if its procedure body scans the entire input string. Non Recursive Predictive Parsing: This type if parsing does not require backtracking. Predictive parsers can be constructed for LL(1) grammar, the first ‘L’ stands for scanning the input from left to right, the second ‘L’ stands for leftmost derivation and ‘1’ for using one input symbol lookahead at each step to make parsing action decisions. Before moving on to LL(1) parsers please go through FIRST and FOLLOW Construction of LL(1)predictive parsing table For each production A -> α repeat following steps – Add A -> α under M[A, b] for all b in FIRST(α) If FIRST(α) contains ε then add A -> α under M[A, c] for all c in FOLLOW(A). Size of parsing table = (No. of terminals + 1) * #variables Eg – consider the grammar S -> (L) | a L -> SL’ L’ -> ε | SL’ For any grammar if M have multiple entries than it is not LL(1) grammar Eg – S -> iEtSS’/a S’ ->eS/ε E -> b Important Notes 1. If a grammar contain left factoring then it can not be LL(1)
Eg – S -> aS | a —- both productions go in a
2. If a grammar contain left recursion it can not be LL(1)
Eg – S -> Sa | b
S -> Sa goes to FIRST(S) = b
S -> b goes to b, thus b has 2 entries hence not LL(1)
3. If a grammar is ambiguous then it can not be LL(1)
4. Every regular grammar need not be LL(1) because
regular grammar may contain left factoring, left recursion or ambiguity. We will discuss Bottom Up parser in next article (Set 2). This article is contributed by Parul Sharma

Frequently Asked Questions about definition of parser

What is parser in programming?

A parser is a compiler or interpreter component that breaks data into smaller elements for easy translation into another language. A parser takes input in the form of a sequence of tokens, interactive commands, or program instructions and breaks them up into parts that can be used by other components in programming.Sep 18, 2020

What is parsing in data science?

Parsing is the process of analyzing all the words in a sentence and identifying their syntactic and/or semantic roles. … Similarly, we use grammar in NLP to decipher how a sentence is constructed and understand its syntactic and semantic role.

What is the role of parser?

Role of the parser : … The parser obtains a string of tokens from the lexical analyzer and verifies that the string can be the grammar for the source language. It detects and reports any syntax errors and produces a parse tree from which intermediate code can be generated.Jun 28, 2021

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