There are basically two types of Parsing methods, namely: Top-down Parsing Bottom-up Parsing Top-down Parsing The top-down parsing can be defined as the parsing method in which there is a parse tree generated from top to bottom. The parsing starts from the root of the derivation tree and then fills in. Bottom-up Parsing The bottom-up parsing can be defined as the inverse of the top-down parsing, which means that the parse tree is generated from bottom to top.
The parsing starts from the leaves and then fills in. The differences between Top-down and Bottom-up Parsing are as follows: Initiated From When it comes to the top-down parsing then this parsing method is always initiated from the Root; whereas; when it comes to the Bottom-up parsing then this parsing method is always initiated from the Leaves. Working Mechanism In the case of the top-down parsing, it so happens that the production is dedicated to derive and find out the similarity in the string; whereas; In the case of the bottom-up parsing, it so happens that the process starts from token and then it proceeds towards the start symbol.
Method Uses When it comes to the top-down parsing then this parsing method is most often used for Backtracking; whereas; when it comes to the bottom-up parsing then this parsing method is most often used for handling and managing the code.
The conflict situations in the state are called RR conflict with 2 reduced productions and SR conflict when 1 shifted and 1 reduced production is present. If no conflict is present its grammar is LR 0 grammar. This parser is the same as the LR 0 bottom up parsing parser but with a reduced productions entry which is written such that it uses the FOLLOW of the reduced production variable. This parser is like the CLR parser. After the operation of minimization, it is called LALR grammar when the parsing table reveals no grammar conflicts.
It is sufficient to look at the GOTO graph and not having to construct the bottom up parsing tables to determine the LR 0 grammar by looking for the conflicts being present or absent. When no conflicts are present, it is called LR 0 grammar. Therefore, it follows a left mot derivation method. Top-down parsing is a parsing strategy that first looks at the highest level of the parse tree and works down the parse tree by using the rules of a formal grammar.
Bottom up parsing is a parsing strategy that first looks at the lowest level of the parse tree and works up the parse tree by using the rules of a formal grammar. The parsing occurs from the starting symbol to the input string, in top down parsing. On the other hand, parsing occurs from the input string to the starting symbol, in bottom up parsing.
Furthermore, the main decision in top down parsing is to select what production rule to use in order to construct the string while the main decision in bottom down parsing is to select when to use a production rule to reduce the string to get the starting symbol. Moreover, top down parsing uses left most derivation and bottom down parsing uses right most derivation.
The difference between top down and bottom up parsing is that top down parsing performs the parsing from the staring symbol to the input string while bottom down parsing performs the parsing from input string to the starting symbol. Available here. Her areas of interests in writing and research include programming, data science, and computer systems.
Your email address will not be published. Leave a Reply Cancel reply Your email address will not be published. Parsing is the technique of examining a text containing a string of tokens, to find its grammatical structure according to the given grammar.
Now, let us discuss the responsibilities of a parser. The parser receives a string of token from the lexical analyser. When the string of tokens can be generated by the grammar of the source language, then the parse tree can be constructed.
Additionally, it reports the syntax errors present in the course thing. After this, the generated parse tree is transferred to the next stage of the compiler. Starts from the token and then go to the start symbol. In Top-down parsing , we know that the parse tree is generated in the top to bottom fashion i.
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