LR Parser

LR parsing is one type of bottom up parsing. It is used to parse the large class of grammars.

"L" stands for left-to-right scanning of the input, "R" stands for constructing a right most derivation in reverse

SLR Parser

SLR parser refers to simple LR Parsing. It is same as LR(0) parsing. The only difference is in the parsing table.To construct SLR (1) parsing table, we use canonical collection of LR (0) item.

In the SLR (1) parsing, we place the reduce move only in the follow of left hand side.

CLR Parsers

CLR refers to canonical lookahead. CLR parsing use the canonical collection of LR (1) items to build the CLR (1) parsing table. CLR (1) parsing table produces the more number of states as compare to the SLR (1) parsing.

In the CLR (1), we place the reduce node only in the lookahead symbols.

LALR Parsers

LALR refers to the lookahead LR. To construct the LALR (1) parsing table, we use the canonical collection of LR (1) items.

In the LALR (1) parsing, the LR (1) items which have same productions but different look ahead are combined to form a single set of items

LR(1) Parser

Create your own LR Parsers and check how they works

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SLR Parser

Create your own SLR Parser and check how it works

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About Parsers

Parser is also known as Syntax Analyzer.

Types of Parsers

Parser is mainly classified into 2 categories: Top-down Parser, and Bottom-up Parser.

1.Top-down Parser

Top-down parser is the parser which generates parse for the given input string with the help of grammar productions by expanding the non-terminals i.e. it starts from the start symbol and ends on the terminals. It uses left most derivation.

2.Bottom-up Parser

Bottom-up Parser is the parser which generates the parse tree for the given input string with the help of grammar productions by compressing the non-terminals i.e. it starts from non-terminals and ends on the stat symbol. It uses reverse of the right most derivation.

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Differences

LR(0) ⊂ SLR(1) ⊂ LALR(1) ⊂ CLR(1)

LL Parser

  • First L of LL is for left to right and second L is for leftmost derivation.
  • It follows the left most derivation.
  • LL starts with only the root non-terminal on the stack.
  • Starts with (S), It reads terminal when its pop one of the stack.
  • LL uses pre-order traversal of the parse tree.

LR Parser

  • L of LR is for left to right and R is for rightmost derivation.
  • It follows reverse of right most derivation.
  • In LR parser, terminals are compressed.
  • Ends with start symbol(S).
  • Post-order traversal of the parser tree.

Top down Parsing

  • It attempts to find the left most derivations for an input string.
  • In this parsing technique we start parsing from top to down in top-down manner.
  • This parsing technique uses Left Most Derivation.
  • It’s main decision is to select what production rule to use in order to construct the string.

Bottom-Up Parser

  • It can be defined as an attempts to reduce the input string to start symbol of a grammar.
  • In this parsing technique we start parsing from bottom to up.
  • Technique uses Right Most Derivation.
  • It’s main decision is to select when to use a production rule to reduce the string to get the starting symbol.

LL Parser

  • First L of LL is for left to right and second L is for leftmost derivation.
  • It follows the left most derivation.
  • LL starts with only the root non-terminal on the stack.
  • Starts with (S), It reads terminal when its pop one of the stack.
  • LL uses pre-order traversal of the parse tree.

LR Parser

  • L of LR is for left to right and R is for rightmost derivation.
  • It follows reverse of right most derivation.
  • In LR parser, terminals are compressed.
  • Ends with start symbol(S).
  • Post-order traversal of the parser tree.

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