1.
What is Bottom up parsing?
1.1.
Example
2.
Classification of Bottom-up parsing
2.1.
Shift Reduce Parsing
2.1.1.
Example
2.1.2.
Parsing table
2.2.
Operator Precedence Parsing
2.2.1.
Precedence table
2.2.2.
Parsing Action
2.3.
Table-driven LR Parsing
2.3.1.
LR algorithm
2.3.2.
LR (1) Parsing
2.3.3.
Augment Grammar
3.
LL vs. LR
4.
4.1.
How does bottom-up parsing work?
4.2.
Write a drawback of the LR parser?
4.3.
What are the algorithms for constructing an LR parser?
4.4.
What is the reduce step in shift-reduce parsing?
4.5.
What are the features of a simple LR Parser?
5.
Conclusion
Last Updated: Jul 3, 2024
Easy

# Bottom-up Parsing

Muskan Gupta
Master Python: Predicting weather forecasts
Speaker
Ashwin Goyal
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Bottom-up parsing begins at the terminal symbols (leaf nodes) of a parse tree and ascends towards the root node, combining smaller units into larger ones until the entire structure is formed. This method is commonly used in syntax analysis to construct parse trees for input strings.

This blog covers the concept of bottom-up parsing, Shift Reduce Parsing, Operator Precedence Parsing, and table-driven L-R Parsing.

## What is Bottom up parsing?

This parser compresses the non-terminals where it starts and moves backwards to the starting symbol of the parse tree using grammar productions to draw the input string's parse tree.

• In this technique, parsing starts from the leaf node of the parse tree to the start symbol of the parse tree in a bottom-up manner.
• Bottom-up parsing attempts to construct a parse tree by reducing input string and using Right Most Derivation.
• Bottom-up parsing starts with the input symbol and constructs the parse tree up to the start symbol using a production rule to reduce the string to get the starting symbol.

### Example

1. E → T
2. T → T * F
3. T → id
4. F → T
5. F → id

Parse Tree representation of input string "id * id" is as follows:

## Classification of Bottom-up parsing

Bottom-up parsing has been classified into various parsing. These are as follows:

1. Shift-Reduce Parsing
2. Operator Precedence Parsing
3. Table Driven LR Parsing

Classification of Bottom-up parsing

### Shift Reduce Parsing

Shift reduce parsing is a process to reduce a string to its grammar start symbol. It uses a stack to hold the grammar and an input tape to hold the string. Shift reduce parsing performs the two actions: shift and reduce. This is why it is known as shift-reduce parsing. The current symbol in the input string is pushed to a stack at the shift action. At each reduction, the symbols will be replaced by the non-terminals. The non-terminal is the left side of the output, and the symbol is the right side of the production.

#### Example

Grammar:

1. S → S+S
2. S → S-S
3. S → (S)
4. S → a

Input string: a1-(a2+a3)

#### Parsing table

There are two main categories in shift-reduce parsing as follows:

1. Operator-Precedence Parsing
2. LR-Parser

### Operator Precedence Parsing

Operator precedence grammar is a category in the shift-reduce method of parsing. It is applied to a class of grammars operators. Operator precedence grammar must have the following two properties:

• No RHS of any product has a∈.
• Two non-terminals must not be adjacent.

Operator precedence can only be fixed between the terminals of the grammar. It generally ignores the non-terminal.

There are the three operator precedence relations:

a ⋗ b means that terminal "a" has higher precedence than terminal "b."

a ⋖ b means that terminal "a" has lower priority than terminal "b."

a ≐ b means that the terminal "a" and "b" both have the same precedence.

#### Precedence table

Following is a precedence table according to which grammar of the operator precedence parsing works.

#### Parsing Action

The sequence in which parsing action is performed in operator precedence parsing is as:

• At first, the \$ symbol is added to both ends of the string.
• Now we scan the input string from left to right until the character ⋗ is encountered.
• Scanning is done towards leftover all the equal precedence until the first leftmost ⋖ is encountered.
• Everything between leftmost ⋖ and rightmost ⋗ is a handle.
•  If it is \$ on \$, then it means parsing is successful.

### Table-driven LR Parsing

LR parsing is also a category of Bottom-up parsing. It is generally used to parse the class of grammars whose size is large. In the LR parsing, "L" stands for scanning of the input left-to-right, and "R" stands for constructing a rightmost derivation in a reverse way.

"K" stands for the count of input symbols of the look-ahead that are used to make many parsing decisions.

Types of LR Parsers:

1. LR( 1 )
2. SLR( 1 )
3. CLR ( 1 )
4. LALR( 1 )

The L stands for scan operations of Bottom-up parsing left-to-right, and R stands for scan right-to-left in Bottom-up parsing.

LR bottom-up parsing parsers have several advantages of Bottom-up parsing like:

• Used in many programming languages except Perl and C.
• Efficient implementation.
• In L-operations, they detect quickly any syntactic errors present in a bottom-up parsing example.

LR parsing is divided into four categories of Parsing:

• LR (0) parsing
• SLR parsing
• CLR parsing
• LALR parsing

Types of LR parser

#### LR algorithm

The LR algorithm requires input, output, stack, and parsing tables. In all types of LR parsing, input, output, and stack are the same, but the parsing table is different. The input buffer indicates the end of input data, and it has the string to be parsed. The symbol of "\$ follows that string."A stack is used to contain grammar symbols' sequence with the symbol \$ at the stack's Bottom.

A parsing table can be defined as an array of two-dimension. It usually contains two parts: the action part and the go-to part.

#### LR (1) Parsing

The various steps involved in the LR (1) Parsing are as follows:

• At first, context-free grammar is written for the given input.
• The ambiguity of the grammar is checked.
• Augment production is added in the given grammar.
• A canonical collection of LR (0) items are created.
• A data flow diagram is drawn.
• An LR (1) parsing table is created.

#### Augment Grammar

Augmented grammar will be generated if we add one more product in the given grammar G. It helps the parser identify when to stop the parsing and announce the acceptance of the input.

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## LL vs. LR

LL (Left-to-Right, Leftmost Derivation) and LR (Left-to-Right, Rightmost Derivation) are two types of parsing techniques used in compiler construction and formal language theory. They belong to the broader category of shift-reduce parsing algorithms. Here's a comparison between LL and LR parsing:

### How does bottom-up parsing work?

Bottom-up parsers work by "shifting" symbols onto a stack till the stack top contains a right-hand side of a production, and then the stack is reduced by replacing the right-hand side of production with the left-hand side.

### Write a drawback of the LR parser?

A major drawback of an LR parser is that it is too much work to construct an LR parser by hand.

### What are the algorithms for constructing an LR parser?

There are mainly three such algorithms:
SLR(1) - Simple LR Parser
LR(1) - LR Parser

### What is the reduce step in shift-reduce parsing?

When the parser finds a complete grammar rule in RHS and replaces it with LHS, it is a reduced step.

### What are the features of a simple LR Parser?

It works on the smallest class of grammar, and they have simple and fast calculations.

## Conclusion

In this article, we have discussed the concepts of bottom-up parsing, shift-reduce Parsing, operator precedence parsing, and table-driven LR parsing and frequently asked questions.