Prerequisite – Classification of top down parsers, FIRST Set, FOLLOW Set

A top-down parser builds the parse tree from the top down, starting with the start non-terminal. There are two types of Top Down Parsers:

- Top Down Parser with Backtracking
- Top Down Parsers without Backtracking

Top Down Parsers without Backtracking can further be divided into two parts:

In this article we are going to discuss about Non-Recursive Descent which is also known as LL(1) Parser.

**LL(1) Parsing:**

Here the 1st **L** represents that the scanning of the Input will be done from Left to Right manner and second **L** shows that in this Parsing technique we are going to use Left most Derivation Tree. and finally the **1** represents the number of look ahead, means how many symbols are you going to see when you want to make a decision.

**Construction of LL(1) Parsing Table:**

To construct the Parsing table, we have two functions:

**1: First():** If there is a variable, and from that variable if we try to drive all the strings then the beginning *Terminal Symbol* is called the first.

**2: Follow():** What is the *Terminal Symbol* which follow a variable in the process of derivation.

Now, after computing the First and Follow set for each *Non-Terminal symbol* we have to construct the Parsing table. In the table Rows will contain the Non-Terminals and the column will contain the Terminal Symbols.

All the **Null Productions** of the Grammars will go under the Follow elements and the remaining productions will lie under the elements of First set.

Now, let’s understand with an example.

**Example-1:**

Consider the Grammar:

E --> TE' E' --> +TE' | e T --> FT' T' --> *FT' | e F --> id | (E) **e denotes epsilon

Find their first and follow sets:

First | Follow | |
---|---|---|

E –> TE’ |
{ id, ( } | { $, ) } |

E’ –> +TE’/e |
{ +, e } | { $, ) } |

T –> FT’ |
{ id, ( } | { +, $, ) } |

T’ –> *FT’/e |
{ *, e } | { +, $, ) } |

F –> id/(E) |
{ id, ( } | { *, +, $, ) } |

Now, the LL(1) Parsing Table is:

id | + | * | ( | ) | $ | |
---|---|---|---|---|---|---|

E |
E –> TE’ | E –> TE’ | ||||

E’ |
E’ –> +TE’ | E’ –> e | E’ –> e | |||

T |
T –> FT’ | T –> FT’ | ||||

T’ |
T’ –> e | T’ –> *FT | T’ –> e | T’ –> e | ||

F |
F –> id | F –> (E) |

As you can see that all the null productions are put under the follow set of that symbol and all the remaining productions are lie under the first of that symbol.

**Note:** Every grammar is not feasible for LL(1) Parsing table. It may be possible that one cell may contain more than one production.

Let’s see with an example.

**Example-2:**

Consider the Grammar

S --> A | a A --> a

Find their first and follow sets:

First | Follow | |
---|---|---|

S –> A/a |
{ a } | { $ } |

A –>a |
{ a } | { $ } |

Parsing Table:

a | $ | |
---|---|---|

S |
S –> A, S –> a | |

A |
A –> a |

Here, we can see that there are two productions into the same cell. Hence, this grammar is not feasible for LL(1) Parser.

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