# How do you calculate first and follow sets?

## How do you calculate first and follow sets?

For any production rule A → αBβ,

1. If ∈ ∉ First(β), then Follow(B) = First(β)
2. If ∈ ∈ First(β), then Follow(B) = { First(β) – ∈ } ∪ Follow(A)

## What is first () and follow () functions?

FIRST and FOLLOW help us to pick a rule when we have a choice between two or more r.h.s. by predicting the first symbol that each r.h.s. can derive.

Why do we calculate first and follow?

So FOLLOW can make a Non-terminal to vanish out if needed to generate the string from the parse tree. The conclusions is, we need to find FIRST and FOLLOW sets for a given grammar, so that the parser can properly apply the needed rule at the correct position.

What are the rules to calculate first information?

Rules to compute FIRST set:

• FIRST(X) = FIRST(Y1)
• If FIRST(Y1) contains Є then FIRST(X) = { FIRST(Y1) – Є } U { FIRST(Y2) }
• If FIRST (Yi) contains Є for all i = 1 to n, then add Є to FIRST(X).

### Is there a standard algorithm for computing first and FOLLOW sets?

The standard algorithm for computing FIRST and FOLLOW sets is discussed in most compiler textbooks and books on parsing algorithms. I would be surprised if you were taking a course where this was covered and had no assigned reading or materials provided on this topic.

### How to calculate first and FOLLOW sets in grammar?

A graphical method has been given to calculate the First and Follow sets of a grammar which is easier to apply for a learner or a student… This graphical method is applied to the Expression Grammar to find the First and Follow sets and we also check its correctness with the help of derivations.

How to calculate the first and follow function?

Before calculating the first and follow functions, eliminate Left Recursion from the grammar, if present. We calculate the follow function of a non-terminal by looking where it is present on the RHS of a production rule. The given grammar is left recursive. So, we first remove left recursion from the given grammar.

How to compute follow ( a ) for all nonterminals?

To compute FOLLOW(A) for all nonterminals A, apply the following rules until nothing can be added to any FOLLOW set: 1. Place \$ in FOLLOW(S), where S is the start symbol and \$ is the input right endmarker. 2. If there is a production A ⇒ αΒβ , then everything in FIRST(β ), except for ε, is placed in FOLLOW(B).