What is first and follow in compiler design?
As the first character in the input is a, the parser applies the rule A->aBb. In RHS of A -> aBb, b follows Non-Terminal B, i.e. FOLLOW(B) = {b}, and the current input character read is also b. Hence the parser applies this rule. And it is able to get the string “ab” from the given grammar.
How do you write first and follow in compiler design?
For any production rule A → αBβ,
- If ∈ ∉ First(β), then Follow(B) = First(β)
- If ∈ ∈ First(β), then Follow(B) = { First(β) – ∈ } ∪ Follow(A)
What is the uses of first and follow in compiler design?
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.
In which phase of compiler do we use first?
Lexical Analysis
Lexical Analysis: Lexical analyzer phase is the first phase of compilation process. It takes source code as input. It reads the source program one character at a time and converts it into meaningful lexemes.
How do you get first and follow sets?
If α is any string of grammar symbols, let FIRST(α) be the set of terminals that begin the strings derived from α. If α ⇒ ε then ε is also in FIRST(α). To compute FIRST(X) for all grammar symbols X, apply the following rules until no more terminals or ε can be added to any FIRST set: 1.
What are first sets?
FIRST(u) is the set of terminals that can occur first in a full derivation of u , where u is a sequence of terminals and non-terminals. In other words, when calculating the FIRST(u) set, we are looking only for the terminals that could possibly be the first terminal of a string that can be derived from u .
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