We are given the grammar rules

  • $A \to F B E$

  • $B\to A C$

These rules are only some of the rules of a larger grammar $G$, but we are not given the remaining rules of $G$. We are told that $A$ is the start symbol of $G$ and that the following holds:

  • $\{\varepsilon, c, d\}\subseteq \text{FIRST}(C)$
  • $\{\varepsilon, e\} \subseteq \text{FIRST}(E)$
  • $\{\varepsilon, f, g\} \subseteq \text{FIRST}(F)$

Recall that end of file is denoted EOF.

Which of the following must hold?

  • $\varepsilon \in \text{FIRST}(B)$
  • $f \in \text{FIRST}(B)$
  • $c \in \text{FIRST}(B)$
  • $d \in \text{FIRST}(B)$
  • $a \in \text{FIRST}(B)$
  • $EOF \in \text{FIRST}(B)$

When I don't know what consist of $A,B,C,D,E,F$ given the condition above, how can I infer what $\text{FIRST}(B)$ would be?

My attempt was something like this: Given the condition, I can infer that $\text{FIRST}(A)$ may contain $f, g$ but absolutely can't have $e$. $\text{FIRST}(B)$ may contain $f, g$, but without knowing what $\text{FIRST}(A)$ is, I can't really figure out what $\text{FIRST}(B)$ is.

Am I missing something? Is there a way to fully infer what $\text{FIRST}(B)$ is without having all the information?


1 Answer 1


Recall that if $X \to Y_1…Y_k$ is a rule of the grammar, then:

  • $\Sigma\cap\text{FIRST}(Y_1)\subseteq \text{FIRST}(X)$
  • if, for some $2\leqslant j \leqslant k$, $\varepsilon \in \bigcap\limits_{i=1}^{j-1}\text{FIRST}(Y_i)$, then $\Sigma\cap\text{FIRST}(Y_j)\subseteq \text{FIRST}(X)$
  • if $\varepsilon \in \bigcap\limits_{i=1}^{k}\text{FIRST}(Y_i)$, then $\varepsilon\in\text{FIRST}(X)$.

Since $A\to FBE$ is a rule, $\Sigma \cap \text{FIRST}(F)\subseteq \text{FIRST}(A)$. Since $B\to AC$ is a rule, $\Sigma \cap \text{FIRST}(A)\subseteq \text{FIRST}(B)$. That means $\Sigma \cap \text{FIRST}(F)\subseteq \text{FIRST}(B)$, and so $f\in \text{FIRST}(B)$, no matter what the other rules of the grammar.

On the other hand, one cannot guarantee that $\varepsilon \in \text{FIRST}(B)$. Indeed, the following grammar:

$A\to FBE\mid a$

$B\to AC$

$C \to \varepsilon \mid c \mid d$

$E \to \varepsilon \mid e$

$F\to \varepsilon \mid f \mid g$

satisfies all the conditions, but $B$ cannot be derivated into $\varepsilon$.

Now that I answered the first two propositions, it is your turn to work on the others (either by finding inclusions or counter-examples).

  • $\begingroup$ Hi! I appreciate this. I have not learnt these rules. I was simply given how to calculate FIRST sets so this is very helpful. Given this rule, FIRST(B) does include f and g and we can't guarantee whether epsilon can be included because of the absence of FIRST(A). Did I understand that correctly? $\endgroup$
    – tmhs
    Apr 15 at 6:34
  • $\begingroup$ I tried to apply this to a new problem set but it only put. me in confusion: A ➝ F C B E which is one rule from a larger grammar G, but we are not given the remaining rules of G. We are told that A is the start symbol of G and that the following holds: {ε, c, d} ⊆ FIRST(C), {ε, e} ⊆ FIRST(E), {ε, a, b} ⊆ FIRST(F) The ask is given this, figure out FIRST(A). According to what you have introduced above, I know that FIRST(A) includes {a,b}. The answer reads that it should also include {c}. But based on how to calculate FIRST set (see following comment), that seems impossible! $\endgroup$
    – tmhs
    Apr 15 at 8:59
  • $\begingroup$ Rule I am referring to : 1. FIRST(x) = { x } if x is a terminal, 2. FIRST(𝜺) = { 𝜺 }, 3. If A → Bα is a production rule, then add FIRST(B) – { 𝜺 } to FIRST(A), 4. If A → B0B1B2…BiBi+1…Bk and 𝜺 ∈ FIRST(B0) and 𝜺 ∈ FIRST(B1) and 𝜺 ∈ FIRST(B2) and … and 𝜺 ∈ FIRST(Bi), THEN add FIRST(Bi+1) – { 𝜺 } to FIRST(A), 5. If A → B0…Bk and FIRST(B0) and 𝜺 ∈ FIRST(B1) and 𝜺 ∈ FIRST(B2) and … and 𝜺 ∈ FIRST(Bk), then add 𝜺 to FIRST(A) $\endgroup$
    – tmhs
    Apr 15 at 8:59
  • $\begingroup$ Rules 3, 4, 5 are exactly the same rules I have stated, just formulated a bit differently… $\endgroup$
    – Nathaniel
    Apr 15 at 11:22
  • $\begingroup$ Also, of course $c\in \text{FIRST}(A)$, since $\varepsilon \in \text{FIRST}(F)$, and using your rule 4. $\endgroup$
    – Nathaniel
    Apr 15 at 11:30

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