I'm solving some past job interview problems. I met an embarrassing question about compilers.

The question is :

Consider the following grammar, with start symbol $E$:

\begin{align*} E &\rightarrow E \ast E\\ &\quad \mid E ~/~ E\\ &\quad \mid E + E\\ &\quad \mid E - E\\ &\quad \mid (E)\\ &\quad \mid a \mid b \mid c \mid \ldots \mid x \mid y \mid z \end{align*}

The following strings are legal derivations from this grammar:

  1. $a \ast b + c$
  2. $( a - b ) \ast c$
  3. $a ~/~ ( b – c)$

Which of the above are rightmost sentential forms?

When I was an undergraduate student, I learned that rightmost(leftmost) sentential form is result of rightmost(leftmost) derivation. so, I think if $1,2,3$ are legal derivations form above grammar then absolutely all of $1,2,3$ are rightmost sentential form. If I'm correct, above question is very worthless.

Is there any counter example that is not rightmost sentential form of the grammar $G$ although it is a legal derivation from $G$?


Is there another definition about rightmost sentential form corresponding to above question? I have googled "rightmost sentential form" but there are few results and most of them is the definition that I already know.

  • $\begingroup$ LR parsers find right-most derivations, LL parsers left-most derivations. I don't know what a "rightmost sentential form" is supposed to be. $\endgroup$
    – Raphael
    Commented Aug 9, 2016 at 21:06

5 Answers 5


I found other answers here confusing, so let me explain it clearly here-


A sentential form is any string derivable from the start symbol. Note that this includes the forms with non-terminals at intermediate steps as well.

A right-sentential form is a sentential form that occurs in a step of rightmost derivation (RMD).

A sentence is a sentential form consisting only of terminals

The examples in your question are all sentences. And since any way of derivation (including RMD) ultimately leads to a sentence, technically all sentences are right-sentential as well as left-sentential forms. So there isn't any counter example which you asked for.

Clarifying Examples : Page 2 of this link

  • $\begingroup$ "since any way of derivation (including RMD) ultimately leads to a sentence, technically all sentences are right-sentential as well as left-sentential forms" -- there is a logical error in this sentence. A proof that all sentences are right-sentential and left-sentential requires some work. (Note that any way of derivation leads to a sentential form, but not all sentential forms are right-sentential, etc.) $\endgroup$
    – Alexey
    Commented Oct 28, 2018 at 11:14

The grammar can be parsed both left-most and right-most.

a∗b+c can be derived left-most:

E * E
E * E + E
a * E + E
a * b + E
a * b + e

and also right-most (correct precedence:)

    E + E 
E * E + E 
E * E + c 
E * b + c 
a * b + c

Only in the following form the grammar is both unambiguously left-recursive and arithmetically correct

S → E

E → T | E + T | E - T

T → F | T * F | T / F | (E)

F → a | b | c | ... | 1 | 2 | 3 | ...
  • $\begingroup$ I don't know how this answer is helping answer the question, except to know that each line in your second example is a right-sentential form. $\endgroup$ Commented Feb 25, 2018 at 4:07

I agree, that all the above examples given are rightmost sentential forms. I am not quite sure, how much this is correct, but I thought of a slight modification to definition of right sentential form. Here it goes,

The right sentential form is the sentential form in which the input string when parsed from right-to-left and reduced whenever possible would give final output as the start symbol

Here reduction operation means to replace the symbol or set of symbols by the non-terminal symbol such that the symbol(s) satisfies some rule corresponding to that non-terminal symbol.

I thought of this definition because when I made the parse tree for the example strings given, I found that first example produced two parse trees, but both of them when parsed from right to left and reduced accordingly will ultimately lead to the start symbol.

  • $\begingroup$ I think the above definition is incorrect. A sentential form is any string (including ones with non-terminals) derivable from the start symbol. A right-sentential form is any string that occurs during making a rightmost derivation. Examples : Page 2 of this link $\endgroup$ Commented Feb 25, 2018 at 3:49

Right derivation: when you select right most non terminal everytime to derive the string from G then it will result in the right sentetntial form If there is a legal derivation for G it must have atmost one right or left derivation, which results in right sentential form or left sentential form.

this can be proved for example consider a grammar G which derives the strings not by left or right derivations but by mixed , so now i can rearrange the productions to produce only the right and then the left, it is a simple arrangement as we do not have any conditions when to apply the the production rules.So i think there will be no grammar G for which there will be a derivation but not a right most derivation tree


The answer is (iii).

Considering precedence rules, * operator has more precedence than + operator, but bracket () has more precedence than *.

So in (i), you start from the left where you have *, as there is no bracket in the string.

In (ii), you still start from the left due to the use of bracket for the + operator which make it to have higher precedence than *.

But in (iii), you must start from the right because of the use of bracket on the operator with the lesser precedence (+).

  • $\begingroup$ So the other answers are wrong? $\endgroup$ Commented Aug 16, 2017 at 19:21
  • $\begingroup$ Also, perhaps you could be a little less playful in your answer, say not starting it with a greeting. $\endgroup$ Commented Aug 16, 2017 at 19:22
  • $\begingroup$ The precedence rules you mention may be standard, but they are in no way implied by the grammar given. $\endgroup$ Commented Aug 24, 2017 at 13:30

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