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comp2022 Assignment 4 s2 2020
This assignment is due on Sunday Nov 22, 23:59 and has a coding part and a written part.
• Submit your written part (Problems 1 and 2) as a single pdf on Gradescope.
• Submit your code (Problem 3) on Ed.
• All work must be done individually without consulting anyone else’s solutions in accordance
with the University’s “Academic Dishonesty and Plagiarism” policies.
• For clarifications and more details on all aspects of this assignment (e.g., level of justification
expected, late penalties, repeated submissions, what to do if you are stuck, etc.) you are
expected to regularly monitor the Ed Forum post “Assignment Guidelines”.
Problem 1. Let Σ = {a, b} and consider the language L1 ⊆ Σ
∗ of all strings of
the form a
mu where m ≥ 0 and u has at most m many as in it. Thus, e.g., aababa
and aabba and aa are in L1, but aababaabb is not in L1.
1. Give a context-free grammar G1 = (V, Σ, R, S) that recognises the language L1.
2. Explain why your grammar is correct.
3. Explain whether or not L1 is regular.
Problem 2. In this problem you will design a context-free grammar that generates
the language associated with a non-negative integer counter. Let Σ = {p, m}. We
say that a string w ∈ Σ

is a valid counter string if, when read from left to right, and
interpreting p as adding 1 and m as subtracting 1, and we start from 0, the running
sum is non-negative. For instance, ppm and pm and ppmmp are valid counter strings,
but pmmp and mpp are not valid counter string.
1. Give a context-free grammar G2 = (V, Σ, R, S) that recognises the language L2
of valid counter strings.
2. Explain why your grammar is correct.
3. Explain whether or not L2 is regular.
Problem 3. In this problem you will implement the CYK algorithm and extend
it to provide rightmost-derivations and detect ambiguous strings. Your algorithm will
have three modes: membership, rightmost-derivation, and ambiguous. Input is read
from standard input (stdin). The first line of input indicates which mode is to be
executed. The remaining input is the data to act on. Examples of usage, and of input
and output are provided in Appendix B.
1. Membership mode
• Input: membership followed by a context-free grammar in Chomsky normalform
followed by a sequence of input strings.
• Output: One line per input string, giving the string, a comma, and then 1
if the string is generated by the grammar and 0 otherwise.
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comp2022 Assignment 4 s2 2020
2. Rightmost-derivation mode
• Input: rightmost-derivation followed by a context-free grammar in
Chomsky normal-form followed by an input string that is generated by the
context-free grammar.
• Output: A sequence of lines, each line containing the next step of a rightmost
derivation of the input string.
3. Ambiguous mode
• Input: ambiguous followed by a context-free grammar in Chomsky normalform
followed by a sequence of input strings.
• Output: One line per input string, giving the string, a comma, and then 1
if the string is ambiguous and 0 otherwise.
A Marking
Problem 1 (25 marks)
• The first part is for 10 marks. For full credit your context-free grammar should not be more
complicated than needed.
• The second part is for 5 marks. For full credit your answer should be clear, and explain why
your grammar only generates strings in L1 and why it generates all strings in L1.
• The third part is for 10 marks. An explanation of regularity requires that you provide a DFA,
NFA or RE for the language and prove it is correct. An explanation of non-regularity requires
that you provide a proof using the PHP as in lectures.
Problem 2 (25 marks)
• The first part is for 10 marks. For full credit your context-free grammar should not be more
complicated than needed.
• The second part is for 5 marks. For full credit your answer should be clear, and should explain
why your grammar only generates strings in L2 and why it generates all strings in L2.
• The third part is for 10 marks. An explanation of regularity requires that you provide a DFA,
NFA or RE for the language and prove it is correct. An explanation of non-regularity requires
that you provide a proof using the PHP as in lectures.
Problem 3 (50 marks)
• For passing the test-cases:
– 20 marks for membership mode
– 10 marks for rightmost derivation mode
– 10 marks for ambiguous mode.
You are not allowed to hard-code any of the test-cases.
• The remaining 10 marks are based on the overall quality of the code. Quality is judged based
on our answers to questions such as:
– How readable is the code? e.g. commenting, variable naming, space, line length, etc.
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comp2022 Assignment 4 s2 2020
– Are the data structures appropriate to the problem? e.g. how the grammars are represented.
– Are the algorithms implemented in a reasonably efficient way?
– How much of the assignment was attempted.
B Input/Output formats
Input context-free grammar in Chomsky normal-form
A sequence of lines:
1. A comma separated list of variable symbols
2. A comma separated list of terminal symbols
3. The start variable
4. One or more lines of the form:
• A -> B C
• A -> a
• A -> epsilon
5. the string end
For example:
✞ ☎
A ,B ,C ,D ,S , T
a , b
T
T -> A B
T -> B A
T -> S S
T -> A C
T -> B D
T -> epsilon
S -> A B
S -> B A
S -> S S
S -> A C
S -> B D
C -> S B
D -> S A
A -> a
B -> b
end
✝ ✆
... represents the grammar:
T → AB | BA | SS | AC | BD | 
S → AB | BA | SS | AC | BD
C → SB
D → SA
A → a
B → b
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comp2022 Assignment 4 s2 2020
Membership mode
1. The input is a sequence of lines:
(a) membership
(b) A context-free grammar in Chomsky normal-form (in the format described above)
(c) Several strings to check, one on each line, followed by end
2. The output is 1 or 0 for each input string, where 1 means the string is generated by the
grammar and 0 means it is not, all followed by end.
Here is an example. If the input is:
Rightmost-derivation mode
1. The input is a sequence of lines:
(a) rightmost-derivation
(b) A context-free grammar in Chomsky normal-form (in the format described above)
(c) A single non-empty string to derive, followed by end
2. The output is a sequence of lines:
(a) Each line should be a non-empty string over the alphabet of variables and terminals
(b) The sequence of lines should represent a rightmost derivation in which each line yields
the next
(c) The last line should be end
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comp2022 Assignment 4 s2 2020
Here is an example. If the input is:
which encodes the rightmost derivation
T ⇒ AB ⇒ Ab ⇒ ab
of the string ab.
Ambiguous mode
1. The input is a sequence of lines:
(a) ambiguous
(b) A context-free grammar in Chomsky normal-form (in the format described above)
(c) Several non-empty strings to check, one on each line, followed by end
2. The output is a sequence of lines:
(a) The output is 1 or 0 for each input string, where a 1 means the string is ambiguous, and
a 0 that it is not
Here is an example. If the input is:
because ab has one rightmost derivation, a has no rightmost derivations, and abab has two rightmost
derivations.
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