编译原理代写 | comp2022 Assignment 4

comp2022 Assignment 4

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.

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