Raymond Smullyan has designed many puzzles involving Knights and Knaves. Knights always tell the truth,
whereas Knaves always lie. We refer to Knights and Knaves as Sirs. A puzzle, which is a set of English sentences,
involves a finite number of Sirs. Solving the puzzle means:
• determining the names of all Sirs involved in the puzzle;
• determining solutions to the puzzle, where a solution qualifies each Sir as either a Knight or a Knave.
Some puzzles have no solution, others have a unique solution, and others have at least 2 solutions. The following
is an example of a puzzle with a unique solution.
One evening as you are out for a stroll, you walk by a doorway labeled no normals
allowed. Some people are talking inside. Curious, you listen, and you hear Sir Paul
who says: "all of us are Knaves." "Exactly one of us is a Knight," replies Sir Jenny.
As for Sir John, who is also inside, he just keeps quiet. Who is a Knight, and who
is a Knave?
The Sirs involved in this puzzle are Sir Jenny, Sir John, and Sir Paul. The unique solution is given by Sir Jenny
being a Knight, Sir John being a Knave, and Sir Paul being a Knave.
2. Detailed description
2.1. Syntax of puzzles. A sentence starts with a capital letter and ends in a full stop, an exclamation mark,
or a question mark, possibly followed by closing double quotes. Sir, Sirs, Sir names, Knight and Knave always
start with a capital letter, and no other word inside a sentence is capitalised. A sentence in a puzzle contains
at most one part enclosed between double quotes. When a sentence contains one part enclosed between double
quotes, the part outside the double quotes contains a single occurrence of the form Sir Sir_Name, and what
occurs between the double quotes is something said by Sir Sir_Name. A sentence that contains no part enclosed
between double quotes might refer to a number of Sirs, always in the form Sir Sir_Name, or Sirs Sir_Name_1
and Sir_Name_2, or Sirs Sir_Name_1, Sir_Name_2, …and Sir_Name_n, where n ≥ 3, and Sir_Name_1, …,
Sir_Name_n are pairwise distinct.
What is between double quotes is a sentence in one of the following forms, ending in either a comma, a full
stop, an exclamation mark, or a question mark:
• At/at least one of Conjunction_of_Sirs/us is a Knight/Knave
• At/at most one of Conjunction_of_Sirs/us is a Knight/Knave
• Exactly/exactly one of Conjunction_of_Sirs/us is a Knight/Knave
• All/all of us are Knights/Knaves
• I am a Knight/Knave
• Sir Sir_Name is a Knight/Knave
• Disjunction_of_Sirs is a Knight/Knave
• Conjunction_of_Sirs are Knights/Knaves
where:
• Disjunction_of_Sirs is in one of the following forms:
– Sir_1 or Sir_2
– Sir_1, Sir_2, … or Sir_n (n ≥ 3)
• Conjunction_of_Sirs is in one of the following forms:
– Sir_1 and Sir_2
– Sir_1, Sir_2, … and Sir_n (n ≥ 3)
• Sir_1, …, Sir_n are pairwise distinct expressions of the form Sir Sir_Name or I.
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2.2. Input and output of programs. Your program will prompt the user for a text file, assumed to be stored
in the working directory, that stores the sentences that make up a puzzle. No assumption should be made on
the number of English sentences provided as input, nor on the length of a sentence, nor on the length of a Sir
name, nor on the number of Sirs involved in the puzzle.
Your program should:
• output the Sirs involved in the puzzle in lexicographic order (5 marks);
• output whether or not there is a solution, and in case there is, how many there are (3 marks);
• in case a unique solution exists, output it, with all Sirs being qualified as either Knight or Knave in
alphabetical order (5 marks).
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2.3. Sample outputs. Here are a few tests together with the expected outputs. The outputs of your program
should be exactly in accordance with the following outputs. Outputs of your program will be matched against
expected outputs line for line.
$ cat test_1.txt
I have just seen Sirs Sanjay and Eleonore!
"I am a Knave," whispered Sir Eleonore.
Who is a Knight and who is a Knave?
$ python3 knights_and_knaves.py
Which text file do you want to use for the puzzle? test_1.txt
The Sirs are: Eleonore Sanjay
There is no solution.
$ cat test_2.txt
I have just met Sirs Frank, Paul and Nina.
Sir Nina said: "I am a Knight," but I am not sure
if that is true. What do you think?
$ python3 knights_and_knaves.py
Which text file do you want to use for the puzzle? test_2.txt
The Sirs are: Frank Nina Paul
There are 8 solutions.
$ cat test_3.txt
Yesterday, I visited Sirs Andrew and Nancy. I asked Sir Andrew
who he was, and he answered impatiently: "Sir Nancy and I
are Knaves!" Then I met Sir Bill who introduced me to his wife
and told me: "at least one of Sir Hilary
and I is a Knave." Should I trust them?
$ python3 knights_and_knaves.py
Which text file do you want to use for the puzzle? test_3.txt
The Sirs are: Andrew Bill Hilary Nancy
There is a unique solution:
Sir Andrew is a Knave.
Sir Bill is a Knight.
Sir Hilary is a Knave.
Sir Nancy is a Knight.