Problem 1189. Knights and Knaves (part 3)

Created by Alfonso Nieto-Castanon

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This is a Matlab adaptation of the <a href = "http://en.wikipedia.org/wiki/Knights_and_Knaves">Knights and Knaves</a> logical puzzles.You are in an island where all inhabitants are either Knights, who always tell the truth, or Knaves, who always lie. The island inhabitants can always tell Knights and Knaves apart by their appearance, but to you, as an outsider, they look exactly the same.Previous problem in this series: <a href = "http://www.mathworks.com/matlabcentral/cody/problems/1180-knights-and-knaves-part-2">Knights and Knaves (part 2)</a>Chapter 3That mob of islanders turned out to be a friendly bunch after all, and with their help you reach the castle in no time. The doors to the castle are guarded by a sentinel who will only let you in if you answer correctly the day's password. Fortunately islanders are not particularly security conscious and the guard is happy to answer as many questions as you see fit. What is today's password?DetailsYou are given a function handle QUESTION that allows you to query the guard (either a Knight or a Knave, you do not know which). The function y=QUESTION(str) takes a string str as input (the 'question'), and returns a logical value (the yes/no 'answer' the guard would give to this question). Valid questions are any valid matlab syntax. The guard has access to the following variables (his pool of 'knowledge'):<ul><li>KNIGHT: Islander's type (true for a Knight, false for a Knave)</li><li>PASSWORD: Today's password (a char array of all-capital letters)</li></ul>You may query the guard by evaluating the function QUESTION. For example x=QUESTION('KNIGHT==true') asks the guard whether he is a Knight.Your function should return the correct password.

_This is a Matlab adaptation of the_ <http://en.wikipedia.org/wiki/Knights_and_Knaves Knights and Knaves> _logical puzzles_.

_You are in an island where all inhabitants are either_ Knights, _who always tell the truth, or_ Knaves, _who always lie. The island inhabitants can always tell Knights and Knaves apart by their appearance, but to you, as an outsider, they look exactly the same_.

Previous problem in this series: <http://www.mathworks.com/matlabcentral/cody/problems/1180-knights-and-knaves-part-2 Knights and Knaves (part 2)>

*Chapter 3*

That mob of islanders turned out to be a friendly bunch after all, and with their help you reach the castle in no time. The doors to the castle are guarded by a sentinel who will only let you in if you answer correctly the day's password. Fortunately islanders are not particularly security conscious and the guard is happy to answer as many questions as you see fit. What is today's password?

*Details*

You are given a function handle *QUESTION* that allows you to query the guard (either a Knight or a Knave, you do not know which). The function y=QUESTION(str) takes a string _str_ as input (the 'question'), and returns a logical value (the yes/no 'answer' the guard would give to this question). Valid questions are any valid matlab syntax. The guard has access to the following variables (his pool of 'knowledge'):

* *KNIGHT*: Islander's type (true for a Knight, false for a Knave)
* *PASSWORD*: Today's password (a char array of all-capital letters)

You may query the guard by evaluating the function *QUESTION*. For example x=QUESTION('KNIGHT==true') asks the guard whether he is a Knight.

Your function should return the correct password.

Solve

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Last Solution submitted on Dec 04, 2021

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Eran Greenberg on 10 Jan 2013

If the guard is a knave, and I ask him what the password is, what response do I get? Do I still get an array of chars? Does he replace 'A' with 'Z', 'B' with 'Y', etc..?

Alfonso Nieto-Castanon on 10 Jan 2013

questions can only be statements that evaluate to a scalar logical value (yes/no questions), so you can ask things like QUESTION('PASSWORD(1)==''A'''); or QUESTION('strcmp(PASSWORD,''ASDF'')'); but if you ask a question that returns a vector you will simply get an error.

Nikolaos Nikolaou on 16 May 2020

Very fun problem Thanks!

Pat Schimoler on 4 Dec 2021

Thank you for taking the time to make up this series of problems.

Solution Comments

Solution 7043310

Solution 651989

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Jan Orwat on 15 Apr 2015

Islander Code injection :)

Rafael S.T. Vieira on 3 Jul 2020

Shame on you, LY Cao, for that hack. :)

Solution 189786

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1 Comment

Alfonso Nieto-Castanon on 17 Feb 2013

Nice way to minimize number of queries!

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Problem 1189. Knights and Knaves (part 3)

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Problem 1189. Knights and Knaves (part 3)

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