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Assignment 1
The learning goals covered in this assignment:
- Prolog: facts, predicates; querying them and writing your own.
- Introducing searching.
- Introducing backtracking.
There are two files you will need to look at:
assignment1_12345.plailp/library/assignment1_library.pl
To find the answers you should not change anything in the library file (though you may want to make a small change in order to test Q5b).
- Run the assignment 1.
- You should see a grid world, and an agent at a particular position in this world.
- The agent can move around the grid horizontally or vertically (north, south, east or west).
- The questions in this assignment look at how we move within and search this world using Prolog.
- The Prolog code is linked with this web page and JavaScript is used to display the world. You don't need to worry about this though, you'll just be using the 2 Prolog files we've supplied for this assignment. Of course, feel free to take a look if you wish!
- You will notice a term
p(X,Y)is used. This represents a position in the grid world.
You should submit a single Prolog file with name in the following format assigment1_candidate-number.pl, e.g. assignment1_12345.pl.
This file will contain the answers to each question in the form of a Prolog fact or predicate.
At the top of the Prolog file give the candidate number fact: candidate_number(12345).
The answers should be of 1 of the following types:
- A Prolog query, e.g.
q1(prolog_query).; - A number, e.g.
q1(1).; - A Prolog list e.g.
q1([1,2,3,4]).; - A Prolog return statement in single quotes e.g.
q1(false).; - A Prolog predicate e.g. an implementation of
q6_spiral/1.
If you load your answer script into Prolog using for example ?-consult('assignment1_12345.pl')., then your answers should be returned when you give the following queries:
>?-q1(A1).
>?-q2(A2).
etc...
Check that this runs before submitting!
After using the ?-reset. query and pressing step in the web page, you should be able to see your agent at its start position. What Prolog query would you use to retrieve this position?
Answer example: q1(prolog_query).
Using the new_pos/3 predicate, answer the following:
Suppose your agent is at position p(1,1) and should move east. Write a query to retrieve the new position of your agent.
Answer example: q2a(prolog_query).
Suppose your agent is at position p(1,1) and wants to move north. Write a query to retrieve the new position of your agent.
As you can see this query DOES NOT succeed. On what line of assignment1_library.pl (ailp/library/assignment1_library.pl) does Prolog fail?
Answer example: q2b(7).
Look at the code and find the predicates (with 1 argument) that specify which direction the agent can move in. If an agent moves by simply calling this predicate, moving to the new position, then backtracking to retrieve another direction by calling this predicate again, what is the order of the directions that the agent will move in?
Answer example: q3([n,e,s,w]).
A path is a sequence of adjacent positions representing how the agent moves in the world.
Run the query ?-next(L).
At some point the agent finds itself at a dead end and starts backtracking. What is the path from the starting position right until the first dead end, just before the agent starts backtracking? (make sure you give the whole path in the correct order in your answer)
Answer example: q4a([p(1, 1), p(1, 2), p(1, 3)]).
After backtracking once and thereby avoiding the first dead end, give the full path from the starting position to the second dead end.
Answer example: q4b([p(1, 1), p(1, 2), p(2, 2), p(2, 3)]).
Give the first full path covering the whole grid found by the agent.
Hint: you may want to disable the visualisation to speed things up, type the following into Prolog interpreter:
toggle_switch(headless).
Answer example: q4c([p(1, 1), p(2, 1), p(3, 1), p(4, 1)]).
Give the second full path covering the whole grid found by the agent.
Answer example: q4d([p(1, 1), p(2, 1), p(3, 1), p(4, 1)]).
Look in assignment1_library.pl and find the predicate ailp_show_move/2. This predicate moves your agent between squares in the grid.
Write a predicate called q5_corner_move/0 that moves the agent between all 4 corner squares in the world, in any order. The agent should not visit any other squares.
Answer: The q5_corner_move/0 predicate you have declared. We should be able to run ?-q5_corner_move. to see the visualisation in the web browser.
The ailp_grid_size/1 predicate changes the size of the grid. If you change this predicate to 5 for example, does your q5_corner_move/0 predicate
still work?
Write another predicate q5_corner_move2/0 that uses this ailp_grid_size/1 predicate so that the agent moves between the corners no matter the size of the grid. (i.e. the locations are not hard coded)
Answer: The q5_corner_move2/0 predicate you have declared. We should be able to run ?-q5_corner_move2. to see the visualisation in the web browser.
NB. Don't forget to change the grid size back after you have tested this!
Write a predicate q6_spiral/1 that has a single argument being the path taken by the agent. The agent should start in one of the outer squares (any square next to the edge of the board), and cover the board in a spiral (either clockwise or anticlockwise) such that it ends in one of the 4 central positions.
NB: The ?-reset. query sets the agents position to your start position, so the first thing your predicate will need to do is move the agent to an outer square (of your choice) if it is not already on one of these.
Answer: The q6_spiral/1 predicate you have declared. We should be able to run ?-q6_spiral(X). to find the path and see the visualisation in the web browser.
NB. Use recursion / backtracking for this question.