INTRODUCTION -
Consider a Toy world, containing a finite number n of objects. Each object is specified by a unique name (a string, such as "A", or "B", or "d1", or "table2"). The current state of the world is described using a (finite) set of properties (propositions) which specify the state the objects are in. In the simple scenario considered here, only 3 possibly propositions are used to describe the state of the world:
ON(x, y) In the current state, object x is on top of object y
CLEAR(x) Object x is clear, i.e. it has nothing (no other object) on top of it
HEAVIER(x, y) Object x is heavier than object y
The current state of the world is specified by a finite set of "ground instances" of the above propositions, i.e., instances obtained by replacing all the variables (x and y) with specific object names. For example, the following state
s1 = {ON(A, table1), HEAVIER(table1, A), HEAVIER(table2, A), CLEAR(A), CLEAR(table2)}
contains 5 propositions involving 3 objects (named "A", "table1", "table2"). Here s1 describes a state in which object A is clear and is on top of table1, table2 is clear, and table1 and table2 are both heavier than A. Note that the order of the propositions is irrelevant (i.e., a state is a set, and not a sequence, of propositions).
In this Toy world only 1 action is possible, namely, moving an object from its current location to a different one. This action is described by the "Move" schema (or operator) below:
Move(x, y, z):
Preconditions: ON(x, y), CLEAR(z), CLEAR(x), HEAVIER(z, x) Add: ON(x, z), CLEAR(y)
Delete: ON(x, y), CLEAR(z)
This operator moves object x (lying on top of y) onto object z, leaving y clear. The Move operator, however, can only be applied if all of its preconditions are true in the current state:
DEFINITION: A ground instance of the "Move" action is APPLICABLE in a state 'S' ONLY IF ALL OF ITS PRECONDITIONS APPEAR IN 'S' (i.e., if they are all true in 'S').
When the operator Move is applied, the current world state is changed in the following way:
- all the propositions in the "Add" list are added to the state (i.e., they become true);
- all propositions in the "Delete" list are removed from the state (i.e., are no longer true).
In the above example, executing Move(A, table1, table2) in s1 produces a new state, s2, where
s2 = {ON(A,table2), CLEAR(table1), CLEAR(A), HEAVIER(table1, A), HEAVIER(table2, A)}
In short, executing a Move action removes two propositions from the state, adds two new propositions, and leaves the rest of the state unchanged.
Given the previous Introduction, write a Python program that executes the following steps:
1. Asks the user to enter a string (Obj) containing all the object names (see example below);
2. Asks the user to enter a string describing the initial state, S0 (see example below);
3. Asks the user to enter a string describing the goal, G (see below);
4. Prints a finite sequence of ground instances of the Move action (with all variables replaced by object names taken from Obj) such that, if applied in the specified order, the action sequence changes the initial state S0 into a final one, Sn, containing all of the propositions listed in G.
5. Terminates.
Attachment:- Assighment File.rar