Description
Overview
We start by dening a simple calculating machine. The machine has one register, R, which can store a double precision oating point value (integers will be automatically converted). The machine has the following
instructions:
Instruction Eect
NOP v R does not change
ADD v R = R + v
SUB v R = R − v
MUL v R = R × v
DIV v R = R/v (oating point division)
Notice that the NOP operation eectively tells the machine to ignore the given number. Here’s an example script
with 5 instructions, listed horizontally for convenience:
✞ ☎
MUL 1, NOP 2 , ADD 3 , SUB 4 , DIV 5
✝ ✆
This saves space on the page for examples. We will assume that the register R is initialized to 0.0 (zero) at
the start of every script. With this assumption, after executing the above sequence of instructions, the register
would contain a oating point value close to −0.2.
Targeted Coding Problem
The Targeted Coding Problem is stated as follows. You are given a oating point target value T, and a list
of numbers, L, of any length. You are to nd a sequence of instructions for our simple machine above that
calculates a value as close to T as possible, using all the numbers in L. You must use every number in L, and
you cannot change the order. This means that the only thing you can do is gure out which operator to give to
each value in the list. For example, given L = [1, 2, 3, 4, 5], you could try:
✞ ☎
MUL 1, ADD 2 , ADD 3 , DIV 4 , ADD 5
✝ ✆
or
✞ ☎
ADD 1, NOP 2 , NOP 3 , SUB 4 , DIV 5
✝ ✆
You may use any of the 5 operators any number of times. These restrictions are articial, but they allow us to
explore the local search ideas without over-complicating the task.
The objective is to minimize the dierence between the target, T, and the result of a sequence of instructions (as described above). To make this precise, let m(s) represent the value in the simple machine’s register
after executing the instruction sequence s. The objective function is dened as follows:
f(s, t) = |t − m(s)|
where s is any sequence of instructions, and t is the target value.
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Programming ideas
The machine It’s easy to write a simple interpreter for this machine. In Python it would look something like
this:
✞ ☎
def machine_exec ( seq ):
register = 0
for instruction in seq :
operator = instruction [0]
operand = instruction [1]
if operator == “ADD “:
register += operand
elif operator == ” SUB “:
register -= operand
elif operator == ” MUL “:
register *= operand
elif operator == ” DIV “:
register = register / operand
elif operator == ” NOP “:
pass
else :
print (’unknown operator ’, operator )
return register
✝ ✆
You can implement the above function in any language. You don’t need to parse a textle containing a script.
You can represent a script using lists or arrays. For example, in Python you might use something like this:
✞ ☎
[( ’MUL ’, 1) , (’ADD ’, 2) , (’MUL ’, 3) , (’SUB ’, 4) , (’MUL ’, 5)]
✝ ✆
For non-Python programmers, 2-dimensional arrays are completely acceptable. Once you are sure you’re
getting correct results, you can even start using a compiled form, by turning operator strings to integers, to
save space and save time.
✞ ☎
[(1 , 1) , (0 , 2) , (1 , 3) , (2 , 4) , (1 , 5)]
✝ ✆
It’s machine code now, and harder to read. But the rst value is the compiled operator, and the second is the
operand. It may even be advantageous to split the operators and and operands into two lists (or arrays):
✞ ☎
operations = [1 , 0 , 1 , 2 , 1]
operands = [1 , 2 , 3 , 4 , 5]
✝ ✆
This might be useful because we’ll be changing the operations, but not the operands. You’ll have to modify
your machine_exec() program.
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Clarication In Assignment 1, we explored a similar problem using a constructive approach: an expression
was constructed from an initially empty expression. It’s rather like the textbook example of solving the 8-
Queens problem starting from an empty board, and placing the queens on the board one at a time. This
approach is what AIMA Ch 3 is all about.
In Assignment 2, we want to explore AIMA Ch 4.1. We’ll take a mutative approach instead. It will be like
the textbook example of solving 8-Queens, putting all 8 queens on the board to start, and then solving the
problem by moving queens around, one at a time.
To be clear, every question in Assignment 2 should start with a complete script that uses all the numbers
in the given order, and has an operator for each one. For example, suppose our target is T = 40, and our list is
L = [1, 2, 3, 4, 5]. You’d start every local search strategy with a random program using the numbers, perhaps:
✞ ☎
Add 1, Mul 2 , Add 3 , Nop 4 , Div 5
✝ ✆
All the numbers are used, in the order given; the script is built all at once, not one step at a time as in A1. Your
Problem class should have a method to create random scripts like this for lists L of any length.
After that, your search algorithms will explore other complete scripts by various means. In Q1, you’ll just
create completely random scripts like this. In Q2, you’ll make a random change to a scripts (by selecting one
of the operations, and changing it at random. In Q3, you’ll look at all scripts that are exactly one operation
change away, and keep the best one. Etc. Each time you have a complete script, and each script will have a
value, namely its score according to the objective function dened earlier.
Software Design As in Assignment 1, it’s a good decision to separate the programming into loosely coupled
modules.
• A Problem State class. An ADT to contain information that changes as the search algorithm explores the
problem space (also called the state space).
Clarication (04/10/2018) The problem state should contain a complete script, as explained above.
• A Problem class. An ADT to contain particulars about the Problem. It should have a small number of
standard methods.
Clarication (04/10/2018) The actions() result() model for AIMA Ch 3 is probably not the most convenient.
Each Question gives a suggestion on what you’ll need.
• A module to contain search algorithms. The search algorithms should interact with the Problem only
through the standard methods.
Assignment 1 Question 1 confused a lot of people, so I am leaving it out this time. You have to do similar work
here, but it’s not ocially a “question.”
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Question 1 (6 points):
Purpose: To implement the Random Guessing search strategy, and apply it to the Targeted Coding problem.
Degree of Diculty: Moderate. This is where you have to get most of the problem class dened, and do
some thinking about design. The search algorithm itself will not be dicult.
The Random Guessing search strategy can be described with the following pseudo-code:
✞ ☎
function random_guessing ( problem )
// Keep generating completely random states
// Remember the best one
best_guess = obtain a random state from the problem
while there ’ s still time
guess = obtain a random state from the problem
if guess is better than best_guess :
best_guess = guess
return best_guess
✝ ✆
To control the time spent searching, implement a simple counter, limiting your algorithm to a given number
of iterations of the main loop.
Clarication (10/10/2018) Ignore time. Just count iterations. More iterations means more time. An
iteration-limit should probably be passed as an argument, meaning you’ll need another parameter in the
implementation.
Implement the search strategy, and demonstrate that your implementation works, by applying it to the
Targeted Coding problem, maybe a few of the simple examples.
Clarication (04/10/2018) For this problem, your problem class should have a method that creates a
complete script randomly.
What to Hand In
This assignment is broken into pieces, and the last question asks for all the source code, so you don’t have
to hand in multiple copies. But pay attention to these requirements:
• A le named a2q1.txt containing a demonstration of the output of your program, which is copy/paste
from a console, or output le. You only need to show a few examples. If this le is missing, the marker
will assume your implementation is incomplete or not working.
• A le named a2q1.LANG containing your implementation of the search algorithm. Use the le extension appropriate for your choice of programming language, e.g., .py or .java. For this question, only
submit the implementation of the search algorithm, not the problem state or problem class implementations. The markers will not attempt to run the code you submit.
Be sure to include your name, NSID, student number, and course number at the top of all documents.
Evaluation
• 6 marks: Your implementation of Random Guessing is correct, and well-documented.
Page 5
Question 2 (6 points):
Purpose: To implement the Random Search search strategy, and apply it to the Targeted Coding problem.
Degree of Diculty: Easy. Hopefully, your problem class denition can be extended easily!
The Random Search search strategy can be described with the following pseudo-code:
✞ ☎
function random_search ( problem )
// Randomly generate successors from the current state
// Move to it if it ’ s better than the current state
best_guess = obtain a random state from the problem
while there ’ s still time
guess = obtain a random neighbour of best_guess
if guess is better than best_guess
best_guess = guess
return best_guess
✝ ✆
To control the time spent searching, implement a simple counter, limiting your algorithm to a given number
of iterations of the main loop.
Clarication (10/10/2018) Ignore time. Just count iterations. More iterations means more time. An
iteration-limit should probably be passed as an argument, meaning you’ll need another parameter in the
implementation.
Implement the search strategy, and demonstrate that your implementation works, by applying it to the
Targeted Coding problem, maybe a few of the simple examples.
Clarication (04/10/2018) For this problem, your problem class should have a method that creates a
complete script by randomly making a change to a given script.
Clarication (10/10/2018) A “neighbour state” is any state you can get to from the current state. In this
context, the neighbours if script S are all the scripts that are dierent from S by exactly one instruction.
What to Hand In
This assignment is broken into pieces, and the last question asks for all the source code, so you don’t have
to hand in multiple copies. But pay attention to these requirements:
• A le named a2q2.txt containing a demonstration of the output of your program, which is copy/paste
from a console, or output le. You only need to show a few examples. If this le is missing, the marker
will assume your implementation is incomplete or not working.
• A le named a2q2.LANG containing your implementation of the search algorithm. Use the le extension appropriate for your choice of programming language, e.g., .py or .java. For this question, only
submit the implementation of the search algorithm, not the problem state or problem class implementations. The markers will not attempt to run the code you submit.
Be sure to include your name, NSID, student number, and course number at the top of all documents.
Evaluation
• 6 marks: Your implementation of Random Search is correct, and well-documented.
Page 6
Question 3 (6 points):
Purpose: To implement the Hill-climbing Search search strategy, and apply it to the Targeted Coding problem.
Degree of Diculty: Easy. You might need to refactor your code a bit, to make things nice, but the search
algorithm is easy.
The Hill-climbing Search search strategy can be described with the following pseudo-code:
✞ ☎
function hill_climbing_search ( problem )
// Starting from a random state
// Obtain the state ’ s best neighbour
// Move to it if it ’ s better than the current state
// Stop if no neighbour is better
best_guess = obtain a random state from the problem
while there ’ s still time
guess = obtain the best neighbour of best_guess
if guess is better than best_guess
best_guess = guess
else if best_guess is better than guess
return best_guess
return best_guess
✝ ✆
To control the time spent searching, implement a simple counter, limiting your algorithm to a given number
of iterations of the main loop.
Clarication (10/10/2018) Ignore time. Just count iterations. More iterations means more time. An
iteration-limit should probably be passed as an argument, meaning you’ll need another parameter in the
implementation.
Implement the search strategy, and demonstrate that your implementation works, by applying it to the
Targeted Coding problem, maybe a few of the simple examples.
Clarication (04/10/2018) For this problem, your problem class should have a method that returns the
best neighbour of a given script. Here, a neighbour is any script that is exactly one change away from the
given script.
Clarication (10/10/2018) A “neighbour state” is any state you can get to from the current state. In this
context, the neighbours if script S are all the scripts that are dierent from S by exactly one instruction.
What to Hand In
This assignment is broken into pieces, and the last question asks for all the source code, so you don’t have
to hand in multiple copies. But pay attention to these requirements:
• A le named a2q3.txt containing a demonstration of the output of your program, which is copy/paste
from a console, or output le. You only need to show a few examples. If this le is missing, the marker
will assume your implementation is incomplete or not working.
• A le named a2q3.LANG containing your implementation of the search algorithm. Use the le extension appropriate for your choice of programming language, e.g., .py or .java. For this question, only
submit the implementation of the search algorithm, not the problem state or problem class implementations. The markers will not attempt to run the code you submit.
Be sure to include your name, NSID, student number, and course number at the top of all documents.
Page 7
Evaluation
• 6 marks: Your implementation of Hill-climbing Search is correct, and well-documented.
Page 8
Question 4 (6 points):
Purpose: To implement the Random-Restart Hill-climbing Search search strategy, and apply it to the Targeted Coding problem.
Degree of Diculty: Easy.
The Random-Restart Hill-climbing Search search strategy simply repeats Hill-climbing a number of times.
You should control the number of restarts and the number of steps that Hill-climbing is allowed, using
separate parameters.
Implement the search strategy, and demonstrate that your implementation works, by applying it to the
Targeted Coding problem, maybe a few of the simple examples.
What to Hand In
This assignment is broken into pieces, and the last question asks for all the source code, so you don’t have
to hand in multiple copies. But pay attention to these requirements:
• A le named a2q4.txt containing a demonstration of the output of your program, which is copy/paste
from a console, or output le. You only need to show a few examples. If this le is missing, the marker
will assume your implementation is incomplete or not working.
• A le named a2q4.LANG containing your implementation of the search algorithm. Use the le extension appropriate for your choice of programming language, e.g., .py or .java. For this question, only
submit the implementation of the search algorithm, not the problem state or problem class implementations. The markers will not attempt to run the code you submit.
Be sure to include your name, NSID, student number, and course number at the top of all documents.
Evaluation
• 6 marks: Your implementation of Random-Restart Hill-climbing Search is correct, and well-documented.
Page 9
Question 5 (6 points):
Purpose: To implement the Stochastic Hill-climbing Search search strategy, and apply it to the Targeted
Coding problem.
Degree of Diculty: Easy.
The Stochastic Hill-climbing Search search strategy is a minor variation on Hill-climbing. Instead of requiring the best neighbour of the current state, you get a random choice from the neighbours that are better
than the current state. The textbook suggests that the probability of choosing a state should depend on
how much better the state is: better states have higher probability of being chosen. But that’s a tricky bit
of code, so just choose any one of the better neighbours, with equal probability.
Implement the search strategy, and demonstrate that your implementation works, by applying it to the
Targeted Coding problem, maybe a few of the simple examples.
What to Hand In
This assignment is broken into pieces, and the last question asks for all the source code, so you don’t have
to hand in multiple copies. But pay attention to these requirements:
• A le named a2q5.txt containing a demonstration of the output of your program, which is copy/paste
from a console, or output le. You only need to show a few examples. If this le is missing, the marker
will assume your implementation is incomplete or not working.
• A le named a2q5.LANG containing your implementation of the search algorithm. Use the le extension appropriate for your choice of programming language, e.g., .py or .java. For this question, only
submit the implementation of the search algorithm, not the problem state or problem class implementations. The markers will not attempt to run the code you submit.
Be sure to include your name, NSID, student number, and course number at the top of all documents.
Evaluation
• 6 marks: Your implementation of Stochastic Hill-climbing Search is correct, and well-documented.
Page 10
Question 6 (20 points):
Purpose: To measure the quality of the search algorithms in terms of error, and time, and make comparisons.
Degree of Diculty: Moderate
Now that you’ve got your search algorithms working, we’ll get some empirical results to demonstrate their
relative performance. We want to assess the general quality of the solutions they provide, and the runtime
costs. You should review the Overview section before reading this question.
In order to quantify the quality of a single script s for target t, we’ll measure the relative error:
err(s, t) = f(s, t)
t
This makes use of the objective function for the problem, but the error function is not part of the problem;
it’s part of the analysis.
To evaluate the quality of a search algorithm, we apply the algorithm to a collection of examples, and measure the average quality. One measure of average quality is called Root Mean Square Error. It’s calculated
as follows:
RMSE = sPN
i=1 (err(si
, ti))2
N
where ti
is the target for the ith example problem, si
is a solution to the ith example problem, and N is the
number of example problems. You’ll recognize the err function from above. You’re basically calculating
the squared error for each example in the le, adding it all up, dividing to get an average square error, and
then the square root. Using the square of the error emphasizes large errors, and prevents negative errors
and positive errors from cancelling each other.
On Moodle, you’ll nd two data les for this question, containing a number of problem instances, one per
line. The target value T is the rst value on the line, a oating point value, and then a sequence of integers,
which is the list L. Run your search algorithms on all the problems in both les, and produce the following
table (one for each le):
Strategy RMSE Ave Time
Random Guessing
Random Search
Hill-climbing
Stochastic Hill-climbing
Random-Restart Hill-climbing (50 × 20)
Random-Restart Hill-climbing (10 × 100)
Limit your search to 1000 steps. In the case of Random-Restart, try two variations:
1. 50 restarts; limit each hill-climbing attempt to 20 steps (50 × 20 = 1000).
2. 10 restarts; limit each hill-climbing attempt to 100 steps (10 × 100 = 1000).
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Answer the following questions about your results:
1. Which algorithm had the lowest RMSE?
2. Did random guessing work well? Explain why (or why not)?
3. Compare the two variations of Random-restart Hill-climbing. Is it better to have lots of short runs, or
fewer long runs?
4. Do you think the RMSE would get bigger or smaller as the length of L increases?
What to Hand In
• A le named a2q6_EXECUTION.txt containing brief instructions for compiling and/or running your
code. See page 1.
• A le named a2q6.txt containing the tables required, and your answers to the questions above. If this
le is missing, the marker will assume your implementation is incomplete or not working.
• A le named a2q6.LANG containing your implementation of the search algorithms. Use the le extension appropriate for your choice of programming language, e.g., .py or .java. You may submit
multiple les, provided that the lenames begin with a2q6_
Be sure to include your name, NSID, student number, and course number at the top of all documents.
Evaluation
• 8 marks: Your two tables are plausible results from executing the search algorithms.
• 12 marks: Your answers to the questions demonstrate understanding of the issues and concepts.
Page 12
Extra work for the ambitious
1. Implement the Simulated Annealing search strategy, and demonstrate that your implementation works.
Add a row to the table from Question 6 giving your results.
2. Implement the Genetic Algorithm search strategy, and demonstrate that your implementation works.
Add a row to the table from Question 6 giving your results.
Page 13
