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# CS 161 Lab 9

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## Description

Lab 9
Get checked off for up to 3 points of incomplete work from the previous lab within the
first 10 minutes of the lab.
In this lab, you can form a group of 2-3 individuals. You must be checked off together
as a group at the end of the lab. Although you perform tasks as a group, ensure that
you understand the work and ask questions to TAs as needed.
Problem statement:
Towers of Hanoi is a simple puzzle that we’re going to be using as practice for recursion and 2D
arrays. The puzzle itself is very simple– it consists of three columns arranged from left to right, and
some number of disks N of different sizes. To begin, the N disks are placed on the 1
st column in order
of their size, with the largest disk at the bottom of the column. The puzzle’s goal is to finish with the
disks arranged in the same order (biggest on the bottom, smallest on the top) on the 2
nd column. Of
course, you can’t just move the disks however you want! You need to follow these rules:
● You can only move one disk at a time by taking it off the top of its peg and putting it onto
another peg.
● You’re not allowed to place a disk on top of another disk that’s smaller– that is, every disk must
be smaller than every disk beneath it on the peg.
You can do the simulation here: https://www.mathsisfun.com/games/towerofhanoi.html
(4 pts) Design
First, begin by writing the steps on a piece of paper that represents the moves among the columns. For
instance, with three disks, the smallest disk from the 1st post will be moved to the second peg, i.e. 1 ->
2. Then, the 2nd disk will be moved to the 3rd peg, i.e. 1->3, etc.
Write the steps for the base case, n = 1 disks, n = 2 disks, and n = 3 disks. You should notice that you
have 2n – 1 moves for each of these cases. Also, note any pattern that you see, i.e. when do you see
the base case.
Here is an outline of the recursive towers function, feel free to add another parameter if needed, i.e.
total_disks.:
void towers(int number_of_disks, int b[ ][3], int from_col, int to_col, int spare) {
If(number of disks is >= 1)
Call Towers with (number_of_disks-1, b, from_ col, spare, to_ col)
Move the disk
Print the board
Call Towers with (number_of_disks-1, b, spare, to_ col, from_ col)
}
As a group with the TAs, walk through the algorithm provided for the towers() function with a board that
has 1 disk and 3 columns, then 2 disks and 3 columns, and 3 disks with 3 columns, e.g. . towers(1, 1,
2, 3);, towers(2, 1, 2, 3);, towers(3, 1, 2, 3);, etc.
Provide the example walk through for the following calls:
towers(1, 1, 2, 3);
towers(2, 1, 2, 3);
towers(3, 1, 2, 3);
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(6 pts) Implementation : Statically Allocated 2-D array
First, you can implement this using a static 2-D array with 3 columns for the 3 posts and 3 rows,
and you can initialize the array with the numbers 1, 2, and 3 in the first column to represent the initial
state of the game. The goal is to print out the board after each move in the game, seeing the following
output. Example with two disks:
1 0 0
2 0 0
———-
0 0 0
2 0 1
———-
0 0 0
0 2 1
———-
0 1 0
0 2 0
———-
The above implementation can also be done using a dynamically allocated array. Let us figure it out in
next lab
Show your completed work and answers to the TAs for credit. You will not get points if
you do not get checked off!
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