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Homework answers / question archive / CS 120 (Spring 22): Introduction to Computer Programming II Long Project #11 - Maze Solver due at 5pm, Tue 12 Apr 2022 REMEMBER: The itertools and copy libraries in Python are banned

CS 120 (Spring 22): Introduction to Computer Programming II Long Project #11 - Maze Solver due at 5pm, Tue 12 Apr 2022 REMEMBER: The itertools and copy libraries in Python are banned

Computer Science

CS 120 (Spring 22): Introduction to Computer Programming II
Long Project #11 - Maze Solver
due at 5pm, Tue 12 Apr 2022

REMEMBER: The itertools and copy libraries in Python are banned.


# & . SHURE 8

%# 88H Ln... #

# # & . #

# 888 BNE. #

# . #

# ...% R#8RRRD

# E.....
1 Overview
In this program, you will solve a maze. You will read the map up from a file,
build a tree to represent the maze, and then search through the tree, to find a
path from the start to the end.

Name your program
1.1 Dump Points
Solving the maze takes several steps. so I've broken it into smaller pieces. For
each piece, [ve added a “dump” option - if I ask for that particular dump. then
you will stop the algorithm at that point. print out what you have so far, and
then terminate the program. I've done this for two reasons. First, I hope that
it will help guide you through the solution - since each step will be relatively
small. Second, if you complete only part of the algorithm but not all of it. you
can still get partial credit. because we'll have testcases that will stop you, and
check your code, at various points in the process.
(Yes, this reduces your flexibility. and locks you into an algorithm I’ve de-
vised. Sorry!)
1.2. User Input
When the program runs. the user will type two lines of input. The first will be
a filename. which gives the map. The second will be the “command:” if it is
one of the “dump” commands, then you will run the program to that point and
dump out your current state. If the command is blank, then you will run the
algorithm all the way to the end, and print out the solution only.

2 Map Format
The maze will be encoded in a text file. which will look something like this:


# & # SHULE 8

# 888 #NERR #

# * # #8 #

%# #82 NNR ON #

# # #


# E*eeed#
In this map the hash symbols represent the paths that you can follow (not
walls!). The S represents the start point for the maze. and the E represents the
end. (Don’t assume that the start and end are at any special locations: they
mnight be anywhere in the maze.)

Your program will find the path through the maze, and report it by printing
out the maze again. but with the path marked with periods, like this:

* # . SHURE 8
% 88H... #
# # & . #
# 88H BNE. #
# . #
# ...8 R#BRRED
# E.....

When you read the file. don’t assume that the lines in the file are a perfect
grid: different lines might be different widths. Instead, simply focus on the
locations where hash symbols exist (plus the start and end symbols).

2.1 Coordinates
When you read the map file, you need to record every “cell” that contains a
path. Assign them (zr. y) coordinates, where (0.0) is the upper-left corner of the
map. In the example map above. we have lots of cells along the top edge: their
coordinates are
(0.0), (1.0). (2,0), (3,0). (4.0), (5. 0), (6,0), ...
There are also quite a few along the left edge:
(0,1). (0,2), (0.3), (0.4), (0.5)...
The start point is at (4.0) and the end point is at (5. 7).

2.2 Map Guarantees - and Things You Must Check

You may assume (without checking) that the map we give you is acyclic (mean-
ing that it doesn’t have any loops). Real mazes sometimes have loops, of course
- but that makes the solver harder to write!

You can also assume that all of the paths in the map are connected to each

other. Together, these two assumptions mean that you know for sure that there
is exactly one path from the start to the end.
However, you must check and confirm that each of the following things is
true about the map. If any of these are not true, you must immediately report
an error and terminate the program. (See the testcases for the exact error
messages. )

e You must be able to read the map file.

e The map must contain exactly one start state - no more, no less.

e The map must contain exactly one end state - no more. no less.

e The only characters allowed in a map file are space, newline, hash symbol,

S. and E.

3 Trees and Mazes

In this program, you will be using a tree - but it won't be a binary tree! Build
your own class, to hold the nodes of this new tree - but design it such that each
node can have an unlimited number of children. While I won't tell you exactly
how this tree must work - and so you can define its methods and data fields -
note that your TA will be checking your design, and good design of this class
will be part of your grade.

It may seem odd to use a tree to solve a maze, but - so long as the maze
doesn't contain any loops - it works remarkably well. The root of the tree will
represent the start position in the maze; the parent/child relationships in the
tree represent moving further away from the start. That is, the start location
itself will be the root, and its children will be the cells (one, or perhaps many)
which are adjacent to the start. The next level of children will be locations that
are two steps away from the start, and so on. The end location, then, will be
one of the nodes, somewhere in the tree - it might be a leaf node. but don’t
count on it!

The cool thing about representing a maze with a tree is that it makes the
maze very easy to solve: to find a path from the start to the end, really all we're
doing is doing a search through an un-ordered. non-binary tree!

(spec continues on next page)

4 Required Dump Points
As noted above, the “command” that the user types will tell you how far to run
the algorithm, before you dump out a result. You must support the following
commands; if the user types anything other than one of these (a blank line
is OK). then report an error and terminate the program. For each of these
commands, check the testcases to find the proper output.
WARNING: For this program, we're going to need to be more
picky about exactly matching the required output than we normally
do. (This is because we need to verify exactly how many spaces
you print out in your map.) So make sure to test your code on
GradeScope early - and be prepared that the autograder will be
quite picky about blank spaces. and blank lines.
4.1 Command: dumpCells
After reading the map in from the file, print out all of the cells in the file, as
(z,y) pairs. Sort the pairs. Mark the START and END cells.
4.2 Command: dumpTree
After converting the set of cells to a tree, print out the tree, using a pre-order
traversal. Use indentation (with spaces and vertical bar characters) to make it
clear which nodes are parents and children.

(If a node has multiple children, the order in which you print the children
mnatters. Make sure. when you build the tree, that you always build the children
in the following order: up,down.left,right. Then you will match the expected

4.3. Command: dumpSolution

After finding the solution (by seaching the tree), print out all of the steps in the
path (including the start and end points)

4.4 Command: dumpSize

The last little step, before actually printing the output from your maze. is to
figure out the height and width of the input maze you were given. This must
be calculated only based on the cells you’ve been checking - don't try to
keep track of how many lines the file had. or how wide they were.

If I ask you to dump this out, then scan through the set of cells, and find
the maximum x and y values.

4.5 No Command

If there is no command, then simply print out the map. back to the user - but
with the path (except for the start and end cells) replaced by periods.

5 Hints: Tree Building

Probably the most challenging single part of this program is buildling the tree.
You won't be able to build it like we build BSTs (adding one node at a time):
instead, you will need to recurse through the maze, building the entire tree
in one recursive algorithm.

The set of cells that you have read from the map will be central to this. Any
time that you want to build a node of the tree, create a new node (its value
should be the (z, y) coordinates of this cell). Then, check, in all 4 directions, to
see which ones have adjacent cells. Where you find adjacent cells, build child
nodes for the tree.

Of course, we're overlooking some details. We must never go back into our
own parent! So, we'll need some way to prevent any node from treating its
own parent as a child. There are several possible strategies for this: feel free to
choose any one of these (or come up with your own):

e Pass, as a parameter, information about your parent node - so you can

know which neighbor you can’t treat as a child.

e Have a set of all of the cells - pass it as a parameter - but modify this

set as you go, removing cells from it as soon as you build them.
Haven't I told you. repeatedly, not to modify an array (or set) that you've
been given as a parameter? Not exactly! I said, don’t modify it unless the
function spec says that you can. Since you get to write the function
spec, you are allowed to make it legal, if you want!

e Build a new set of nodes (passed around as a parameter), that gets larger
every time you add a node. These are the already-built cells - and so
you should never build a new node for any of them, a second time.

5.1 Child Order

Which child should you explore first? If it weren't for the autograder, you
actually would have complete freedom - any order of the children would work
as well as any other. But, since the autograder expects you to print out the
nodes in a very specific order in the dumpTree command, you have to create
your children in the same order as I did. (Sorry!)

Always, from any given node, look for children in the following order: up.down, left right.

6 Hints: Tree Searching

We already know how to search through an unordered tree: we have to check
each of the children, to see if they have the value we want. But this program
requires a more advanced search than you've done before: instead of returning
a boolean value (True or False), you need to return an entire path through
the maze.

I'm not going to give you the solution here, but here are some things to be
thinking about:

e What sort of data structure would you use to store the path. if you had

your choice? Try returning exactly that from the search function.

e Many searches through the tree will fail, because we made a wrong turn
in the maze. What should your search function return in that case?

e If a node recurses into a child, and the child finds the thing we're looking
for, it will return some information about how to get to the node. But
that information will (probably) only get us from the current node, to
the destination. It doesn’t show the whole path from the root. What new
information can the current node add, before it returns an answer to its
own parent?

7 Hints: Printing Out the Solved Maze

While the input file is not required to be a perfect grid of characters, your
output must be. It should be a grid of characters, of exactly the required width
and height. This will often include some trailing spaces, at the end of a line. to
fill up space on the line - and perhaps leading spaces, too - if the current line
doesn't have cells all the way at the edges.

There are a couple ways to do this (both pretty good). First. we could declare
a 2D grid. and fill the grid up with individual characters. This is sometimes a
cool trick, since you can fill the grid up with some starting default (say, spaces?)
and then “draw” into the grid by changing individual cells.

Second, we could iterate through the grid, one line and character, at a time,
and ask ourselves each time: “what character should be at location (z, y)?”
There are only a few characters that you have to print: open space, paths not
in the solution, paths that are part of the solution, the start location, and the
end location.

8 Turning in Your Solution
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