CSci 1100 Lab 6 — Sudoku


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Lab Overview
This lab uses the game of Sudoku to investigate the use of logic, nested lists, and nested
loops. Please download the file from the Piazza site. This includes a
Python utility for reading files, and several “games”. You will use the utility function only
in the final checkpoint. Until then, you will work with a given board. Of course, given we
will cover files in Lecture 13, you can write this utility function yourself easily.
The Game of Sudoku
Sudoku is a popular logic puzzle, often called a “wordless crossword”. There are many books
and websites for Suduko. The following puzzle was taken from,
where you can learn a bit more about the rules of the puzzle
In a Sudoku solution, each row, each col, and each 3×3 block has each of the numbers 1-9
appearing exactly one time. A Sudoku puzzle starts with some of squares having numbers,
and there is generally only one way the remaining squares may be filled in legally. Sometimes
finding this solution is easy. Other times it seems impossible.
Checkpoint 0: Double loops
Before you start this lab, we will do a small exercise that will allow you to complete the rest
of the lab much faster. This checkpoint is meant for an exercise and it will not be checked
The idea is that you will need to write a few loops that simply generates pairs of values
of different types. Once you have these loops in place, you can easily use them as indices
for your code later. The best idea is to write them in a separate file. Try doing these both
with while and for loops.
1. Write a loop to output the digits from 0 up to (and including) 8, all on one line.
0 1 2 3 4 5 6 7 8
As a hint, create an empty string called line, and then write a for loop that appends
a string for each digit to line. After the for loop ends, print line.
2. Write a loop to generate pairs of values from 0 up to 8 (basically, for each value above,
you will generate a second value between 0-8). As a special challenge, we added a
space and line to separate each 3×3 block.
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8
1,0 1,1 1,2 1,3 1,4 1,5 1,6 1,7 1,8
2,0 2,1 2,2 2,3 2,4 2,5 2,6 2,7 2,8
3,0 3,1 3,2 3,3 3,4 3,5 3,6 3,7 3,8
4,0 4,1 4,2 4,3 4,4 4,5 4,6 4,7 4,8
5,0 5,1 5,2 5,3 5,4 5,5 5,6 5,7 5,8
6,0 6,1 6,2 6,3 6,4 6,5 6,6 6,7 6,8
7,0 7,1 7,2 7,3 7,4 7,5 7,6 7,7 7,8
8,0 8,1 8,2 8,3 8,4 8,5 8,6 8,7 8,8
These will serve as the indices for the Sudoku board entries.
3. Write a loop to generate all the items in a given row, say row=2.
2,0 2,1 2,2 2,3 2,4 2,5 2,6 2,7 2,8
4. Write a loop generate all items in a single column, say column=5.
0,5 1,5 2,5 3,5 4,5 5,5 6,5 7,5 8,5
5. Finally, write a loop to generate the valid indices for the first 3×3 piece of the board.
0,0 0,1 0,2
1,0 1,1 1,2
2,0 2,1 2,2
Think about how you would modify this to generate the other 3×3 blocks. What are
the starting and end indices?
You can see the patterns of how we can write these loops. Now, we will use these to actually
solve the lab.
Checkpoint 1: Representing and Building the Board
We will represent the Sudoku board as a list of lists of single character strings. Start by
looking at the code in It has an example board, stored in the variable bd. Each
‘.’ is an empty location on the Sudoku board. The code prints the length of bd, the length
of the 0-th list stored in bd, the entry in row 0, column 0, and finally the entry in row 8,
column 8. Go ahead and run this code, and make sure you understand the output you are
Write nested for or while loops to print the whole board on the screen. You will first go
through each row with one loop, then for each row, you will go through each column using
a second loop (see index range 2 from Checkpoint 0). Print each item with space on both
sides, and a | after every third item and third row. Remember, you have exactly 9 rows
and 9 columns.
Here is the expected output:
| 1 . . | . 2 . | . 3 7 |
| . 6 . | . . 5 | 1 4 . |
| . 5 . | . . . | . 2 9 |
| . . . | 9 . . | 4 . . |
| . . 4 | 1 . 3 | 7 . . |
| . . 1 | . . 4 | . . . |
| 4 3 . | . . . | . 1 . |
| . 1 7 | 5 . . | . 8 . |
| 2 8 . | . 4 . | . . 6 |
Hint. Double loops can be difficult, so we recommend you start slowly and add complexity.
This will also help you in the other parts.
First, read each row as a list, and print each list on a single line. This is doable with a
single loop.
Now, add the second loop for formatting each row. Go through each item in the row with
a second loop, and construct a string containing your whole line. For each item, you will
append a space before and after the item, and | at the beginning and end. Once done,
print this string.
Now that you are printing reasonable lines, figure out how to add the | after every third
column in the row.
Finally, add the code to print the line of hyphens (-) as needed. Always do things in small
steps, and add complexity.
To complete Checkpoint 1: Show your code and output once you are finished.
Checkpoint 2: Assigning Numbers to Cells
Recall that the completed Sudoku board has no repeated numbers in a row, in a column,
or in any 3×3 block. In Checkpoint 2, your code will ask the user of the program to enter
a row (starting at index 0), a column (starting at index 0), and a number. It will then
call function ok_to_add, which you must write, to check to see if the number can safely
be added to that particular row and column based on the current contents of the Sudoku
board. You will then either tell the user, This number cannot be added, or if it can be
added, change the board and reprint it.
To start Checkpoint 2, copy and paste your code from Checkpoint 1.
The actual work of Checkpoint 2 is the function ok_to_add. It has quite a few checks you
will need to write. For example, if the user asks to put a 2 in row 1, column 8, the function
• check if row 1 contains a 2 already,
• check if column 8 contains a 2 already, and
• check if the 3×3 block starting at row 0, column 6 contains a 2 already.
What about the location (1,8)? Well, you need to check that there is nothing there currently.
But, it is better to move this check to outside of this function. This way, we can use this
function for multiple purposes.
For the Sudoku board from Checkpoint1, ok_to_add should return False because there
is already a 2 in the 3×3 block. ok_to_add should also check to see if the row index, the
column index and the number are also legal values — remember that users make typing
The function ok_to_add will have separate loops to check the row, to check the column,
and to check the 3×3 block. The latter is the hardest because you need to find the lowest
row and column indices in the block and then write nested loops to check all 9 locations.
The code should return False immediately when it finds a mistake, but it should wait until
all checking is complete before returning True.
Note that when ok_to_add returns True, it does not mean that the placement of the
number is actually correct. It is really just a sanity check.
To complete Checkpoint 2, show a TA or mentor your code, and the results of testing
a full range of possible mistakes.
Checkpoint 3: Sudoku Verifier
You are going to make a few changes to your code from Checkpoint 2 to complete this last
checkpoint. First, you will use the utility given to you to read a board from file. Import
lab06_util, prompt the user for a file name and read the board from the file using the
read_sudoku function.
Next, you will write a Sudoku solution verifier by using the ok_to_add function. The
verification of a correct solution to a Sudoku board can be broken down into two steps:
• Verify that there are no empty (‘.’) spaces in the board.
• Verify that for every number in the solution, it is ok_to_add in its current position. For example, if there is a 3 in position (0,1), we want to make sure that
If all locations have a number and if ok_to_add is true for each position, then the solution
is valid. Your job in Checkpoint 3 is to write the function verify_board. To verify your
solution, you can use the files solved.txt, unsolved1.txt and unsolved2.txt.
To complete Checkpoint 3, combine all your code into a while loop that does the
ask user file name
read board from file
while (board is not solved according to verify_board):
ask user for input to the puzzle
if ok_to_add
add value
print the board
When you have tested your code properly, show it to a TA or mentor.
Extra Challenges
Here are two extra challenges for those of you who are ambitious:
1. In theory, a Sudoku can be any NxN block grid, where N > 0. The most common
Sudoku is a 3×3 block board, but there do exist 4×4 and 5×5 Sudoku puzzles. As
an extra challenge, rewrite Checkpoints 1b, 2 and 3 so that they support any sized
Sudoku boards. You might start by modifying print_board, and include a parameter
for the block_size (in an ordinary 9×9 Sudoku, block_size is 3). You can test your
code with the file bigger.txt, which is a 4×4 block Sudoku.
2. If you are really ambitious, write an automatic solver. For each cell, keep a list of the
possible digits that can be assigned to that cell. When there is only one possibility,
say the digit 3, then 3 must be removed from the list of possible digits for the other
cells in the row, in the column and in the block. The trick is to keep a list of which
cells have been reduced to one entry.
This solution will not allow you to automatically solve all Sudoku puzzles — multi-cell
reasoning is needed for this — but it should be able to solve almost anything labeled
“medium” or easier.
Unfortunately, no extra credit is being offered for implementing this except for the satisfaction of great accomplishment!