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Sudoku [9 marks]
In these questions you will be working on writing code that will
solve Sudoku puzzles.
All these questions are part of the Core assignment, due in at the
For the avoidance of doubt -- with the exception of the
SudokuSquareSet question, you may add extra #include and using
statements, if you need to.
Make sure you don't commit any compiled code to your GitHub
repository; or if you choose to use an IDE, any large project
directories created by your IDE. You can make these on your machine,
but don't commit or add them to your repository -- this isn't what
git is designed for.
a) Making a Sudoku board class
In the file Sudoku.h make a class Sudoku that holds an incomplete
Sudoku solution.
It should have a constructor that takes a single argument -- the size
of the board. For instance, for a 9x9 Sudoku, the constructor would
be given the value 9. Or, for a 16x16 board, the constructor would be
given the value 16.
You need to store the incomplete solution as a member variable. The
recommended way to do this, to start with, is to have a vector of
vectors (a square array), in which each square is represented as a
set that holds the values that could possibly go in that square.
Initially, for a 9x9 Sudoku, if the grid is completely blank, each
set will contain the values {1,2,3,4,5,6,7,8,9}. When a square is
given some value, the set is cleared and replaced with a set
containing just that one value -- the other options are removed.
Write a function getSquare(int row, int col) that returns the value
in the cell in the square at the given row and column:
• If there is only one value in the set for that square, return
the number in that set.
• Otherwise, return -1 (a dummy value to indicate we don't know
what should be in that square yet)
b) Setting the value of a Sudoku
square
Write a function setSquare(int row, int col, int value) that sets the
value in the cell in the square at the given row and column, then
updates the sets of possible values in the rest of the grid to remove
choices that have been eliminated. For instance, if we put a '3' on a
given row, then nothing else on that row can have the value 3.
The implementation of setSquare is split into two parts.
First, the easy part: the set of possible values for that cell is
cleared, and value is inserted. This forces that cell to have that
value.
Then, a loop begins that does the following:
• Loop over the entire grid
• For each square that has only one value in it, remove that
value from the sets of possible values for:
o All the other squares on that row
o All the other squares in that column
o All the other squares in the same box. A 9x9 grid is
divided into 9 boxes, each 3x3: no two values in the same
box can have the same value. For larger grids (e.g.
16x16), the size of the box is always the square root of
the size of the grid.
If at any point the set of values for a square becomes empty, the
function should return false: it has been shown that there is no
value that can go in a square.
The loop should continue whilst values are still being removed from
the sets of possible values. The reason for this is that after
setting the given square, we might end up with only one option being
left for some other squares on the grid. For instance, if on a given
row some of the squares had the sets:
{3,4} {3,5} {4,5}
...and we call setSquare to set the middle square to have the value
3, then before the loop:
{3,4} {3} {4,5}
On the first pass of the loop, we would find the square containing 3
and remove this from the other sets on the row (and the other sets in
the same column and box). The row then looks like:
{4} {3} {4,5}
We then start the loop again, and find the square containing the
value '4'. This is removed from the other sets on the row (and column
and box) to give:
{4} {3} {5}
We then start the loop again, and find the square containing the
value '5'.
This process stops when, having looped over the board, and updated
the sets by removing values, our sets have stopped getting any
smaller. At this point the function returns true.
For simple Sudodu puzzles, this process here is enough to solve the
puzzle. No guesswork is needed: setting the squares of the board to
hold the initial values specified in the puzzle, is enough to cause
all the other squares of the board to have only one option left.
You can test this by compiling and running BasicSudoku.cpp:
g++ -std=c++11 -g -o BasicSudoku BasicSudoku.cpp
This calls setSquare for the values in a simple Sudoku puzzle; then
c) Searching for a solution
For more complex puzzles, after putting in the initial values using
setSquare, some of the squares on the board have more than one value
left in their set of possible values -- using logic alone, we cannot
deduce what the value has to be; we have to make a guess and see what
happens.
For this, we are going to use the Searchable class. This is an
abstract class for puzzles, containing the following virtual
functions:
• isSolution(): this returns true if the puzzle has been solved.
For Sudoku, this means all the squares contain just one value.
• write(ostream & o): a debugging function to print the board to
screen.
• heuristicValue(): an estimate of how far the puzzle is from
• successors(): in a situation where a guess is needed, this
returns several new puzzle objects, each of which corresponds
to a different guess having been made.
Make your Sudoku class inherit from Searchable, by changing the
opening of the class definition to class Sudoku : public Searchable
Implement isSolution() to only return true if the puzzle has been
solved; i.e. every set in every square is of size 1.
Implement a write() function to print the board. You can display the
board however you like. A reasonable implementation is to print out
the board one row at a time:
• If the square has more than one value in its set, print a space
character
• Otherwise, print the value from the set.