Q: How do we use constraint propagation to solve the naked twins problem?
A: We iterate through all our unsolved units (rows, columns and squares) to identify any two boxes that both contain the same only two choices. If we find a pair, they are known as naked twins. Even though we do know which box contains which value, we know together they must take both values, therefore we can eliminate their values from any other boxes in the same units, due to the constraints that we've been given.

Q: How do we use constraint propagation to solve the diagonal sudoku problem?
A: A diagonal sudoku is simply adding two additional units, one for each diagonal as additional constraints such that we have a total of 29 constraints (9 rows, 9 columns, 9 boxes and two diagonals).

This project requires Python 3.

We recommend students install Anaconda, a pre-packaged Python distribution that contains all of the necessary libraries and software for this project. Please try using the environment we provided in the Anaconda lesson of the Nanodegree.

Optionally, you can also install pygame if you want to see your visualization. If you've followed our instructions for setting up our conda environment, you should be all set.

If not, please see how to download pygame here.

• solution.py - You'll fill this in as part of your solution.
• solution_test.py - Do not modify this. You can test your solution by running python solution_test.py.
• PySudoku.py - Do not modify this. This is code for visualizing your solution.
• visualize.py - Do not modify this. This is code for visualizing your solution.

To visualize your solution, please only assign values to the values_dict using the assign_value function provided in solution.py

Before submitting your solution to a reviewer, you are required to submit your project to Udacity's Project Assistant, which will provide some initial feedback.

The setup is simple. If you have not installed the client tool already, then you may do so with the command pip install udacity-pa.

To submit your code to the project assistant, run udacity submit from within the top-level directory of this project. You will be prompted for a username and password. If you login using google or facebook, visit this link for alternate login instructions.

This process will create a zipfile in your top-level directory named sudoku-.zip. This is the file that you should submit to the Udacity reviews system.