78_SolverForQueensPuzzle

represent a solver for the queens problem

Yifan's remarks

citations / worked with / received help from

I have received help from George Zhou from the csDojo with guiding me through parts of the solution. I also worked with Emma Choi in class to discuss and work on the drawings.

This list may omit sources common to everyone in the course, including...

• Piazza threads
• class-wide discussions
• communication with Holmes via the class web site or GitHub repos.

Holmes's remarks

no parameters and no return in the recursive method

Unlike many recursive solutions, recordSolutionsStarted in SolverForQueensPuzzle...

• takes no parameters and
• returns no value.

That's ok, because all that information is communicated via the state of an instance of SolverForQueensPuzzle:

• No parameters are required because the inProgress board "knows" which rank recordSolutionsStarted should work on, namely, that board's most-recently populated rank.

• No return value is required because an invocation of recordSolutionsStarted communicates its results by updating the solutions collection.

We have seen this kind of communication elsewhere: No return value is required in the Towers of Hanoi puzzle, when you call on Monk3 to move 3 platters. Monk3 returns nothing to you. Rather, Monk3 communicates their results to you by updating the towers / disks instance.

testing via UserOfSolver

UserOfSolver can take two command-line arguments. Ignore the second one temporarily.

The first command-line argument is required and gives the number of ranks in the largest board to be analyzed. UserOfSolver will analyze all boards from size 0 up to the given size.

You are the kind of person who tests base cases first when debugging recursive programs, right? So what size boards will test only the base cases?

After the base cases, I verified my program's operation on a the largest recursive case I could stand: I was proud to make it through boards of size 3 and part way into 4.

Once you have seen both base cases and a couple recursive cases work, clean up your output by commenting out the debugging statements. With the debugging commented out, I checked the solutions for boards of size 4 and 6.

You weren't going to delete useful debugging statements, were you? What if your program stops working some time in the future? This may not matter to students, but in the real world, programs change constantly. That's why it's called "software". With improvements come bugs, so it is worth saving good debugging code.

John Bruestle taught me that one should print no more output than one is willing to read, since extraneous junk hides problems and answers that you want to see. So unless you're going to look at all 40 solutions to a 7-queens puzzle, benefit from the second command line argument.

The optional second command line argument imposes a cap on the number of solutions displayed, saving both time and screen real estate. For example, the command...

java UserOfSolver 7 3

Shows the up to 3 solutions for each of the puzzles of size 0 through 7. You can limit output to just summaries (no boards shown) with a command like...

java UserOfSolver 13 0

The number of solutions is given in this table in N.J.A. Sloane's amazing The On-Line Encyclopedia of Integer Sequences