music

# eight queens

## summary

This subchapter looks at the eight queens problem.

The eight queeens problem is a common computer science problem, usually used to teach backtracking in recursion.

## free computer programming text book project

If you like the idea of this project,
then please donate some money.

### stub section

This subchapter is a stub section. It will be filled in with instructional material later. For now it serves the purpose of a place holder for the order of instruction.

Professors are invited to give feedback on both the proposed contents and the propsed order of this text book. Send commentary to Milo, PO Box 1361, Tustin, California, 92781, USA.

# eight queens

This subchapter looks at the eight queens problem.

The eight queeens problem is a common computer science problem, usually used to teach backtracking in recursion.

The problem is to place eight queens on a standard eight by eight chess board in such a pattern that no queen attacks any other queen.

The problem can be extended to n number of queens on an n × n board.

### a few algorithms

The brute force method is to try every possible combination of placement of eight queens on a chess board. Each possible arrangement is individually checked for any mutual attacks (disqualifying it as a solution). You can continue this process until you find one solution, or even continue through every combination in order to find every solution.

One obvious improvement on the brute force method is to immediately eliminate all combinations where queens are on the same row or column. These cases are easy to identify and all failures.

A greedy method is to start with a random placement (limited to one queen per row). Identify the queen with the most attacks and move that queen to the position on its row with the lowest number of attacks (for the queen moved). identify the new worst case queen and move it in a similar manner. Continue until you achieve success. If this method works, it is the mathematically fastest known solution method. Unfortunately, there is no guarantee of success. It is possible to get locked into an infinite loop. One could potentially identify an infinite loop and start over with a new random placement.

The method of concern to us is recursive backtracking because it illustrates this important computer science topic.

# free music player coding example

Coding example: I am making heavily documented and explained open source code for a method to play music for free — almost any song, no subscription fees, no download costs, no advertisements, all completely legal. This is done by building a front-end to YouTube (which checks the copyright permissions for you).

View music player in action: www.musicinpublic.com/.

Create your own copy from the original source code/ (presented for learning programming).

## view text bookHTML file

Because I no longer have the computer and software to make PDFs, the book is available as an HTML file, which you can convert into a PDF.

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## free computer programming text book project

Building a free downloadable text book on computer programming for university, college, community college, and high school classes in computer programming.

If you like the idea of this project,
then please donate some money.

send donations to:
Milo
PO Box 1361
Tustin, California 92781

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