1. CS 101 – Nov. 4 Wrap up OS Examples User interface Hierarchy of files Techniques for searching –Breadth first and depth first Continue with Access in lab

2. Popular OS’s 1971 UNIX, Linux 1979 DOS 1983 Microsoft Windows 1984 MacOS 1980 QNX (a real-time OS)

3. User interface Text-based –Enter commands at the keyboard –Program called a “shell” awaits your commands. GUI –Popular since mid 1980s –Uses mouse –Can see multiple applications on screen –Examples: http://toastytech.com/guis

4. File Organization Hierarchical –Files in folders, folders inside bigger folders –E.g. C:\work\sp09\cs101\excel\calorie.xlsx –Can visualize entire file system as a tree. How to search for a file? –First, need some “key” info to search on. –Then, traverse the tree in organized fashion until you find file.

5. Traversing a tree 2 basic strategies Breadth-first search (BFS) –Start at the root (top) of tree –Fan out in all directions simultaneously –Good if you think what you’re looking for is near the top. Depth-first search (DFS) –Start at the root, as usual –Go as far as you can down one path of the tree. –If not found, back up and try another path. –Good if you have an idea what area to search first.

6. Example Suppose we’re looking for file at node 9. We visit the nodes of the tree in the following order. BFS: 1, 2, 3, 4, 5, 6, 7, 8, 9 DFS: 1, 2, 4 – back up 5 – back up 3, 6, 7, 9 1 23 45 6 78 91011