and Recursion Recursive Structures
Definition in the dictionary: in mathematics an expression in which a value is calculated by using preceding terms of the expression Pretty worthless, doesnt convey details or power Consider folders and files in a computer, whats in a folder? Files, other folders, anything else? Is there a distinction between a folder and a file? Viewpoint is everything, who is the viewer? The viewee? What does this have to do with physics? How do we look at files via programming
Genome Revolution: COMPSCI 006G 15.1
Files and Folders: Files and Directories
OS X view of Directory Structure
Genome Revolution: COMPSCI 006G
15.2
Different OS, Same View
Windows 2000 Files and Directories
Genome Revolution: COMPSCI 006G
15.3
java.io.File
An abstract representation of file and directory pathnames C:\eclipse\workspace\dirstuff\DirectoryViewer.java /Users/ola/Desktop/eclipse/workspace/dirstuff/DV.java What is a pathname? Given the path, can you find/navigate to the file/folder? Is there more than one way to get to a folder? Why? What accessor methods exist for Files/Directories getName() length(), lastModified() isDirectory(), isFile()
Genome Revolution: COMPSCI 006G 15.4
Problem? How 'large' is a directory?
The size accessor method and file-system view tells how much diskspace the directory entry needs This is small, it requires name, date, list-of-files The list-of-files isn't really part of the directory size, but it doesn't matter, we want SIZE How does the 'search' command work to find a file? Search everything from scratch each time Cache results with indexing to avoid re-searching How do interact programmatically with file system in platform independent way? Need an abstraction (hint: see java.io.File)
Genome Revolution: COMPSCI 006G
15.5
Code to visit a Folder (dirstuff)
public void visit(File f, int level){ if (f.isDirectory()){ System.out.println(tab(level)+f.length() + " **** " + f.getName()); File[] files = f.listFiles(); for(int k=0; k < files.length; k++){ visit(files[k],level+1); } } else { System.out.println(tab(level)+f.length() + "\t" + f.getName()); } }
Genome Revolution: COMPSCI 006G
15.6
Reasoning about File/Directory code
What is the purpose of the method visit? What does visit do? What methods does visit call? How does visit use java.io.File? Does the method call itself? No, but it calls a method named visit, with a different parameter Think of hundreds of little visit clones, do some work, pass other work off to clone How can we modify the method to print directory last? How can we modify method to calculate "real" size?
Genome Revolution: COMPSCI 006G 15.7
Who is Alan Perlis?
It is easier to write incorrect an program than to understand a correct one Simplicity does not precede complexity, but follows it If you have a procedure with ten parameters you probably missed some If a listener nods his head when you're explaining your program, wake him up Programming is an unnatural act Won first Turing award http://www.cs.yale.edu/homes/perlis-alan/quotes.html
Genome Revolution: COMPSCI 006G 15.8
What is recursion (revisited)?
Structure in which a container can hold/use other containers A directory holds file and directories A Russian doll holds other russian dolls? A method that calls 'itself' or clones Do russian dolls nest infinitely? Do folders hold infinite subfolders? Can a method call methods infinitely? We need a way out of the mess Last doll, directory with no subdirectories Method that makes no calls
Genome Revolution: COMPSCI 006G 15.9
Mathematical Definition of Factorial
6! = 1 x 2 x 3 x 4 x 5 x 6 What's n! If n equals 1 or 0, then it's 1 Otherwise it's n x (n-1)! Does this get the job done? Is it constructive? What does the method below return for 5? What about 1?
public static int factorial(int n){ if (n == 1 || n == 0) return 1; else return n * factorial(n-1); }
Genome Revolution: COMPSCI 006G 15.10
Recursive Structure and Methods
There must be some way out of the recursion A structure with no substructure A method with no recursive method calls This is the base-case of the recursion The substructure must be similar to the original structure Russian dolls don't hold machine guns (recursively) Each russian doll holds a smaller doll Each call of visit() gets closer "to the bottom&quo...