Refining Functions and Programs

The goal of the following sequence of exercises is to develop several common utility functions for directory and file systems, using our third and most refined model. Even though these functions process Scheme-based representations of files and directories, they give us a good idea how such real-world programs work.

Exercise 16.3.1

Translate the file system in figure  into a Scheme representation. Remember to use empty for the content of the files. Solution

To make the exercise more realistic, DrScheme supports the teachpack dir.ss. It introduces the two necessary structure definitions and a function to create representations of directories according to our third model:

;; create-dir : string -> dir
;; to create a representation of the directory that a-path specifies: 
;; 1. Windows: (create-dir ``c:\\windows'')  
;; 2. Mac: (create-dir ``My Disk:'')  
;; 3. Unix: (create-dir ``/home/scheme/'') 
(define (create-dir a-path) ...) 

Exercise 16.3.2

Develop the function how-many, which consumes a dir (according to model 3) and produces the number of files in the dir tree. Test the function on the directories created in exercise . Why are we confident that the function produces correct results? Solution

Exercise 16.3.3

Develop the function du-dir. The function consumes a directory and computes the total size of all the files in the entire directory tree. This function approximates a true disk-usage meter in that it assumes that directories don't require storage.

Refine the function to compute approximate sizes for subdirectories. Let's assume that storing a file and a directory in the content field costs 1 storage unit. Solution

Exercise 16.3.4

Develop the function find?, which consumes a dir and a file name and determines whether or not a file with this name occurs in the directory tree.

Challenge: Develop the function find. It consumes a directory d and a file name f. If (find? d f) is true, it produces a path to the file; otherwise it produces false. A path is a list of directory names. The first one is that of the given directory; the last one is that of the subdirectory whose files list contains f. For example:

(find TS 'part3)
;; expected value:  
(list 'TS 'Text) 

(find TS 'read!)
;; expected value:  
(list 'TS) 

Which read! file in figure  should find discover? Generalize the function to return a list of paths if the file name occurs more than once. Each path should lead to a different occurrence, and there should be a path for each occurrence. Solution