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 representation of files and
directories, they give us a good idea how such real-world functions work.
<#19213#>Exercise 16.3.1<#19213#>
Translate the file system in figure~#figaccountant#19215> into a Scheme
representation. Remember to use <#64028#><#19216#>empty<#19216#><#64028#> for the content of the
files.~ 
Solution<#64029#><#64029#>
To make the exercise more realistic, DrScheme supports the teachpack
<#19222#>dir.ss<#19222#>. It introduces the two necessary structure definitions
and a function to create representations of directories according to our
third model:
<#71120#>;; <#64030#><#19227#>create-dir<#19227#> <#19228#>:<#19228#> <#19229#>string<#19229#> <#19230#><#19230#><#19231#>-;SPMgt;<#19231#><#19232#><#19232#> <#19233#>dir<#19233#><#64030#><#71120#>
<#71121#>;; to create a representation of the directory that <#64031#><#19234#>a-path<#19234#><#64031#> specifies:<#71121#>
<#71122#>;; 1. Windows: <#64032#><#19235#>(create-dir<#19235#> <#19236#>``c:\<#19236#><#19237#>\<#19237#><#19238#>windows'')<#19238#><#64032#> <#71122#>
<#71123#>;; 2. Mac: <#64033#><#19239#>(create-dir<#19239#> <#19240#>``My<#19240#> <#19241#>Disk:'')<#19241#><#64033#> <#71123#>
<#71124#>;; 3. Unix: <#64034#><#19242#>(create-dir<#19242#> <#19243#>``/<#19243#><#19244#>home/<#19244#><#19245#>scheme/<#19245#><#19246#>'')<#19246#><#64034#><#71124#>
<#19247#>(define<#19247#> <#19248#>(create-dir<#19248#> <#19249#>a-path)<#19249#> <#19250#>...)<#19250#>
Use the function to create some small and large examples based on the
directories in a real computer. <#19254#>Warning<#19254#>: For large directory trees,
DrScheme may need a lot of time to build a representation. Use
<#64035#><#19255#>create-dir<#19255#><#64035#> on small directory trees first.
<#19256#>Exercise 16.3.2<#19256#>
Develop the function <#64036#><#19258#>how-many<#19258#><#64036#>, which consumes a <#64037#><#19259#>dir<#19259#><#64037#> (according to
model~3) and produces the number of files in the <#64038#><#19260#>dir<#19260#><#64038#> tree. Test the
function on the directories created in exercise~#exfileexample3#19261>. Why are we
confident that the function produces correct results?~ Solution<#64039#><#64039#>
<#19267#>Exercise 16.3.3<#19267#>
Develop the function <#64040#><#19269#>du-dir<#19269#><#64040#>. 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
directory 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 <#64041#><#19270#>content<#19270#><#64041#> field costs <#64042#><#19271#>1<#19271#><#64042#>
storage unit.~ Solution<#64043#><#64043#>
<#19277#>Exercise 16.3.4<#19277#>
Develop the function <#64044#><#19279#>find?<#19279#><#64044#>, which consumes a <#64045#><#19280#>dir<#19280#><#64045#> and a file name
and determines whether or not a file with this name occurs in the directory tree.
<#19281#>Challenge<#19281#>: Develop the function <#64046#><#19282#>find<#19282#><#64046#>. It consumes a
directory <#64047#><#19283#>d<#19283#><#64047#> and a file name <#64048#><#19284#>f<#19284#><#64048#>. If <#64049#><#19285#>(find?<#19285#>\ <#19286#>d<#19286#>\ <#19287#>f)<#19287#><#64049#> is true,
it produces a path to the file; otherwise it produces <#64050#><#19288#>false<#19288#><#64050#>. 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 <#64051#><#19289#>files<#19289#><#64051#> list contains <#64052#><#19290#>f<#19290#><#64052#>. For
example:
<#19295#>(find<#19295#> <#19296#>TS<#19296#> <#19297#>'<#19297#><#19298#>part3)<#19298#>
<#19299#>=<#19299#> <#19300#>(list<#19300#> <#19301#>'<#19301#><#19302#>TS<#19302#> <#19303#>'<#19303#><#19304#>Text)<#19304#>
<#19312#>(find<#19312#> <#19313#>TS<#19313#> <#19314#>'<#19314#><#19315#>read!)<#19315#>
<#19316#>=<#19316#> <#19317#>(list<#19317#> <#19318#>'<#19318#><#19319#>TS)<#19319#>
assuming <#64053#><#19323#>TS<#19323#><#64053#> is <#64054#><#19324#>define<#19324#><#64054#>d to be the directory in
figure~#figaccountant#19325>.
Which <#19326#>read!<#19326#>\ file in figure~#figaccountant#19327> should <#64055#><#19328#>find<#19328#><#64055#>
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<#64056#><#64056#>