Solving a problem: scandir and Unix ls Access all the entries in a - - PowerPoint PPT Presentation

Duke CPS 108

• 9. 1

Solving a problem: scandir and Unix ls

• Access all the entries in a directory, or selected entries

➤ ls *.cc, ls -l *, ls -lt foo.* ➤ process the entries in some way (add sizes, print, …)

• What is in a directory?

➤ ., .., files, subdirectories ➤ what is a file in Unix? What kind of resources are

associated with a file? How do you create/remove a file from within a running program?

➤ How are files and directories accessed from a program?

• How is a directory tree backed-up to disk/tape?

➤ What are the issues?

Duke CPS 108

• 9. 2

Solving another problem: search for words

• Web-based search engines

➤ Search for “Titanic” + “Linux” in Alta vista

• File-based search engines

➤ What’s the file that has the reference to “elephants”?

• What are the issues in the searches?

➤ User’s perspective ➤ Programmer’s perspective

• What about approximate matches

➤ search for “pincake” or “pancake” or “pancakes” or … ➤ are approximate/fuzzy matches a good idea?

Duke CPS 108

• 9. 3

Coping with transferable ownership

• Factory creates a class, who deletes it?

Foo * f = FooFactory::makeFoo(); f->doFooStuff(); delete f;

➤ Why might this be a problem? What are alternatives?

• Instead of using a Foo pointer, use a proxy, a class that stands

in for a Foo pointer

➤ looks like a Foo pointer, acts like a Foo pointer, but cleans

up after itself (SmartStrings in sortall/cps 100): heap vs. stack allocation

FooProxy f(FooFactory::makeFoo()); f->doFooStuff();

Duke CPS 108

• 9. 4

Program parameters and errors

• What are alternatives to large if/else chains to process argc and

argv?

➤ What are argc/argv used for? ➤ What helper functions cope with arg parsing? ➤ What’s wrong with if (argv[k] == “-f”) ➤ Other options to getting info into program

• How do you signal an error condition?

➤ Files don’t open, times (wag) out of order, … ➤ cerr << ➤ other alternatives?