Home Accessibility Courses Twitter The Mouth Facebook Resources Site Map About Us Contact
 
For 2023 (and 2024 ...) - we are now fully retired from IT training.
We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk.

Lisa and I (Graham) now live in what was our training centre in Melksham - happy to meet with former delegates here - but do check ahead before coming round. We are far from inactive - rather, enjoying the times that we are retired but still healthy enough in mind and body to be active!

I am also active in many other area and still look after a lot of web sites - you can find an index ((here))
Traversing a directory in Perl

As part of the revision of our Perl Programming course, we're adding an example on directory handling in with our basic file handling module. That's because, over recent years, Perl's use in parsing directories and searching around for data - systems admin tasks - has increased disproportionately to its heavy data handling use and web use.

So there's a new example - source code [here] - which parses a directory and produces a summary of the data for each item it contains, and a brief overall report.

In Perl, you open a directory with an opendir function call:
  opendir DH,".";
and you then read from the directory with a readdir:
  $item = readdir DH;
with each read returning the name of the next item. Once you've read the whole directory, you'll get a false value returned, so that you can easily write a loop to check a directory:
  while ($item = readdir DH) { [etc]

Characteristics of individual items in the file system can be checked with operators such as -d ("is it a directory") and -f ("is it a plain file"):
  if (-d $item) {
and other operators in the same family can be called to get more data, such as -s to return the size of a file, and -M to return its age in days:
  $result .= "Size is ",-s $item," bytes\n";




When you're traversing a directory, or reading a file of data, you'll often want to generate multiple reports on your output. Rather than traversing your data multiple times, for efficiency's sake you'll want to traverse it just once, and store each of your reports, as you generate it, in another variable. You can then print out each of these reports once you have completed your data traversal.

The example above uses this technique to store up a report until the work is completed. At that start, a scalar variable is initialised to being empty:
  $result = "";
then throughout the traversal, information is added onto the end of that variable using the .= operator, for example:
  $result .= "Is a plain file\n";
  $result .= "Size is ",-s $item," bytes\n";
  $result .= sprintf "Modified %.2f%% of a day ago\n\n", 100 * (-M $item);
and finally the variable is printed out:
  print $result;

Saving the data in a variable in this way has other advantages - it allows the report to be written to two diffrerent destinations very easily, and it allows the program to have a "rethink" - i.e. to generate a string that will probably be output, but then supress or modify that output later on if some condition towards the end of the data dictates that should be the case.

There's one downside of this approach when used with a long process - the user won't see any results until the data has been completely traversed. There are techniques for dealing with this sort of issue - the whole topic of handling huge data is covered on our more advanced Perl for Larger Proects course.
(written 2012-08-08, updated 2012-08-18)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P207 - Perl - File Handling
  [12] How many people in a room? - (2004-08-12)
  [114] Relative or absolute milkman - (2004-11-10)
  [255] STDIN, STDOUT, STDERR and DATA - Perl file handles - (2005-03-23)
  [616] printf - a flawed but useful function - (2006-02-22)
  [618] Perl - its up to YOU to check your file opened - (2006-02-23)
  [702] Iterators - expressions tha change each time you call them - (2006-04-27)
  [867] Being sure to be positive in Perl - (2006-09-15)
  [1312] Some one line Perl tips and techniques - (2007-08-21)
  [1416] Good, steady, simple example - Perl file handling - (2007-10-30)
  [1442] Reading a file multiple times - file pointers - (2007-11-23)
  [1467] stdout v stderr (Tcl, Perl, Shell) - (2007-12-10)
  [1709] There is more that one way - Perl - (2008-07-14)
  [1841] Formatting with a leading + / Lua and Perl - (2008-10-15)
  [1860] Seven new intermediate Perl examples - (2008-10-30)
  [1861] Reactive (dynamic) formatting in Perl - (2008-10-31)
  [2233] Transforming data in Perl using lists of lists and hashes of hashes - (2009-06-12)
  [2405] But I am reading from a file - no need to prompt (Perl) - (2009-09-14)
  [2818] File open and read in Perl - modernisation - (2010-06-19)
  [2821] Chancellor George Osborne inspires Perl Program - (2010-06-22)
  [2833] Fresh Perl Teaching Examples - part 2 of 3 - (2010-06-27)
  [3326] Finding your big files in Perl - design considerations beyond the course environment - (2011-06-14)
  [3548] Dark mornings, dog update, and Python and Lua courses before Christmas - (2011-12-10)
  [3839] Spraying data from one incoming to series of outgoing files in Perl - (2012-08-15)


Back to
Training courses - rest of 2012, 2013 and January 2014
Previous and next
or
Horse's mouth home
Forward to
Our Melksham Hotel is not quiet - but we are waiting to give you a quiet welcome. Big difference!
Some other Articles
Geekmas 2012 - celebrating open source languages such as Perl, PHP and Python
Learning to use existing classes in Perl
A busy day at Well House Manor - so come in and make it even busier!
Our Melksham Hotel is not quiet - but we are waiting to give you a quiet welcome. Big difference!
Traversing a directory in Perl
Training courses - rest of 2012, 2013 and January 2014
Pimms and Croquet in Edwardian Melksham - 8th September 2012 - Food Festival Event
Melksham Community Apple Pressing Day
TransWilts trains - what the next franchise period will bring
Well House Manor - direct hotel bookings help us improve the customer experience
4759 posts, page by page
Link to page ... 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96 at 50 posts per page


This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

Link to Ezine home page (for reading).
Link to Blogging home page (to add comments).

You can Add a comment or ranking to this page

© WELL HOUSE CONSULTANTS LTD., 2024: 48 Spa Road • Melksham, Wiltshire • United Kingdom • SN12 7NY
PH: 01144 1225 708225 • EMAIL: info@wellho.net • WEB: http://www.wellho.net • SKYPE: wellho

PAGE: http://www.wellho.net/mouth/3830_Tra ... -Perl.html • PAGE BUILT: Sun Oct 11 16:07:41 2020 • BUILD SYSTEM: JelliaJamb