Home Accessibility Courses Diary The Mouth Facebook Resources Site Map About Us Contact
Writing more maintainable Perl - naming fields from your data records

Perl is the Practical Extraction and Reporting Language, and the data from which you'll want to extract data often comes in the form of CSV (Comma separated variables), or space or tab delimited records.

Opening and reading a file of such records, in a loop, is easy:

  open (FH,"trains") or die;
  while ($service = <FH>) {
    # act in each record here
  }


and within the loop, you can split each line into its individuak fields; if the line's tab delimited, for example, you migh write:

  @flds = split(/\t/,$service);

That's short and sweet, and I can then refer to individual elements. for example:

  print "Train to $flds[2] at $flds[0]\n";

This means, however, that in a complex piece of extraction and analysis code you're likely to be making a large number of references to elements by their position in the original lines, making the bulk of the code harder to follow, and making it difficult to reuse / update the code if the format / field order changes in future data files.

A Better Way

In Perl, you can give a list of scalars on the left of an assignment to name each element of a list (from something like a split) in one go:

  ($time, $cars, $place, $capa) = split(/\t/,$service);

and you can then refer to the elements by name during your extraction and analysis phase:

  print "Train to $place at $time\n";

Although the initial splitting line is (a little) longer, you can now write code that's much more self-documuenting, with meaningful variable names, in the analysis. And if the field order should even change, you've just got a single splitting line to recode, rather that having to re-engineer the whole analysis phase.

Full example (including sample data and output) is [here] on our web site. And this example is as taught on this week's Learning to program in Perl course.
(written 2012-09-25, updated 2013-01-01)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P208 - Perl - Lists
  [3939] Lots of ways of doing the same thing in Perl - list iteration - (2012-12-03)
  [3906] Taking the lead, not the dog, for a walk. - (2012-10-28)
  [3669] Stepping through a list (or an array) in reverse order - (2012-03-23)
  [3548] Dark mornings, dog update, and Python and Lua courses before Christmas - (2011-12-10)
  [3400] $ is atomic and % and @ are molecular - Perl - (2011-08-20)
  [2996] Copying - duplicating data, or just adding a name? Perl and Python compared - (2010-10-12)
  [2833] Fresh Perl Teaching Examples - part 2 of 3 - (2010-06-27)
  [2813] Iterating over a Perl list and changing all items - (2010-06-15)
  [2484] Finding text and what surrounds it - contextual grep - (2009-10-30)
  [2295] The dog is not in trouble - (2009-07-17)
  [2226] Revision / Summary of lists - Perl - (2009-06-10)
  [2067] Perl - lists do so much more than arrays - (2009-03-05)
  [1918] Perl Socket Programming Examples - (2008-12-02)
  [1917] Out of memory during array extend - Perl - (2008-12-02)
  [1828] Perl - map to process every member of a list (array) - (2008-10-09)
  [1703] Perl ... adding to a list - end, middle, start - (2008-07-09)
  [1316] Filtering and altering Perl lists with grep and map - (2007-08-23)
  [1304] Last elements in a Perl or Python list - (2007-08-16)
  [968] Perl - a list or a hash? - (2006-12-06)
  [928] C++ and Perl - why did they do it THAT way? - (2006-11-16)
  [773] Breaking bread - (2006-06-22)
  [762] Huge data files - what happened earlier? - (2006-06-15)
  [622] Queues and barrel rolls in Perl - (2006-02-24)
  [560] The fencepost problem - (2006-01-10)
  [463] Splitting the difference - (2005-10-13)
  [355] Context in Perl - (2005-06-22)
  [240] Conventional restraints removed - (2005-03-09)
  [230] Course sizes - beware of marketing statistics - (2005-02-27)
  [140] Comparison Chart for Perl programmers - list functions - (2004-12-04)
  [28] Perl for breakfast - (2004-08-25)


Back to
How have Melksham shops changed in 60 years?
Previous and next
or
Horse's mouth home
Forward to
On getting noticed for the right reasons when you ask about job availability
Some other Articles
Using Perl to read an RSS feed off a web site and extract data - via LWP and XML modules
Henbury loop, Bristol - a freight railway line with passenger potential?
Trains across Wiltshire - an update on the TransWilts
On getting noticed for the right reasons when you ask about job availability
Writing more maintainable Perl - naming fields from your data records
How have Melksham shops changed in 60 years?
How to make an ohno cake
The difficult interface between the business, council and voluntary sectors
Celebrate! Buses from Meksham to Bath
What makes Well House Manor different?
4255 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 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., 2014: Well House Manor • 48 Spa Road • Melksham, Wiltshire • United Kingdom • SN12 7NY
PH: 01144 1225 708225 • FAX: 01144 1225 899360 • EMAIL: info@wellho.net • WEB: http://www.wellho.net • SKYPE: wellho

PAGE: http://www.wellho.net/mouth/3870_Wri ... cords.html • PAGE BUILT: Sun Mar 30 15:20:58 2014 • BUILD SYSTEM: WomanWithCat