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For 2021 - online Python 3 training - see ((here)).

Our plans were to retire in summer 2020 and see the world, but Coronavirus has lead us into a lot of lockdown programming in Python 3 and PHP 7.
We can now offer tailored online training - small groups, real tutors - works really well for groups of 4 to 14 delegates. Anywhere in the world; course language English.

Please ask about private 'maintenance' training for Python 2, Tcl, Perl, PHP, Lua, etc.
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
  [28] Perl for breakfast - (2004-08-25)
  [140] Comparison Chart for Perl programmers - list functions - (2004-12-04)
  [230] Course sizes - beware of marketing statistics - (2005-02-27)
  [240] Conventional restraints removed - (2005-03-09)
  [355] Context in Perl - (2005-06-22)
  [463] Splitting the difference - (2005-10-13)
  [560] The fencepost problem - (2006-01-10)
  [622] Queues and barrel rolls in Perl - (2006-02-24)
  [762] Huge data files - what happened earlier? - (2006-06-15)
  [773] Breaking bread - (2006-06-22)
  [928] C++ and Perl - why did they do it THAT way? - (2006-11-16)
  [968] Perl - a list or a hash? - (2006-12-06)
  [1304] Last elements in a Perl or Python list - (2007-08-16)
  [1316] Filtering and altering Perl lists with grep and map - (2007-08-23)
  [1703] Perl ... adding to a list - end, middle, start - (2008-07-09)
  [1828] Perl - map to process every member of a list (array) - (2008-10-09)
  [1917] Out of memory during array extend - Perl - (2008-12-02)
  [1918] Perl Socket Programming Examples - (2008-12-02)
  [2067] Perl - lists do so much more than arrays - (2009-03-05)
  [2226] Revision / Summary of lists - Perl - (2009-06-10)
  [2295] The dog is not in trouble - (2009-07-17)
  [2484] Finding text and what surrounds it - contextual grep - (2009-10-30)
  [2813] Iterating over a Perl list and changing all items - (2010-06-15)
  [2833] Fresh Perl Teaching Examples - part 2 of 3 - (2010-06-27)
  [2996] Copying - duplicating data, or just adding a name? Perl and Python compared - (2010-10-12)
  [3400] $ is atomic and % and @ are molecular - Perl - (2011-08-20)
  [3548] Dark mornings, dog update, and Python and Lua courses before Christmas - (2011-12-10)
  [3669] Stepping through a list (or an array) in reverse order - (2012-03-23)
  [3906] Taking the lead, not the dog, for a walk. - (2012-10-28)
  [3939] Lots of ways of doing the same thing in Perl - list iteration - (2012-12-03)
  [4609] Mapping an array / list without a loop - how to do it in Perl 6 - (2016-01-03)


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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.

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