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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.
Perl - the duplicate key problem explained, and solutions offered

Do you want to hold data in a hash in Perl - but you can't do so directly because you have more than one piece of data that should have the same key?

The problem ...

while (<DIAL>) {
  ($dial,$place) = split
  $codes{$place} = $dial;
  }


And each line that contains a repeated place name OVERWRITES the previous record for that names place. The problem is shown - this code above in a worked example - [here]. When we run it:

-bash-3.2$ perl stdhash
4898 lines read
4742 entries in hash
Florida: 001954
-bash-3.2$


we only get one Florida entry, and we notice that rather more lines were read than records created.

The solution ...

This is Perl and there are more than one way of doing anything, so take your pick!

1. Store all the possible codes in a string or list within the hash. [source example]

or 2. Make up a new key if the one you want has already been used. [source example]. But note that will only report on the original key, you you'll need to add something to find the other generated keys too - see [source example].

The data file used in these examples may be found [here].
(written 2010-06-28)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P211 - Perl - Hashes
  [240] Conventional restraints removed - (2005-03-09)
  [386] What is a callback? - (2005-07-22)
  [738] (Perl) Callbacks - what are they? - (2006-05-30)
  [930] -> , >= and => in Perl - (2006-11-18)
  [968] Perl - a list or a hash? - (2006-12-06)
  [1334] Stable sorting - Tcl, Perl and others - (2007-09-06)
  [1705] Environment variables in Perl / use Env - (2008-07-11)
  [1826] Perl - Subs, Chop v Chomp, => v , - (2008-10-08)
  [1856] A few of my favourite things - (2008-10-26)
  [1917] Out of memory during array extend - Perl - (2008-12-02)
  [2833] Fresh Perl Teaching Examples - part 2 of 3 - (2010-06-27)
  [2915] Looking up a value by key - associative arrays / Hashes / Dictionaries - (2010-08-11)
  [2920] Sorting - naturally, or into a different order - (2010-08-14)
  [3042] Least Common Ancestor - what is it, and a Least Common Ancestor algorithm implemented in Perl - (2010-11-11)
  [3072] Finding elements common to many lists / arrays - (2010-11-26)
  [3106] Buckets - (2010-12-26)
  [3400] $ is atomic and % and @ are molecular - Perl - (2011-08-20)
  [3451] Why would you want to use a Perl hash? - (2011-09-20)
  [3662] Finding all the unique lines in a file, using Python or Perl - (2012-03-20)


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Just pass a pointer - do not duplicate the data
Software versions used - June 2010
Respecting our customers anonimity
Lorry Parking in Melksham
Perl - the duplicate key problem explained, and solutions offered
A course is more than just a chap giving a lecture
Teaching examples in Perl - third and final part
Are you learning Perl? Some more examples for you!
Recording (a macro) in vi
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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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