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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.
Looking up a value by key - associative arrays / Hashes / Dictionaries

In any substantial programming task, you'll want to store a whole series of values in some sort of table, so that you can process / reprocess / handle each of the elements in turn - and to do that, you'll use a variable construct that's called an "array", or a "list", a "tuple" or a "vector". (The exact word depends on the particular language you're using ... and how it's been implemented internally).

Arrays, lists, tuples and vectors are what we call "ordered collections" - in other words, they're bundles of variables which are keyed / looked up by position numbers. The numbering usually starts at 0 (exceptions - Lua and Fortran which start at 1), and in many cases that's ideal - but not in all cases.

Imagine that I have a data set which shows the UK ports from which you can take a ferry over the English Channel, and their destinations - [here]. The sort of question I'll be asking is "where can I go from Portsmouth" and if I keep the data in an array or list - keyed by position number - I'll have to do a search of some sort every time I make an inquiry; chances are that I'll need two arrays which have to be kept in step. Ironically, my user really won't care if Portsmouth is at position 0, 1, 2, or 3 ... nor if Newhaven comes first ... There has to be a better way!

There is. It's called an "Associative Array" (PHP), a "Dictionary" (Python), a "Hash" (Perl), (a table in Lua, a Hash, Hashmap or hashset in Java ...). In dictionaries and hashes, the elements are not orderable but can be looked up by key. There's a source code example (in Perl) [here] showing the setup and use of a hash for looking up our channel ports from our channel file - and we cover this in detail on our Learning to program in Perl course.

In PHP, things are a bit different. The Associative array CAN be ordered (sorted) - which overcomes one of the limitations of hashes and dictionaries, at the expense of efficiency on enormous data tables. But since PHP is going to be used primarily for web page generation, where the time taken / data sent can't be huge, this loss of efficiency in exceptional cases is no big deal. What does this mean?

It means that in PHP, I can set up a table of data - key, value pairs. I can then sort that table, and output the results in order. And I can do it VERY EASILY! I wrote an example of this - using fictional data for the number of deer in various national parks in the UK - on yesterday's PHP course, and I've shared the source code [here]. The code is robust (it has to be if it's going to be run on a public web site!) and you can run it [here].

(written 2010-08-11, updated 2010-08-20)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
Y107 - Python - Dictionaries
  [103] Can't resist writing about Python - (2004-10-29)
  [955] Python collections - mutable and imutable - (2006-11-29)
  [1144] Python dictionary for quick look ups - (2007-04-12)
  [1145] Using a list of keys and a list of values to make a dictionary in Python - zip - (2007-04-13)
  [2368] Python - fresh examples of all the fundamentals - (2009-08-20)
  [2986] Python dictionaries - reaching to new uses - (2010-10-05)
  [2994] Python - some common questions answered in code examples - (2010-10-10)
  [3464] Passing optional and named parameters to python methods - (2011-10-04)
  [3488] Python sets and frozensets - what are they? - (2011-10-20)
  [3554] Learning more about our web site - and learning how to learn about yours - (2011-12-17)
  [3555] Football league tables - under old and new point system. Python program. - (2011-12-18)
  [3662] Finding all the unique lines in a file, using Python or Perl - (2012-03-20)
  [3934] Multiple identical keys in a Python dict - yes, you can! - (2012-11-24)
  [4027] Collections in Python - list tuple dict and string. - (2013-03-04)
  [4029] Exception, Lambda, Generator, Slice, Dict - examples in one Python program - (2013-03-04)
  [4409] Setting up and using a dict in Python - simple first example - (2015-01-30)
  [4469] Sorting in Python 3 - and how it differs from Python 2 sorting - (2015-04-20)
  [4661] Unique word locator - Python dict example - (2016-03-06)
  [4668] Sorting a dict in Python - (2016-04-01)

Q101 - Object Orientation and General technical topics - Programming Principles
  [2001] I have not programmed before, and need to learn - (2009-01-19)
  [2022] Pre and post increment - the ++ operator - (2009-02-03)
  [2228] Where do I start when writing a program? - (2009-06-11)
  [2310] Learning to write high quality code in Lua - (2009-07-30)
  [2327] Planning! - (2009-08-08)
  [2415] Variable names like i and j - why? - (2009-09-22)
  [2510] The music of the stock market - (2009-11-22)
  [2550] Do not copy and paste code - there are much better ways - (2009-12-26)
  [2586] And and Or illustrated by locks - (2010-01-17)
  [2737] Improving your function calls (APIs) - General and PHP - (2010-04-24)
  [2769] Easy - but for whom? - (2010-05-18)
  [2878] Program for reliability and efficiency - do not duplicate, but rather share and re-use - (2010-07-19)
  [2964] An introduction to file handling in programs - buffering, standard in and out, and file handles - (2010-09-21)
  [3026] Coding efficiency - do not repeat yourself! - (2010-11-02)
  [3456] Stepping stones - early coding, and writing re-usable code quickly - (2011-09-24)
  [3542] What order are operations performed in, in a Perl expression? - (2011-12-07)
  [3548] Dark mornings, dog update, and Python and Lua courses before Christmas - (2011-12-10)
  [3551] Some terms used in programming (Biased towards Python) - (2011-12-12)
  [3673] Object oriented or structured - a comparison in Python. Also writing clean regular expressions - (2012-03-26)
  [3878] From Structured to Object Oriented Programming. - (2012-10-02)
  [3928] Storing your intermediate data - what format should you you choose? - (2012-11-20)
  [3954] Lesson 1 in programing - write clean, reuseable and maintainable tidy code - (2012-12-16)
  [4003] Web and console - same principle, same code - Ruby example - (2013-02-14)
  [4061] Seamless, integrated IT - we have a long way to go! - (2013-04-11)
  [4090] Test Driven Development in Python - Customer Comes First - (2013-05-16)
  [4118] We not only teach PHP and Python - we teach good PHP and Python Practice! - (2013-06-18)
  [4153] Rooms available tonight - how to code an algorithm from first principles - (2013-08-19)
  [4206] Writing the perfect program in Tcl? - (2013-11-13)
  [4325] Learning to program - what are algorithms and design patterns? - (2014-11-22)
  [4611] Hungarian, Camel, Snake and Kebab - variable naming conventions - (2016-01-03)
  [4632] Remember to ask the question before you listen for the answer - (2016-01-26)
  [4645] What are callbacks? Why use them? An example in Python - (2016-02-11)

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)
  [2836] Perl - the duplicate key problem explained, and solutions offered - (2010-06-28)
  [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)

H106 - PHP - Arrays
  [409] Functions and commands with dangerous names - (2005-08-11)
  [603] PHP - setting sort order with an associative array - (2006-02-13)
  [773] Breaking bread - (2006-06-22)
  [832] Displaying data at 5 items per line on a web page - (2006-08-14)
  [1116] PHP adding arrays / summing arrays - (2007-03-23)
  [1199] Testing for one of a list of values. - (2007-05-22)
  [1451] More PHP sample and demonstration programs - (2007-12-01)
  [1614] When an array is not an array - (2008-04-17)
  [2215] If nothing, make it nothing. - (2009-06-02)
  [2274] PHP preg functions - examples and comparision - (2009-07-08)
  [3004] Increment operators for counting - Perl, PHP, C and others - (2010-10-18)
  [3379] Sorting data the way YOU want it sorted - (2011-08-05)
  [3534] Learning to program in PHP - Regular Expression and Associative Array examples - (2011-12-01)
  [4068] Arrays in PHP - contain different and even mixed data types - (2013-04-24)
  [4072] Splitting the difference with PHP - (2013-04-27)
  [4244] Disambiguation - PHP List - (2014-03-07)


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Testing the robustness of our hotel and training systems - holiday and sickness times
Some other Articles
London to Calne, Corsham, Melksham, Bradford-on-Avon, Chippenham by public Transport
Downloading a report from the web for further local analysis
Upload Image, Store in database, redisplay in browser. PHP and MySQL
Testing the robustness of our hotel and training systems - holiday and sickness times
Looking up a value by key - associative arrays / Hashes / Dictionaries
Older picture.
Six languages in one file - an HTML++ web page
Predictions for the seagull population
Feeding the Grockles
Robust - testing the system
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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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