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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.
Stepping through a list (or an array) in reverse order

If you want to iterate through a list in reverse order, you can do so by stepping over (iterating through) the indexes of the list in reverse order. So in Python, for example, you would use range (or xrange for a potentially long list) with an increment of -1, giving the final index number as the start point and the beginning as the end:

  have = [20,40,60]
  for p in range(len(have)-1,-1,-1):
    print p, have[p]


Remember that Python's range starts at the first value given, but stops short of the terminating value. So you need to stop at "-1" to ensure that you process element number 0

If you're happy to have the list altered, many languages include a reverse function. Again in Python:

  have.reverse()
  for val in have:
    print val


Or you could use a "pop" fucntion to keep removing and returning the final item until the list is empty. Let's see that in Perl for a change:

  @numbers = (4,8,7,34);
  while ($val = pop @numbers) {
    print "$val\n";
    }


Full Python example is [here]. These techniques also apply to other languages with "ordered collections" - whether they're known as lists, arrays, tuples, tables, or vectors.

(written 2012-03-23, updated 2012-03-24)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
Y104 - Python - Lists and Tuples
  [383] Overloading of operators on standard objects in Python - (2005-07-19)
  [657] The ternary operator in Python - (2006-03-25)
  [899] Python - extend v append on a list - (2006-10-20)
  [955] Python collections - mutable and imutable - (2006-11-29)
  [1220] for loop - how it works (Perl, PHP, Java, C, etc) - (2007-06-06)
  [1641] Tektronix 4010 series / Python Tuples - (2008-05-13)
  [1789] Looking for a value in a list - Python - (2008-09-08)
  [2280] Creating and iterating through Python lists - (2009-07-12)
  [2284] Strings as collections in Python - (2009-07-12)
  [2368] Python - fresh examples of all the fundamentals - (2009-08-20)
  [2719] Traffic lights in Python - (2010-04-13)
  [2996] Copying - duplicating data, or just adding a name? Perl and Python compared - (2010-10-12)
  [3118] Arrays of arrays - or 2D arrays. How to program tables. - (2011-01-02)
  [3181] Beware - a=a+b and a+=b are different - Python - (2011-02-23)
  [3257] All possible combinations from a list (Python) or array (Ruby) - (2011-04-23)
  [3348] List slices in Python - 2 and 3 values forms, with an uplifting example - (2011-07-06)
  [3763] Spike solutions and refactoring - a Python example - (2012-06-13)
  [4027] Collections in Python - list tuple dict and string. - (2013-03-04)
  [4368] Shuffling a list - Ruby and Python - (2014-12-28)
  [4722] Embedding more complex code into a named block - (2016-11-04)

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)
  [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)
  [3870] Writing more maintainable Perl - naming fields from your data records - (2012-09-25)
  [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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Some other Articles
Object oriented or structured - a comparison in Python. Also writing clean regular expressions
Melksham Train Service - please support the TransWilts case for regular trains
Future train services in Wiltshire - please write in to support a better service
Reading Google Analytics results, based on the relative populations of countries
Stepping through a list (or an array) in reverse order
Kings Cross - new concourse - between Python in Cambridge and Objective C in London
A modern area of Cambridge - some thoughts provoked?
Makefile variables - defined internally, from the command line and from the environment
Will will smile?
Error checking in a Python program - making your program robust via exceptions
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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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