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Shuffling a list - Ruby and Python

Shuffling a list / array is quite a common requirement. The best way to do it is to take a list of all the items to be shuffled, and to choose one at random as the first member of the output list / array. Then move the last item on the incoming array into the gap left, and choose another from the now shorter incoming array to add to the shuffled array. Keep going until the incoming list is empty.

Newcomers to coding may simply start by choosing incoming elements at random, but without the moving up of the remaining elements (housekeeping) they rapidly find themselves selecting some of the incoming items multiple times while missing out others, or in a loop of select and reject as they keep sampling and saying "no, we've had that one already".

A number of years ago, I wrote a shuffling program in Python as a demonstration of list manipulation - and also to help my son Chris and his (then) Girlfriend Delene play the tracks on their computer in a random order. The code is [here]. If you want to tell a story against someone, tell it against yourself - and so I do on courses, for I should have looked closer at Python as it has a built in shuffle method and much of my code was needless - there's a much shorter answer [here].

Years later, I had 72 images, sequentially numbered, that I wanted to mix up and rename, and as the course I'm starting the new year with is Ruby Programmng, I've elected to write the code in that language. Code [here].

The langauges turn out to be remarkably similar

Python shuffle code:

  shuffle = []
  while len(basehand):
    posn = int(random()*len(basehand))
    shuffle.append(basehand[posn])
    basehand[posn] = basehand[-1]
    basehand.pop()


Ruby shuffle code:

  shuffled = []
  while pix.length > 0 do
    position = rand() * pix.length
    shuffled.push(pix[position])
    pix[position] = pix[-1]
    pix.pop()
  end

(written 2014-12-28, updated 2014-12-30)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
Y104 - Python - Lists and Tuples
  [4722] Embedding more complex code into a named block - (2016-11-04)
  [4027] Collections in Python - list tuple dict and string. - (2013-03-04)
  [3763] Spike solutions and refactoring - a Python example - (2012-06-13)
  [3669] Stepping through a list (or an array) in reverse order - (2012-03-23)
  [3348] List slices in Python - 2 and 3 values forms, with an uplifting example - (2011-07-06)
  [3257] All possible combinations from a list (Python) or array (Ruby) - (2011-04-23)
  [3181] Beware - a=a+b and a+=b are different - Python - (2011-02-23)
  [3118] Arrays of arrays - or 2D arrays. How to program tables. - (2011-01-02)
  [2996] Copying - duplicating data, or just adding a name? Perl and Python compared - (2010-10-12)
  [2719] Traffic lights in Python - (2010-04-13)
  [2368] Python - fresh examples of all the fundamentals - (2009-08-20)
  [2284] Strings as collections in Python - (2009-07-12)
  [2280] Creating and iterating through Python lists - (2009-07-12)
  [1789] Looking for a value in a list - Python - (2008-09-08)
  [1641] Tektronix 4010 series / Python Tuples - (2008-05-13)
  [1220] for loop - how it works (Perl, PHP, Java, C, etc) - (2007-06-06)
  [955] Python collections - mutable and imutable - (2006-11-29)
  [899] Python - extend v append on a list - (2006-10-20)
  [657] The ternary operator in Python - (2006-03-25)
  [383] Overloading of operators on standard objects in Python - (2005-07-19)

R107 - Collections (Arrays and Hashes) in Ruby
  [4502] Reading and parsing a JSON object in Ruby - (2015-06-01)
  [4499] Significant work - beyond helloworld in Ruby - (2015-05-27)
  [3757] Ruby - a teaching example showing many of the language features in short but useful program - (2012-06-09)
  [3435] Sorta sorting a hash, and what if an exception is NOT thrown - Ruby - (2011-09-12)
  [3255] Process every member of an array, and sort an array - Ruby - (2011-04-21)
  [3253] Is this number between? Does this list include? - Ruby - (2011-04-18)
  [2976] Creating, extending, traversing and combining Ruby arrays - (2010-09-30)
  [2621] Ruby collections and strings - some new examples - (2010-02-03)
  [2618] What are Ruby Symbols? - (2010-02-02)
  [2606] Sorting arrays and hashes in Ruby - (2010-01-30)
  [2291] Collection objects (array and hash) in Ruby - (2009-07-16)
  [991] Adding a member to a Hash in Ruby - (2006-12-16)


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