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)

 R107 - Collections (Arrays and Hashes) in Ruby
  [991] Adding a member to a Hash in Ruby - (2006-12-16)
  [2291] Collection objects (array and hash) in Ruby - (2009-07-16)
  [2606] Sorting arrays and hashes in Ruby - (2010-01-30)
  [2618] What are Ruby Symbols? - (2010-02-02)
  [2621] Ruby collections and strings - some new examples - (2010-02-03)
  [2976] Creating, extending, traversing and combining Ruby arrays - (2010-09-30)
  [3253] Is this number between? Does this list include? - Ruby - (2011-04-18)
  [3255] Process every member of an array, and sort an array - Ruby - (2011-04-21)
  [3257] All possible combinations from a list (Python) or array (Ruby) - (2011-04-23)
  [3435] Sorta sorting a hash, and what if an exception is NOT thrown - Ruby - (2011-09-12)
  [3757] Ruby - a teaching example showing many of the language features in short but useful program - (2012-06-09)
  [4499] Significant work - beyond helloworld in Ruby - (2015-05-27)
  [4502] Reading and parsing a JSON object in Ruby - (2015-06-01)

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)
  [3348] List slices in Python - 2 and 3 values forms, with an uplifting example - (2011-07-06)
  [3669] Stepping through a list (or an array) in reverse order - (2012-03-23)
  [3763] Spike solutions and refactoring - a Python example - (2012-06-13)
  [4027] Collections in Python - list tuple dict and string. - (2013-03-04)
  [4722] Embedding more complex code into a named block - (2016-11-04)

Back to A year of rail memories!

Previous and next or Horse's mouth home

Forward to Ruby - the second rung of learning the language

Template / design pattern for C++ constructor and accessors
Why are people using the TransWilts?
Conditionals, loops and methods in Ruby - a primer with simple examples
Ruby - the second rung of learning the language
Shuffling a list - Ruby and Python
A year of rail memories!
Changing what operators do on objects - a comparison across different programming languages
The changing face of Christmas
Christmas day in Melksham
A quiet evening in Melksham - awaiting Santa