Collections (Arrays and Hashes) in Ruby

Ruby module R107

Exercises, examples and other material relating to training module R107. This topic is presented on public courses Learning to program in Ruby, Ruby Programming

Background

If you want to hold many values in a Ruby object, you'll use and array or a hash - two powerful structures each of which has a mass of methods to make them flexible, and a great deal of syntactic sugar to make them easy to use.

Articles and tips on this subject		updated
4502	Reading and parsing a JSON object in Ruby Here's a series of three new examples, building up from one another, showing the reading of a JSON feed from a remote web site in Ruby, and the analysis and presentation of that data. The first example - [here] - shows the loading of extra modules to handle reading from the web and the JSON format, ...	2015-06-02
4499	Significant work - beyond helloworld in Ruby A little program can do a LOT of work! Scenario - I have a web server log file of some 50 Mbytes (the data from one particular day on our server) and the secon field in each line tells me which of our hosted web sites was being visited. The question I was asked - "how many hits on each host?". Solution ...	2015-05-28
4368	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 ...	2014-12-30
3757	Ruby - a teaching example showing many of the language features in short but useful program Although the main publicity and driver for the Ruby language has been the Rails web framework (see previous article here), it's an excellent data manipulation language too - with many of the short and efficient coding techniques that you would have available to you in Perl, yet additionally with an object ...	2012-06-16 (longest)
3435	Sorta sorting a hash, and what if an exception is NOT thrown - Ruby Some gems from last week ... 1. Ruby's begin and rescue clauses allow you to catch anything exceptional that could derail your program. But did you know that you can also add an else clause to be run only in the event of there being no exception? There's an example from last week's Ruby course which ...	2011-09-12
3257	All possible combinations from a list (Python) or array (Ruby) If there are eight of us at a meeting, how many possibilities are there for a pair of people to stongly disagree? The answer turns out to be 28. And if you look at subgoups of 3, looking for everyone to have the same opinion within the subgroup, the answer is many more. If there are eight stations ...	2011-04-23
3255	Process every member of an array, and sort an array - Ruby If you're wanting to process every member of an array, you've a choice ... you can write a loop to step through the key of each member, you can write a loop to step through each member itself, or you can call a method which operates on the array as a whole (i.e. the loop is hidden within a method. ...	2011-04-21
3253	Is this number between? Does this list include? - Ruby There's often a requirement in a program to see if one value is between two others, and in most languages you'll write that as as double condition: if (n >= 5 && n <= 8) printf("Yesssss!\n"); in C or C++, for example. Some languages give you further options / methods you ...	2011-04-19
2976	Creating, extending, traversing and combining Ruby arrays Ruby shares many of its eclecticism's with Perl - lots of ways of doing the same thing, though it doesn't go quite so far (some would say so far beyond the reasonable) as Perl does. So when you come to create or manipulate an array (really an ordered list) there's a big choice. indian = ...	2010-10-01
2621	Ruby collections and strings - some new examples From yesterday's Ruby course - a whole set of new examples on Ruby collections - arrays (which are ordered lists) and hashes (which are unordered). In other words, you look somethng up in an array by its position number, whereas you look something up in a hash based on its unique key, and the way it's ...	2010-02-03
2618	What are Ruby Symbols? Ruby Symbols are names that follow a : character. They're a quick way of getting at data, where (for example) you have a fixed key or name in a hash, and you'll find them commonly used in many definitions such as in Ruby on Rails. I've put a short example [here] ...	2010-02-02 (short)
2606	Sorting arrays and hashes in Ruby In Ruby, you can use the sort method on an array to sort that array - but you cannot sort a hash. That's because of the techniques used within a hash to make it very fast to look up individual elements. So what do you do if you want to sort something like this: ? stuff = {"Steve" => "Kent", "Graham" ...	2010-01-30
2291	Collection objects (array and hash) in Ruby Ruby has two collection objects - Arrays (where you access elements based on their numeric position) and hashes (where you can access elements based on a key, rarely numeric). That's similar in all but name to many other languages, although the names may be different (arrays or lists; hashes or dictionaries ...	2009-07-16
991	Adding a member to a Hash in Ruby In Ruby, you must initialise your variables - in other words, you cannot use the content of a variable that doesn't exist and have the language assume it will be 0, as happened with Perl. So if - for example - you're using a Hash to keep tabs of a number of counters, you can't just +=1 members and have ...	2006-12-21

Examples from our training material

a1.rb	First Array
a2.rb	Basic Array manipulation
a3.rb	methods on arrays
amaze4	Array of counters - log file statistics
arar.rb	Arrays of arrays
bone	Combining arrays
d3_4	Arrays, and iterating through them
d3_5	Array methods such as grep
d3_6	Array of arrays
d3_7	Setting up and using a hash
d3_8	Simple but practical hash example
d3_9	Using a hash to count web server accesses
ddd	Symbols v strings
favr	Defining, adding to, iterating through an array
h1.rb	First hash in Ruby
h2.rb	First hash in Ruby
kvp	Sorting in Ruby
logan	Read an Apache httpd access log file and count statuses
lunch	array sorting and iterating
mar	Combining arrays - element by element "and" and "or"
others	Symbols
pophand.rb	Read, interpret a data file
pw.rb	%w to set up an array, and compact
pwc.rb	Copying an object in Ruby
randimages	Randomiser of images
rrs	Sorting the keys of a hash
swapper	Swapping two variables
tessa	all possible combinations from an array
tim	A Hash of arrays in ruby
toppers.xx	Hash to count server accesses

Background information

Some modules are available for download as a sample of our material or under an Open Training Notes License for free download from [here].

Topics covered in this module

What is a collection?
Arrays and hashes.
Constructing an array. The %w shortcut.
Nesting arrays.
Hash keys, iterators, etc.

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