From: Matthew Moss <matthew.moss@...>
Date: 2008-07-19T01:03:08+09:00
Subject: [QUIZ] Records and Arrays (#170)

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## Records and Arrays (#170)

In a typical application, you might use what is commonly referred to
as an "array of records." Essentially, this is a single array
containing multiple objects that represent the same kind of structured
data, either as defined class or OpenStruct instances. Take, for
example, this bit of code that reads in some data regarding people:
their names, ages and favorite colors.

    require 'ostruct'
    data = []
    File.open("data.txt").each do |line|
      name, age, color = line.chomp.split(/,/)
      d = OpenStruct.new
      d.name = name
      d.age = age
      d.color = color
      data << d
    end

The variable `records` is considered an "array of records", since to
get any particular piece of data, you must first access it as an array
(to get a particular record), then access it as a record (to get a
particular field).

    > p data[3].name
    => "Matthew"

    > p data[3].age
    => 36

However, at times, it is more convenient to store data as a "record of
arrays". Instead of one array containing multiple records, you have
one object (i.e. record) containing multiple, parallel arrays. Access
to data then is done first as a record, then as an array.

    > p data.name[3]
    => "Matthew"

    > p data.age[3]
    => 36

This sort of arrangement is useful when you want to access series of
data at a time. For example, if I have a graphing component that takes
two arrays -- one for the domain axis, and another for the range axis
-- a "record of arrays" will make accessing that data trivial.

*Your task this week* is to define two functions that move data
between "array of records" storage and "record of arrays" storage.

    def aor_to_roa(arr)
      # This method accepts an array of records, and
      # should return a single record of arrays.
      #
      # This is your task!
    end

    def roa_to_aor(rec)
      # This method accepts a record of arrays, and
      # should return a single array of records.
      #
      # This is also your task!
    end

You should make this work with [OpenStruct][1]; do not limit yourself
to the example records shown above.

There are two, optional extra-credits for this week.

1. Extend these two functions to accept arbitrary classes.

2. As an alternative to these two functions, create an adapter class
that can wrap around "array of records" data to provide a "record of
arrays" interface without actually moving data about.


[1]: http://www.ruby-doc.org/stdlib/libdoc/ostruct/rdoc/classes/OpenStruct.html




-- 
Matthew Moss <matthew.moss@gmail.com>