Week 4 10/24/2015
In the fourth week of Dev Bootcamp, we reviewed basic Ruby. For this entry, I want to discuss arrays and hashes. Please note that this is only a brief discussion. I do not cover the methods associated with arrays and hashes.
What are arrays and hashes?
An array is a list of objects indexed by the integers 0, 1, 2, ..., n-1 (called indices), where n is the number of objects in the array. For example:
empty_array = [] # This is an empty array
scores = [74, 81, 83, 75, 97, 98, 61, 59, 38, 100, 99, 82] # This is an array of integers - e.g. test scores
dist = [10.5, 11.2, 14.9, 12.7] # This is an array of floats - e.g. miles walked in one day
metal = ['Mayhem', 'Portal', '1349', 'Marduk', 'Yob'] # This is an array of strings - e.g. your favorite metal bands
stuff = [2.78, 'Pickle Juice', false, [2, 3, 5, 7, 11]] # This is an array of objects that are not all of the same type
The things to notice are that arrays can be empty and the members of an array do not need to be of the same type. In fact, in the array stuff we see that arrays can be objects inside of arrays. Members of an array are accessed through their indices. For example ...
scores[0] = 74 # Note that what most would consider to be the first element of scores is actually the first element
# is the 0th element of scores.
dist[1] = 11.2 # What most would consider to be the second element is actually the 1st element of dist.
empty_array[0] = nil # Since this array is empty there is nothing to return
metal[5] = nil # Remember that even though metal has five elements, these are labeled by the indices 0, 1, 2, 3, 4.
# There is no 5th element of metal.
Confusion about indices leads to a lot of bugs in programming.
A hash is a list of pairs of objects. The first object of the pair is the key. The second object is the value. This pair is denoted {key => value}. For example:
metal_hash = {} # This a new, empty hash which can be populated as follows:
metal_hash['Mayhem'] = 'Norway'
metal_hash['Portal'] = 'Australia'
metal_hash['1349'] = 'Norway'
metal_hash['Marduk'] = 'Sweden'
metal_hash['Yob'] = 'United States'
# The resulting hash is ...
{"Mayhemm"=>"Norway", "Portal"=>"Australia", "1349"=>"Norway", "Marduk"=>"Sweden", "Yob"=>"United States"}
# where the key is the band name and the value is the home country of the band.
One very important thing to notice is that the keys must be unique, however the values do not need to be unique. In the above example, each key appears exactly once, but the value Norway appears more than once. As with arrays, there are no resticitons on the types of objects that can be either keys or values.
When should I use an array and when should I use a hash?
Choosing an array or a hash depends on what you want to do with your data. If you want to be able to search and retrieve data, first you need to find the location of your data, then retrieve it. The amount of time it takes to search an array depends on the size of the array, whereas searching in a hash is indepenedent of the size of the hash. In more formal language, the complexity of searching in an array (of length n) is O(n) - aka linear time. The complexity of searching in a hash is O(1) - aka constant time. In other owrds, the bigger the array, the longer it takes to search, On the other hand, it takes the same amount of time to search in a hash, no matter how big the hash may be. So, for a lot of data-driven applications, hashes are probably prefereable.
However, some tasks are easier with arrays, than hases. For example, if you need to perform a task that requires iterating through a list of data, then it is easy to iterate through an array. Let us compare the array metal with the hash metal_hash.
metal.each do |band|
p band
end
# This gives us the ouput ...
"Mayhem"
"Portal"
"1349"
"Marduk"
"Yob"
# Iterating over a hash returns the output
metal_hash.each_pair { |key, value| puts "#{key} maps to #{value}"}
Mayhem maps to Norway
Portal maps to Australia
1349 maps to Norway
Marduk maps to Sweden
Yob maps to United States
# In this instance, we got lucky. Iterating over a hash returns the pairs in random order.
A rough guideline then is to use arrays for iteration and hashes for making your data searchable.