Week 5 11/01/2015
In the fifth week of Dev Bootcamp, we looked a bit more in depth into Ruby. In particular, we had our first taste of classes. For this entry, I want to discuss how the Enumerable module can enrich your classes. As usual, this is not an exhaustive survey, but rather a taste of what you can do.
The Enumerable module
When we first learn to program (in any language), we learn to write code that repeats/iterates a task - usually by means of a for-loop or some other control structure. For example,
for i in 1..5
p "Splat! "
end
# This will output
"Splat! "
"Splat! "
"Splat! "
"Splat! "
"Splat!
So far, so good, but what if we wanted to print out the contents of an array. For example,
metal = ["Portal", "1349", "Yob", "Marduk"]
for i in 1..metal.length
p metal[i-1]
end
# This will output
"Portal"
"1349"
"Yob"
"Marduk"
This example leads to the same output, a list of things printed one at a time, but notice that the code is slightly more complicated. We have to make sure we have gotten the array's indices correct. However, in Ruby, we learn that arrays have a special method each that allows us to iterate without worrying about the indices.
metal = ["Portal", "1349", "Yob", "Marduk"]
metal.each do |band|
p band
end
# This will output
"Portal"
"1349"
"Yob"
"Marduk"
We get the same output, but at greater simplicity. This is because the class Array has the each method. However, there are other things we can do to an array. For example, we can apply any? to see if an object is an element of the array. These methods are not unique to arrays, but can be given to other classes by including the Enumerable module in the class definition and then defining an instance method called each.
class Class_Name {
include Enumerable
def each
# put code here
end
# define the rest of your class
}
By including the Enumerable module in our class, we gain access to a wide variety of methods, such as the searching and sorting methods that we see for arrays. In other languages, we would have to construct our own object-specific methods. Let's look at a particular method.
Example Enumerable#group_by
The group_by method takes a block of code and returns a hash, where the block is executed for each object. For each unique block return value, the returned hash gets a key; the value for that key is an array of all the objects for which the block returned that value. For example,
metal = ["Portal", "1349", "Yob", "Marduk", "Mayhem", "Cultes des Ghoules", "Blut Aus Nord", "Tsjuder", "Dodheimsgard"]
# We can ask if any bands in the above array have the letter 'M'
metal.group_by {|band| band =~/M/}
# The resulting hash is
{nil=>["Portal", "1349", "Yob", "Cultes des Ghoules", "Blut Aus Nord", "Tsjuder", "Dodheimsgard"],
0=>["Marduk", "Mayhem"]}
# where the first array is consists of those bans that do not have an 'M' (i.e. band =~/M/ is false),
# while the second array consists of # those
# that do. I need to figure out how to specify a meaningful key. In this case, 'nil' means 'false' and '0' means 'true'.
# Another example - sorry, I tried to find something other than an array to apply group_by to.
# I couldn't make it work for hashes (yet!)
(1..16).group_by { |i| i%7 }
# The output is hash of integer keys (between 0 <= i <= 6 - i.e. possible remainders upon division by 7)
# whose values are arrays of integers with remainder i.
{1=>[1, 8, 15], 2=>[2, 9, 16], 3=>[3, 10], 4=>[4, 11], 5=>[5, 12], 6=>[6, 13], 0=>[7, 14]}
Once we start constructing our own classes, then we should be able to figure out good implementations of the methods offered to us by the Enumerable module.