This lesson will give a deeper dive on how to create, manipulate, and retrieve data from arrays.
- Describe how arrays are useful in storing multiple pieces of information.
- Create and populate an array.
- Add items to an array using different methods.
- Remove items from an array using different methods.
- Operate on an array using the sort, reverse, and include methods.
- Identify elements in an array based on their index number.
- Retrieve items from an array.
- Reference the Ruby documentation on arrays.
So far, we've used variables to store information. For example, I could create a variable called my_name and set it equal to my name: my_name = "Severus Snape". However, variables only allow us to store one piece of information at a time.
What if my boss, Headmaster Dumbledore, asks me to deliver the names of all of my students? I could create a bunch of variables like this:
student1 = "Harry Potter"
student2 = "Ron Weasley"
student3 = "Hermione Granger"
student4 = "Draco Malfoy"
etc...Then, I could write a program that passes around these variables one at a time. This seems messy though. I could easily forget about a student, for example. Or need to create a new student and then have to hunt through my program for every place I ever passed around all of these individual variables.
If this was real life, Professor Snape would probably just write down all the students in list form and hand that list to Dumbledore. Well, in Ruby, we can do the same thing using an array.
An array is like a list but in code form. It is a way for your program to store pieces of data as a collection. Arrays can contain any data types in any combination––strings, integers, other arrays, hashes, etc.
Arrays are declared by listing variable names or literals separated by commas (,) and wrapped in square brackets [``]. To save our four student from above into an array, we write that in our code like this:
students = ["Harry Potter", "Ron Weasley", "Hermione Granger", "Draco Malfoy"]There are a few different ways to make a new array. You can use the literal constructor or the class constructor.
my_array = []my_array = Array.new
# └── []Advanced: *A class is like a template, or blueprint, for creating objects in Ruby. An "object" is simply a bundle of information and behaviors. For example, a string is an object, because it contains information (i.e. the text inside the " ") and because it has behaviors––it can do things/have things done to it. For example:
"hi".reverse
=> "ih"There is an Array class that serves as the blueprint for every array that you will make. This means that all arrays are capable of certain shared behaviors and are responsive to certain methods.
To create a new array object from the Array class, you can call .new on Array––the name of the class. This creates a brand new, empty array. Don't worry about understanding objects and classes, or the .new method, just yet. They are all part of something called Object Oriented Programming, which is a big topic. We'll be building up to it through this and the next few units.*
To make an array that isn't empty, you can separate each item, known as an element, by a , ("comma") and wrap all the elements inside [``] ("square brackets").
puppies = ["bulldog", "terrier", "poodle"]
# └── ["bulldog", "terrier", "poodle"]
random_numbers = [ 2, 5, 6, 8, 30050]
# └── [ 2, 5, 6, 8, 30050]
mixed = ["this", "array", 7, 20, "has", 45, "integers", "&", "strings", 309]
# └── ["this", "array", 7, 20, "has", 45, "integers", "&", "strings", 309]It is possible to create an array that contains disparate data types, but that is generally discouraged. It's best to keep your arrays populated with only one kind of element.
If an array is a storage container for a list of data, then we can imagine adding and removing individual items from it. There are several ways to accomplish either.
The shovel method employs the "shovel" operator (<<) and allows you to add ("shovel") items onto the end of an array:
famous_cats = ["lil' bub", "grumpy cat", "Maru"]
famous_cats << "nala cat"
puts famous_cats.inspect
=> ["lil' bub", "grumpy cat", "Maru", "nala cat"]Note: The .inspect method returns a string containing a human-readable representation of an object. In this case, the list of the strings held in the array.
The shovel method (<<) is the preferred syntax for adding elements to an array, however you might see other methods used in examples online:
Calling .push on an array with an argument of the element you wish to add to that array, will also add that element to the end of the array. It has the same effect as the shovel method explained above:
famous_cats = ["lil' bub", "grumpy cat", "Maru"]
famous_cats.push("nala cat")
puts famous_cats.inspect
=> ["lil' bub", "grumpy cat", "Maru", "nala cat"]To add an element to the front of an array, you can call the .unshift method on it with an argument of the element you wish to add:
famous_cats = ["lil' bub", "grumpy cat", "Maru"]
famous_cats.unshift("nala cat")
puts famous_cats.inspect
=> ["nala cat", "lil' bub", "grumpy cat", "Maru"]Calling .pop on an array will remove the last item from the end of the array. The .pop method will also supply the removed element as its return:
famous_cats = ["lil' bub", "grumpy cat", "Maru"]
maru_cat = famous_cats.pop
puts famous_cats.inspect
=> ["lil' bub", "grumpy cat"]
puts maru_cat
=> MaruCalling .shift on an array will remove the first item from the front of the array. The .shift method will also supply the removed element as a return:
famous_cats = ["lil' bub", "grumpy cat", "Maru"]
lil_bub = famous_cats.shift
puts famous_cats.inspect
=> ["grumpy cat", "Maru"]
puts lil_bub
=> lil' bubThere are a number of other methods available for manipulating arrays. You can learn more about them here, but we'll look at just a few examples together.
This method rearranges the contents of the array by, well, sorting them. For strings, this means alphabetically, for numerical values, this means from smallest number to highest number.
Advanced: The .sort method works by implicitly comparing elements with the "spaceship" operator <=> (because it looks like a flying saucer, swoosh!) and moving them accordingly.
famous_cats = ["lil' bub", "grumpy cat", "Maru"]
famous_cats.sort
=> ["grumpy cat", "lil' bub", "Maru"]This method reverses an array.
famous_wizards = ["Dumbledore", "Gandalf", "Merlin"]
famous_wizards.reverse
=> ["Merlin", "Gandalf", "Dumbledore"] This method will return a boolean of whether or not the array contains (or includes) the element submitted to it inside the parentheses:
famous_cats = ["lil' bub", "grumpy cat", "Maru"]
famous_cats.include?("Garfield")
=> false
famous_cats.include?("Maru")
=> trueWhen you write out a list on a notepad, you typically write each item on its own line. Whether or not the list is explicitly numbered, the list has a numerology to it based on the sequence of the lines that the items are listed upon.
Just like the items in our notepad lists, elements in an array are associated with a number that represents their order. In programming, this number is called an index. While humans typically start their lists at "1.", arrays begin their indexes at 0 (zero). So, the first item in array will always be "at index 0". If we have an array of famous (fictional) cats:
famous_cats = ["Cheshire Cat", "Puss in Boots", "Garfield"]The "Cheshire Cat" is at index 0 in the array, "Puss in Boots" is a index 1, and "Garfield" is at index 2. Indexes will always be one less than the count.
To access one of these items in the famous_cats array, we can type the name of the array immediately followed by the relevant index number wrapped in square brackets ([]).
famous_cats = ["Cheshire Cat", "Puss in Boots", "Garfield"]
famous_cats[1]
# └── "Puss in Boots"
famous_cats[0]
# └── "Cheshire Cat"
famous_cats[2]
# └── "Garfield"
famous_cats[20]
# └── nilTo discover the index number of an element within an array, we can use the .index() method. Calling .index() on an array with an argument inside the parentheses will return the first index number of an element matching that argument. If no elements match the argument, then this method will return nil.
famous_cats = ["Cheshire Cat", "Puss in Boots", "Garfield"]
famous_cats.index("Puss in Boots")
# └── 1
famous_cats.index("Maru")
# └── nilThis is not an operation you will perform very often. Arrays are used to store data and usually you will use the index number of an item to access it, not the other way around.