Skip to content

module4.md

Latest commit

 

History

History
140 lines (95 loc) · 3.14 KB

module4.md

File metadata and controls

140 lines (95 loc) · 3.14 KB

Learning Goals

  1. Understand the use cases for arrays
  2. Become familiar with the most widely used array methods

Arrays

Arrays store sequences of objects, separated by commas. Why is that useful? Well, this data structure provides us a handy way to store lists of values that are related in a single variable.

cool_things = ["Racecars", "Lasers", "Aeroplanes"]
four_primes = [2, 3, 5, 7]
an_empty_array = []

Indexing

Arrays wouldn't be very useful if we couldn't get things out of them. So how are we going to access a specific value within the array?

The answer: indices.

Think of an index as the position of a value in the list. Indices ALWAYS start at 0 in programming. So if you want the first value in the list (reading from left to right), you'll access the Oth position. The second value from the first and so on.

You can also change a specific position's value, so in that way, an array is very much like a group of ordered variables.

Note:

Tell them it's normal to think that's odd. Maybe tell them why that's the case. Reassure them it'll come naturally with practice.

cool_things = ["Racecars", "Lasers", "Aeroplanes"]

puts(cool_things[0])
puts(cool_things[1])
puts(cool_things[2])

cool_things[2] = "Airplanes"
puts cool_things[2]

Push + Pop

Although indexing into an array allows us to access and manipulate an element in a specific position of the array, we often need to just add or remove from the end of the array.

favorite_animals = []

favorite_animals.push("dog")

puts favorite_animals

favorite_animals.push("cat")

puts favorite_animals

favorite_animals.pop

puts favorite_animals

Unshift + Shift

These two functions are the same as push and pop, respectively, but will add or remove from the beginning of the array. Let's take a moment to try this out again in the REPL environment.

Looping through Array

# You can write an array across many lines like this.
presidents = [
  "George Washington",
  "John Adams",
  "Thomas Jefferson",
  "James Madison",
  "James Monroe",
  "John Quincy Adams"
]

# Prints 6, the number of items in the array
puts(presidents.length)

# Print each one of the presidents in the array.
idx = 0
while idx < presidents.length
  puts(presidents[idx])
  idx = idx + 1
end

puts("PRESIDENTS LOOP COMPLETED!")

Note: point out the array length method as intro to methods on data structures this is a common use case for using arrays (iterating through lists)

CODE

  1. Write a program that creates an array and prints a list of cities you have lived in. Keep in mind puts array will print the list element by element.

  2. You are given the array [nil,nil,nil,nil,nil]. Write a program that loops through the array (from beginning to end) and prints the phrase "I love App Academy" every time. You should see the phrase five times.

  3. You are given the array [1,2,3,4,5]. You are asked to loop through the array and replace each position with the product of the number at that position and the index. Print out the new array.

Review

  • Array Indexing
  • Push / Pop
  • Unshift / Shift
  • Looping through an Array