Data Structure Review - Full Lecture

Motivation / Why Should You Care?

Sometimes it takes a bit of repetition for a concept to click. Coding is like any new skill you might pick up -- the more you do it, the better you get at it. Last unit we covered arrays and hashes. Let's review what we learned.

Lesson Plan

Difference between an array and a hash -
- Think of an array as a list of items. We access information in an array from an index, starting with 0.
- A hash is more specific way of labeling information. A hash is made up of key-value pairs. Keys have to be unique. Access information in a hash through the key.

Syntax for arrays and hashes *Array:

  snax = ["chips", "oreos", "celery"]
  snax[0] #returns "chips"
  snax.delete("celery") #removes celery
  snax << "popcorn" #adds popcorn to the end of the array)]
  snax.push("trail mix") #adds trail mix to the end of the array
  snax.last #returns trail mix
  snax.first #returns chips
  snax[3] = "Cotton Candy" #replaces trail mix with cotton candy

Hashes

  todo = {
    :homework => "Algebra problems",
    :chores => "make my bed",
    :shopping => "buy new shoes"
  }

  todo[:homework] #returns Algebra Problems
  todo.delete(:homework) #removes that key-value pair from the hash
  todo[:travel] = "Go to Spain" #adds that key value pair to the hash
  todo[:chores] = "laundry" #changes the value of the :chores key to laundry

Iterating over arrays and hashes

Arrays:

  snax = ["chips", "oreos", "celery"]
  snax.each do |snack|
    "I love #{snack}"
  end

Hashes:

  todo = {
    :homework => "Algebra problems",
    :chores => "make my bed",
    :shopping => "buy new shoes"
  }

  todo.each do |category, task|
    "I have so much to do! #{category}: #{task}"
  end

  todo.each_key do |category|
    "I have to do #{category}"
  end

  todo.each_value do |task|
    "I have to #{task}!!!"
  end

Create array with the different ages in the class. Then calculate:

The sum of all the ages

  total = 0
  ages = [15, 16, 17, 19]
  ages.each do |age|
    total += age
  end
  total

The average age

  ages = [15, 16, 17, 19]
  sum = 0.0
  ages.each do |age|
    sum += age
  end
  puts sum/ages.length

The oldest age

  ages = [15, 16, 17, 19]
  biggest_age = ages[0]
  ages.each do |age|
    if biggest_age < age
      biggest_age = age
    end
  end
  puts biggest_age

Turn ages array into a hash with students' names as keys. Then calcuate:

The sum of all the ages

  ages = { :beth => 15,:sarah => 16,:jeff => 17,:carrie => 19}
  array = ages.values
  sum = 0
  array.each do |number|
    sum += number
  end
  puts sum

The average age

  ages = { :beth => 15,:sarah => 16,:jeff => 17,:carrie => 19}
  array = ages.values
  sum = 0.0
  ages.each_value do |age|
    sum += age
  end
  puts sum/array.length

The youngest

  youngest_student = ages.sort_by {|keys,values| values}[0]

Organize the students alphabetically.

  alphabetical_student = ages.keys.sort

Write a method that takes in any array of numbers as an argument and gives back the average?

def find_average(array)
  sum = 0.0
  array.each do |number|
    sum += number
  end
  puts sum / array.length
end

Nested hashes and arrays
- How do we pull individual items out if this is the case?

school_subjects = [
  ["algebra", "geometry", "trigonometry"],
  ["physics", "biology", "chemistry"],
  ["European history", "world civilizations", "U.S. history"]
]

Algebra"

school_subjects.first.first
school_subjects[0][0]

World Civilizations:

school_subjects[2][1]

Convert school_subjects to a hash.

 school_subjects = {
  :math => ["algebra", "geometry", "trigonometry"],
  :science => ["physics", "biology", "chemistry"],
  :social_studies => ["European history", "world civilizations", "U.S. history"]
}

Now how do I access "biology"?

  school_subjects[science][1]

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Data Structure Review - Full Lecture

Motivation / Why Should You Care?

Lesson Plan

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