Hashes

Question 1

What other data types can we use for hash keys? (apart from symbols)

Answer 1

String, array, integer, float or even another hash. They are bizarre and not really used though.

e.g. {“height” => “6 ft”} (string)
{3.56 => “whatever”} (float)

Question 2

Initiate a hash called “car” with 3 elements.

Answer 2

car = {ford: “focus,
honda: “civic”,
toyota: “corolla”
}

Question 3

What a hash is?

Answer 3

A hash is a data structure that stores items by associated keys.
Key-value pairs.

Question 4

Delete the second element from the hash below!

currency = {yen: “¥”, euro: “€”, dollar: “$”}

Answer 4

currency.delete(:euro)

Question 5

How do you access the third element of the hash below?

currency = {yen: “¥”, euro: “€”, dollar: “$”}

Answer 5

currency[:dollar]

Question 6

Merge the two hashes below to return a new merged hash!

currency = {yen: "¥", euro: "€", dollar: "$"}
ball= {basketball: "orange", tennis: "yellow"}

Answer 6

currency.merge!(ball)

=> {yen: “¥”, euro: “€”, dollar: “$”, basketball: “orange”, tennis: “yellow”}

Question 7

Iterate through the hash below and print the results!

currency = {yen: “¥”, euro: “€”, dollar: “$”}

Answer 7

currency.each do |key, val|
puts “The symbol of #{key} is #{val}.”
end

The symbol of yen is ¥.
The symbol of euro is €.
The symbol of dollar is $.

Question 8

When would you choose to use hash over array?

Answer 8

When data need to be associated with a specific label.

Question 9

When would you choose to use array over hash?

Answer 9

When order matters.

When I need a “stack” or a “queue” structure.

Question 10

Use a hash to accept optional parameters when creating methods!

Answer 10

def greeting(name, option = {})
if options.empty?
puts "hi my name is #{name}"
else
puts "hi my name is #{name} and I'm #{options[:age]}" + " years old and I live in #{options[:city]}."
end
end

greeting(“Tim”)
=>hi my name is Tim
greeting(“Tim”, age: 53, city: “Boston”)
=>hi my name is Tim and I’m 53 years old and live in Boston.

Question 11

What do you use the “has_key?” method for?

Answer 11

It allows you to check if a hash contains a specific key. It returns a boolean value.
e.g.
currency = {yen: “¥”, euro: “€”, dollar: “$”}
currency.has_key?(“euro”)
=>true

has_value? works exactly the same way.

Question 12

Which method allows you to pass a block and will return any key-value pairs that evaluate to true when ran through the block?

Answer 12

“select”

e.g.
currency = {yen: “¥”, euro: “€”, dollar: “$”}
currency.select {|k, v| k== “yen” || v == “$” }
=> {yen: “¥”, dollar: “$”}

Question 13

Which method allows you to pass a given key and it will return the value for that key if it exists?

Answer 13

“fetch”

e.g.
currency = {yen: “¥”, euro: “€”, dollar: “$”}
currency.fetch(“yen”)
=> “¥”

Question 14

What does the “to_a” method do?

Answer 14

It returns an array version of your hash when called.

e.g.
currency = {yen: “¥”, euro: “€”, dollar: “$”}
currency.to_a
=>[[:yen, “¥”], [:euro, “€”], [:dollar, “$”]]

It doesn’t mutate the caller.

Question 15

How can you retrieve all the keys or values out of a hash?

Answer 15

With “keys” and “values” methods.

e.g.
currency = {yen: "¥", euro: "€", dollar: "$"}
currency.keys
=>[:yen, :euro, :dollar]
currency.values
=>["¥", "€", "$"]

Question 16

Write a program that prints out groups of words that are anagrams.

words = [‘demo’, ‘none’, ‘tied’, ‘evil’, ‘dome’, ‘mode’, ‘live’, ‘fowl’, ‘veil’, ‘wolf’, ‘diet’, ‘vile’, ‘edit’, ‘tide’, ‘flow’, ‘neon’]

Answer 16

result = {}

words.each do |word|
  key = word.split('').sort.join
  if result.has_key?(key)
    result[key].push(word)
  else
    result[key] = [word]
  end
end

result.each_value do |v|
puts “——”
p v
end

Question 17

Use Ruby’s built-in select method to gather only immediate family members’ names into a new array.

family = { uncles: [“bob”, “joe”, “steve”],

        sisters: ["jane", "jill", "beth"],
        brothers: ["frank","rob","david"],
        aunts: ["mary","sally","susan"]

Answer 17

immediate_family = family.select do |k, v|
k == :sisters || k == :brothers
end

arr = immediate_family.values.flatten

p arr

Question 18

What is the difference between the array#each and hash#each methods?

Answer 18

With array#each there is one block parameter, but with hash#each there are two.

e. g.
numbers. each {|number| number * 2}

numbers.each {|key, val| val * 2}

Question 19

What is the differenae between Enumerator#map and Array#map?

Answer 19

Enumerable#map can accept two block parameters instead of one (key,val).
However, Enumerable#map doesn’t returns a hash when invoked on a hash, but it returns an array.

e.g.
numbers = {high:100, medium:50, low:10}
half_numbers = numbers.map do |key,val|
val / 2
end

p half_numbers
=> [50,25,5]

Question 20

How to nest the following two hashes?

drinks = {
pepsi: "cola",
juice: "orange",
tea: "lemon"
}
food = {
pizza: "salami",
burger: "McDonalds",
pasta: "spagetti"
}

Answer 20

mix = {
drinks: {pepsi: “cola”, juice: “orange”, tea: “lemon”},
food: {pizza: “salami”, burger: “McDonalds”,pasta: “spagetti”}
}

=> {
:drinks=> {:pepsi=> “cola”, :juice=> “orange”, :tea=> “lemon”},
:food=> {:pizza=> “salami”, :burger=> “McDonalds”,:pasta=> “spagetti”}
}

Question 21

Turn this hash into an array containing only Barney’s name and number!

flintstones = {“Fred” => 0, “Wilma” => 1, “Barney” => 2, “Betty” => 3, “BamBam” => 4, “Pebbles” => 5}

Answer 21

flintstones.assoc(“Barney”)
=>[“Barney”, 2]
Hash#assoc searches through the hash, comparing obj with the key. It returns the key-value pair (two elements array) or nil.

flintstones. slice(“Barney”).to_a.flatten
flintstones. keep_if{|key,val| key == “Barney”}.to_a.flatten