# RubyKin

## Answers to Practice Problems

Chapter 2 - Numbers

1) 2 + 3 + 5 => 10

2) 10 - 3 => 7

3) 9 / 3 => 3

4) 4 * 2 => 8

5) 4 ** 2 => 16

6) What is the result of 11 % 5 ? => 1

5 goes into 11 only twice, for a total of 10. 11 minus 10 leaves a remainder of 1

7) What is 14 % 3 ? => 2

8) A wizard carries two numbers (one even and one odd) in each hand. He won't open his hands to show you, but he will let you use modulo. Your task is to find out which hand holds an even number, and which holds the odd.

If we mark each number as "X" we can use modulo: if x % 2 == 0 is true, it’s an even number. if x % 2 == 0 is false, it’s an odd number.

Chapter 3 - Strings

1) puts "What is the result" + " of " + "this operation?" What is the return result of this operation? => nil

The above puts method concatenated the strings, putting the result to the screen and returning nil.

2) "This string minus" - "That string"

NoMethodError: undefined method -' for
"This string minus":String


There is no '-' method for Strings, so the operation cannot be done. Remember, - is for numbers!

3) "1234.55".to_i => 1234

4) "1234.55".to_f => 1234.55

5) "Not a number".to_i => 0

6) puts "1\n2\n3\n"

  1
2
3
=> nil


7) Why might it be useful that the 'to integer' (to_i) method return zero on strings that can’t be represented as numbers?

"to_i" can still be called on a string without numbers, and so it returns a zero, noting the successful call (no error) and the value zero, which could represent zero numbers in the string.

8) What do you think the length method does? "Count".length => 5 Counts the number of letters or items in the string.

9) How about the split method? "Count".split("") => ["C", "o", "u", "n", "t"]

Calling the 'split' method with no space ("") as the argument causes the string to be split at each character and placed inside of an array.

10) What do you think slice does? "Count".slice(2) => "u"

Slice grabs the character from the index argument passed in, which in this case is an index of 2. The letter "u" is at index 2 in our string "Count". string: ["C", "o", "u", "n", "t"] indexes: [ 0 , 1 , 2 , 3 , 4 ]

Chapter 4 - Variables

1) Store the value of 54 / 3 into the variable x. What is the value of x? x = 54 / 3 => 18

2) Give the value of x (from problem 1) to y, a new variable. Now make x equal to itself divided by 3. What’s the value of x? What’s the value of y?

  y = x
=> 18  # y is assigned the x value, 18
x = x / 3   # or written as x /= 3
=> 6   # x is now the value of 18 / 3


3) What if we performed some math on our variables? If we set x to 12, what’s x divided by 3?

  x = 12
x / 3
=> 4


4) What’s the value of x now? => 12 When we divided X by 3, we didn't store the result, so X's value does not change since it was assigned 12.

Chapter 5 - If and Else

1) Is 3 > 5 ? => false

2) 3 < 5 => true

3) 5 == 5 => true

4) 10 >= 10 => true

5) 10 <= 12 => true

6) 10 != 10 => false

7) 10.object_id == 10.object_id => true

What about strings? 8) "dog" == "cat" => false

9) "cat" == "cat" => true

10)

  "cat".object_id == "cat".object_id
=> false
# Numbers have object id's that don't change.


Everytime a string is created, however, a new object_id is created for that string.

Chapter 6 - Loops

1) What’s the current value of the time variable? => 8

Even though our last value outputted was 7, the next code block (current_time = current_time + 1) is executed, and so our variable increments by 1 to equal 8.

Chapter 7 - Collections

1) Create a hash for the Wizard’s magic bag. Inside the bag, we’ll put 3 frogs, 5 herbs, and 10 scrolls. Assign the hash to the variable "bag". What does your hash look like?

  bag = {
"frogs" => 3,
"herbs" => 5,
"scrolls" => 10
}


2) Remember, hashes consist of key value pairs of any data, not just numbers. Let’s add a wizard’s spell and its result (which the wizard can never seem to remember). Add a spell to our wizard’s bag with the key: "shazam" and the value "turns subject into a frog". Now what's in our bag?

  bag["shazam"] = "turns subject into a frog"
bag
=> {"frogs" => 3,
"herbs" => 5,
"scrolls" => 10,
"shazam" => "turns subject into a frog"}


3) Our wizard has a change of heart and decides he never wants to turn anyone into a frog. How can we remove the spell "shazam" from our wizard bag?

  bag.delete("shazam")
=> "turns subject into a frog"


4) Our wizard has recently acquired 3 different types of potions. 4 orange potions, 5 blue potions and 7 red potions. How might we add another hash of potions to our bag? Hint: remember that we can have collections within a collection.

  bag["potions"] = {
"orange" => 4,
"blue" => 5,
"red" => 10
}

bag
=> { "frogs"=>3, "herbs"=>5,
"scrolls"=>10,
"potions"=> {
"orange"=>3,
"blue"=>5,
"red"=>10
}
}


5) Now that our bag has four keys (frogs, herbs, scrolls and potions), we can use these keys to access our data. In order to make a new spell, our wizard needs 2 frogs, 3 herbs, 1 scroll and 2 blue potions. How can we remove these items from our hash? Hint: We can set the value of a key item to itself, minus how many items removed.

  bag["frogs"] = bag["frogs"] - 2
=> 1
bag["herbs"] = bag["herbs"] - 3
=> 2
bag["scrolls"] = bag["scrolls"] - 1
=> 9
bag["potions"]["blue"] = bag["potions"]["blue"] - 2
=> 3
bag
=> {
"frogs" => 1,
"herbs" => 2,
"scrolls" => 9,
"potions" => {
"orange" => 4,
"blue" => 3,
"red" => 10
}
}


Chapter 8 - Methods

1) What is the name of the method below?

   def multiply(num1,num2)
num1 * num2
end
=> multiply


2) How many arguments does this method have?

  def meeting(place, time, day)
...
end
=> 3


3) What does this method return when called? (What's its output?)

  def calculus
numbers = (25 * 37) / 42
numbers / 12 * 25
"Programming is not math"
end
=> "Programming is not math"


Ruby methods return their last line by default

4) Write a method that takes a word as an argument. Make the method return the word and the string "is awesome!".

  def awesomify(word)
word + " is awesome!"
end


Chapter 9 - Enumerables

1) What numbers will the following code output?

     [1,2,3,4,5].each { |num| puts num if num.odd? }
1
3
5
=> [1, 2, 3, 4, 5]


2) What would this each method output from this string?

  "ThisdmakesdmoredsensedwithoutdD's".split("d").each {
|word| puts word
}

This
makes
more
sense
without
D's
=> ["This", "makes", "more", "sense", "without", "D's"]


3) What does select do for this hash?

     food = {
"apple" => "fruit",
"carrot" => "vegetable",
"tomato" => "fruit"
}
food.select do |item, category|
category == "vegetable"
end
=> {"carrot"=>"vegetable"}


4) What will map do?

     numbers = [1,2,3,4,5]
numbers.map { |num| num * 5 }
=> [5, 10, 15, 20, 25]


5) Now what is the value of numbers?

  => [1, 2, 3, 4, 5]
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