ruby_control_flow.md

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Control Flow in Ruby

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Learning Objectives

• Identify the three main types of Control Flow

• Use if & unless to perform branching

• Use while & until to perform looping

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Roadmap

• Control Flow Review (5 mins)

• Setup (5 mins)

• Boolean Logic - Review (5 mins)

• Branching Statements (15 mins)

• Looping Statements (15 mins)

• Other Useful Operators (5 mins)

• Closing Questions (5 mins)

• Practice Exercises (20 mins)

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Control Flow Review (5 mins)

"The execution sequence of instructions in a program determined at run time with the use of control structures"

• The Good News: Thanks to the JS knowledge you've acquired, you already have a nice grounding in the programming concepts of conditional expressions and control flow.

• The Bad News: There are more options in Ruby, and thus more to learn. However, the basics will suffice and serve you well.

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### Control Flow
Three Primary Types

• Sequence:

  • Statements executed one at a time in sequence.

     car = Car.new
     car.start
     car.drive

• Branching (selection):
  • if & unless Statements
• Looping (iterative):
  • while & until Loops
  • each, times, for, etc., Iterating Methods

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Setup (5 mins)

• We won't write much code during this lesson, however, to complete the exercises, you will write code in a Ruby (.rb) file and run it in Terminal.

• In your daily working directory:

   $ touch cf.rb
  

• Then open in your code editor.

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Setup

• Type a bit of Ruby inside cf.rb and save it:

   puts "Hello WDI"

• Run cf.rb in Terminal:

   $ ruby cf.rb
   > Hello WDI
  

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Boolean Logic - Review (5 mins)

• True/Truthy & False/Falsey

• Comparison Expressions

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Boolean Logic
True/Truthy & False/Falsey

• Everything in Ruby is considered to be truthy except for ________ and ________.

• As compared to JS, there are a couple of values that have different truthiness. What are they?

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Boolean Logic
Comparison Operators

• These Comparison Operators work exactly like they do in JS:

  • ==
  • !=
  • <
  • >
  • <=
  • >=

• So do these logical combinators:

  • ||
  • &&

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Boolean Logic
Comparison Operators

• Here's an Operator we don't have in JavaScript, the Spaceship Operator:

  • <=>

     > 5 <=> 10
     => -1
    

• The Spaceship Operator is unique in that it returns one of three values:

  • -1 if the expression on the left is less than the expression on the right.
  • 1 if the expression on the left is greater than the expression on the right.
  • 0 if the expressions are equal.

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Branching Statements (15 mins)

• Branching in Ruby is similar to that in JS, but with a few more options.

• In this lesson, we'll touch upon these:

  • if statement

  • unless statement

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Branching Statements
if (single path)

• Single path if:                

   if val == 1
     puts "is one"
   end

• Parens surrounding the logical expression are optional.

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Branching Statements
if (inline conditional)

• Remember, when possible, Ruby tries to read like English.

• If you have a single path if, the following code

   if val == 1
     puts "is one"
   end

  can be written as

   puts "is one" if val == 1

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Branching Statements
unless

• Speaking of reading like English, the unless statement is designed to do just that. Think of it as the opposite of if.

• For example, if you don't like using a not logical expression, you can use unless to remove the not logic:

   if val != 1
     puts "is not one"
   end

  can be written as

   unless val == 1
   	puts "is not one"
   end

• You can also use unless inline just like with if. Like many things in Ruby, it's a matter of preference.

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Branching Statements
if (dual path)

• Dual paths if with else:

   if val == 1
   	puts "is one"
   else
   	puts "not one"
   end

• then is optional unless you want to write an entire if statement on one line:

   if val == 1 then puts "is one" else puts "not one" end

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Branching Statements
if (three or more paths)

• Three or more paths if with elsif and optionally else:

   if val == 1
   	puts "is one"
   elsif val == 2
   	puts "is two"
   elsif val == 3
   	puts "is three"
   else
   	puts "not one, two, or three"
   end

• That's not a typo, it's actually spelled as elsif!

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Branching Statements
if is an expression

• Unlike in JavaScript, the if statement is actually an expression and can be used to return a value. For example:

   # set the message variable to the appropriate string
   message = if val == 1
   	"is one"
   elsif val == 2
   	"is two"
   elsif val == 3
   	"is three"
   else
   	"not one, two, or three"
   end

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Question - Branching Statements

Discuss the following with your pair and be prepared to share your code:

• How could we write an if statement, on one line, that puts the text "Congrats!" if a variable named correct equals true?

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Looping Statements (15 mins)

• Again, lots of options. We'll just look at these:

  • while

  • until

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Looping Statements
while

• The first looping statement we'll look at is while:

   word = ""
   words = []
   while word != "end"
     print "Enter a word ('end' to quit): "
     word = gets.chomp
     words.push(word) if word != "end"
     puts "You've entered: #{words}"
   end

• Use while when you want to continue to execute a block of code while a condition is true.

• Beware of infinite loops!

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Looping Statements
until

• The next looping statement we'll look at is until:

   word = ""
   words = []
   until word == "end"
     print "Enter a word ('end' to quit): "
     word = gets.chomp
     words.push(word) if word != "end"
     puts "You've entered: #{words}"
   end

• until works like a mirror image of while - it continues to execute a block of code until a condition is true.

• Again, beware of infinite loops!

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Looping Statements
modified while/until

• You can also put the while/until condition at the bottom of the code block as follows:

   words = []
   begin
     print "Enter a word ('end' to quit): "
     word = gets.chomp
     words.push(word) if word != "end"
     puts "You've entered: #{words}"
   end until word == "end"

• This approach guarantees that the code block will be executed at least once.

• Notice also with this construct that there's no longer a need to initialize the word variable to prevent an error from occurring.

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Looping Statements
for loop

• Where's my for loop you ask?

• Ruby does have a for loop, but it isn't much like that of JavaScript's, nor as commonly used.

• It operates more like an each method.

• Learning it's specifics is left up to you.

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Question - Looping Statements

• When using a while or until loop, we must be careful not put the program's execution into an __________ loop.

• How do we avoid the above scenario?

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Other Useful Operators (5 mins)

• ||= (or equals operator)

• Ternary Operator:
     <condition> ? <exp-if-true> : <exp-if-false>

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### Other Useful Operators
or equals

• The or equals operator is used to assign a value to a variable if it currently has no value, or is equal to either nil or false.

• What will name equal after this code runs?

   name = "Fred"
   name ||= 25

• After this code runs?

   name = false
   name ||= "Cool"

• After this code runs?

   # assume name has never been used...
   name ||= false

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Other Useful Operators
ternary

• Our useful friend, the ternary operator works just like it does in JavaScript.

• Just in case you forgot, it's ideal when you need to return one of two values depending upon a condition:

   message = score > 100 ? "You rock!" : "Keep trying!"

• It can also be used to evaluate one of two expressions, so you can even run a method if you'd like:

   score > 100 ? game.winner : game.loop

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Closing Questions (5 mins)

I'll give you a couple of minutes to review the following questions, meanwhile, I'll warm up the picker :)

• In your own words, how would you describe Control Flow?

• The three primary types of control flow are:
  1) Sequence
  2) ___________
  3) ___________

• Why might you decide to use the modified version of the while/until loop, where the condition test is put at the end of the code block?

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Practice Exercises (20 mins)

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Practice Exercises

Exercise 1 - Branching

• The following Ruby code will accept string input from the user, remove the automatic EOL character with chomp and store the string in a variable named choice:

   print "Enter a, b or c: "
   choice = gets.chomp

• Write an if statement that puts the following messages:

  • a entered - "a is for apple"
  • b entered - "b is for banana"
  • c entered - "c is for cantaloupe"
  • anything else - "you're a rebel"

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Practice Exercises

Exercise 2 - Looping

• Use one of the looping statements to continue to execute the code you wrote in the previous exercise until no more fruit is entered by the user.