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Control Flow

The control flow is the order in which the computer executes the statements in our text editor. Code normally runs from the first line in the file to the last line, unless something changes that!


  • Read and make sense of unfamiliar code
  • Use if statements to respond dynamically to user input
  • Write your own code and problem-solve through any errors

What is Control Flow?

You probably noticed the program we wrote before isn’t very flexible. We can ask the user a question and store their input, but we pretty much always say the same thing in response. What if we wanted the flexibility to change the behavior in reaction to the data?

Control flow gives us the flexibility we’re looking for. We can change the response depending on the information the user types. Take a look at the code below and see if you can guess what the output might look like. Be prepared to share.

puts "How old are you?"
age = gets.chomp.to_i

if age >= 21
  puts "Welcome to Sierra Nevada Brewing Company!"
  puts "Sorry, you’re not old enough to access our site. Come back later!"

if Statements

Ruby’s if statement is always followed by an expression, which is a fancy way of saying something that evaluates to true or false. If the expression evaluates to true, anything inside that code block is executed. However, if the expression is false, Ruby skips over that block of code. Here’s an example of what that might look like:

if 5 > 4
  print "You will see this statement in the console, because five is greater than four!"

The way our code is written now, if the expression evaluates to false, we don’t see any output in the console. In order to have another statement that runs if the expression is false, we need an else statement. Here’s an example:

if 5 < 4
  print "This statement won’t print, because five is NOT less than 4."
  print "This statement will print instead!"

We can also check a second condition if we want using an elsif statement.

if 5 < 4
  print "This statement won’t print, because five is NOT less than 4."
elsif 5 > 4
  print "This statement will print, because five is greater than 4."
  print "This statement won’t print, because a true expression was already found!"

Try It: Control Flow with if Statements

Back in your sandbox replit, write a program that will check the user’s age and determine whether or not they can rent a car using the following guidelines:

  • If the driver is below 20 years old, they cannot rent a car.
  • If the driver is between 20 and 24 years old, they can rent the car, but they need to pay an underage driver fee.
  • If the driver is over the age of 25, they can rent the car.

Ready for another challenge? Write a program that asks the user for the product of 5 and 10 and collects their response. Give the user some feedback based on their response. You decide what feedback makes sense!

Building a "Guess the Number" Game

Use this replit as a starting point. We are going to build a “Guess the Number” game for a user in the console. In the starter kit, you already have a secret_number variable assigned to 6 and a puts statement that shows the user the title of game. Follow the steps below to keep going!

  1. Next, prompt the user to enter a number from 1 to 10.
  2. If the guess is less than the secret number, tell the user, "Not quite. Too low."
  3. If the guess is greater than the secret number, tell the user, "Oops. Too high."
  4. Otherwise, tell the user they guessed the number with the statement, "You did it!"
  5. Test your code a few times to make sure you can generate all 3 responses.
  6. Finally, update the value assigned to secret_number to a random number from 1 to 10.

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