The While Construct

while is a little different from the first two constructs we looked at earlier. while will keep executing a block as long as a specific condition is true.

Remember our long and repititious counter code that used if statements to count from 0 to 20? Well, we can refactor that into simple, readable, short code with the while keyword:

counter = 1
while counter < 20
  puts "The current number is less than 20".
  counter += 1
end

Think about the above code like this:

• While it is true that the variable counter is set to a value that is less than or equal to 20, execute the code in the block.
• Inside the block, puts out a phrase, and increment the counter by one
• Go back to the top! check to see if the counter is < 20. If it is true that the value is less than 20, go back into the block. Otherwise, break out of the loop and don't execute the code inside the loop.

We can acheive all of that with just a few lines of code utilizing while.

Examples

Basic While Example: Hot Dog Eating Context

Let's say you are a world famous competitive eater participating in the Coney Island Nathan's Hot Dog Eating Contest. You're kind of new to the competitive eating game though, so you only have the capacity for 7 hot dogs.

num_of_hotdogs_eaten = 0
while num_of_hotdogs_eaten <= 7
  num_of_hotdogs_eaten += 1
  puts "You have now eaten #{num_of_hotdogs_eaten} hot dog(s)"
end

puts "You ate a total of #{num_of_hotdogs_eaten} hot dogs!"

# => "You have now eaten 1 hot dog(s)"
# => "You have now eaten 2 hot dog(s)"
# => "You have now eaten 3 hot dog(s)"
# => "You have now eaten 4 hot dog(s)"
# => "You have now eaten 5 hot dog(s)"
# => "You have now eaten 6 hot dog(s)"
# => "You have now eaten 7 hot dog(s)"
# => "You have now eaten 8 hot dog(s)"

# => "You ate a total of 8 hot dogs"

Until

Until is simply the inverse of a while loop. An until keyword will keep executing a block until a specific condition is true. In other words, the block of code following until will execute while the condition is false. One helpful way to think about it is to read until as "if not".

counter = 0
until counter == 20
  puts "The current number is less than 20"
  counter += 1
end

• The counter once again starts at 0. If it is not true that the counter is equal to 20, the program will execute the code in the block.
• Inside the block, we will puts out a phrase and increment the counter by 1.
• Then, the program will go back to the top of the until loop, and once again check to see if the counter is equal to 20. If it is not true that the counter is euql to 20, the program will execute the code in the block. Otherwise, the program will break out of the loop.

Using while and until

Guess what? We're still struggling to master that levitation charm! But we are so close. Currently, our we have a levitation force of 6. We need to have a levitation force of 10 in order to actually levitate that feather.

First, we'll write a while loop that will continue to puts out the phrase "Wingardium Leviosa" while our levitation force is less than 10. Everytime we puts out that phrase, we should increment our levitation force by one.

Then, we'll write an until loop that will puts out the same phrase until the levitation force is equal to 10, incrementing the levitation force by 1 each time we puts out the phrase.

Instructions

1. Fork and clone this lab.

2. Run the test suite to get started.

3. Let's get the first test passing by coding our solution in while.rb:

   • Fill out the content of the using_while method so that calling it will puts out the desired phrase while your levitation force is less than 10. Remember, every time you puts out the phrase, you should increment your levitation force by one.

4. Let's get the second test passing by coding our solution in until.rb:

   • Fill out the content of the using_until method to puts out the desired phrase, "Wingardium Leviosa", until our levitation force equals 10. Remember, every time you puts out the phrase, you should increment your levitation force by one.