Loop

Objectives

1. Introduce the concept of a basic loop.

2. Introduce the break keyword.

3. Introduce the concept of an incrementing counter.

4. break out of a loop based on a counter.

Introduction

There are a number of different ways to accomplish looping––the task of telling our program to do something a certain number of times. Here, we'll be looking at the most basic way to build a loop: using the loop keyword.

The loop Keyword

The first looping construct that we'll discuss is loop. This is the simplest looping construct that we have in Ruby. It simply executes a block (the code that is between the do and end keywords). Try this in IRB in your Terminal:

loop do
  puts "I have found the Time Machine!"
end

This will output I have found the Time Machine! an infinite number of times in your Terminal. Typically, you can use Control+C to break out of the loop in your terminal, but in IRB this is not the case. Exit the terminal session to break the infinite loop.

Loops start with the loop keyword and are opened by the following do and end block. All the code that goes inside the do and end is considered the loop's body or block; that's the code that will execute on repeat.

Stopping Loops with Break and Counters

Infinite loops will break our program. The loop keyword alone will create an infinite loop. Generally, we want to loop only a certain number of times. We can use the break keyword inside the body of the loop to exit or abort the loop and continue with the rest of our code. Consider:

loop do
  puts "You'll see this exactly once."
  break # Exit the Loop
end

puts "And the Loop is Done"

Our loop starts, it prints our message, and then the next line of code, break will actually end the loop. A loop that only runs once isn't useful. Neither is a loop that runs forever. So how do we actually build a useful loop, say, that runs exactly 10 times? Well first, we need a counter. Then we need to conditionally break out of the loop when the counter reaches 10. Then we need to increment the counter at every iteration (or execution of the loop).

counter = 0 # Start our counter at 0, we have never run the loop
loop do # Start our loop
  # increment our counter by 1 and set it equal to the sum of its current value, plus 1. 
  counter = counter + 1

  # Do Something
  puts "Iteration #{counter} of the loop"

  if counter >= 10 # If our counter is 10 or more
    break # Stop the loop
  end
end

If you copy this to IRB you'll see:

Iteration 1 of the loop
Iteration 2 of the loop
Iteration 3 of the loop
Iteration 4 of the loop
Iteration 5 of the loop
Iteration 6 of the loop
Iteration 7 of the loop
Iteration 8 of the loop
Iteration 9 of the loop
Iteration 10 of the loop

This is a common basic loop. With this construct we can break a loop based on any condition, but the iteration count is a very common condition for stopping the loop.

Advanced: The Add-And-Assignment (or Plus-Equals) Operator +=

Above, we use the addition operator (+) and the assignment operator = separately to reset the counter variable to the sum of its old value, plus one, every time we repeat the loop. The add-and-assignment operator combines the functionality of the addition operator and the assignment operator. For example, let's say that our favorite cat Maru has just had a birthday:

adorable_cat = "Maru"
age = 7

# you've just had a birthday! add one year to your age:
age = 7 + 1

Let's take another look at our age variable and the operation of incrementing it by 1:

age = 7
# age starts at 7 and will get incremented after the birthday
age = age + 1
age #=> 8

Here, we have one variable, age which starts at 7. Then, we reassign age to hold the original value of age plus 1. age + 1 is evaluated first, returning 8, and then we are assigning the result of that expression (everything on the right of age =, which again is age + 1, which just means 8), as the new value for age. We can make this even more elegant by using the add-and-assignment operator += instead:

age = 7
age += 1
age #=> 8

Here, += serves the purpose of the above line: age = age + 1. It's simply condensing that action. It adds a numerical value (or other variable) to a numerical variable, and reassigns that variable to hold the sum of that variable's original value plus the added value (or other variable).

When we use +=, we call this action "incrementing". We are adding a new increment to a known value. Why is that useful? For looping.

Let's re-write our loop from earlier, this time using the += operator:

counter = 0

loop do 
  counter += 1
  puts "Iteration #{counter} of the loop"
  if counter >= 10 
    break
  end
end

If you copy this to IRB you'll see the same input as above:

Iteration 1 of the loop
Iteration 2 of the loop
Iteration 3 of the loop
Iteration 4 of the loop
Iteration 5 of the loop
Iteration 6 of the loop
Iteration 7 of the loop
Iteration 8 of the loop
Iteration 9 of the loop
Iteration 10 of the loop

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