Loops

Computers are like robots. They are good at doing things over and over and over and over again.

A LOOP is when we ask a program to do something many times.

loop

If you want something to keep going forever, use a special loop called loop.

loop do
  puts "Hello"
end

To stop it, hold down the CONTROL key and press the C key.

Note well! The lines between do and end are INDENTED. Indentation is very important to you and other humans. It lets our eyes follow the patterns and helps us quickly see what parts of the program go with each other.

One Infinite Loop

Fun Fact: The address of Apple HQ is

1 Infinite Loop
Cupertino, CA 95014

Image from the Wikimedia Commons, licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license. Attribution: Joe Ravi

LAB: Infinite Hello

Let's change hello.rb so that it keeps saying hello over and over again.

loop do
  puts "What is your name?"
  name = gets.strip
  puts "Hello, " + name + "!"
end

LAB: Infinite Food

Write a program called food.rb that

1. asks the user for a food -- say, "pizza"
2. prints "Yum, I love pizza!"
3. asks again, and prints again, forever and ever

Remember, CONTROL-C means "Stop everything!!!"

Who wants to loop forever?

Next, we will change your food.rb program so that if the user types "return" -- meaning the string is empty -- then the program exits.

Please try this yourself! But it's kind of tricky, so on the next slide I'll show you one solution.

Exiting a Loop with break

The magic word break stops a loop immediately.

loop do
  puts "What is your name?"
  name = gets.strip
  break if name.empty?
  puts "Hello, " + name + "!"
end

The magic word break means "stop the current loop". It's less dangerous -- but still kind of weird, since it's not always clear which loop you mean.

LAB: Exiting a Loop

Change your food.rb program so that if the user types "return" -- meaning the string is empty -- then the program exits.

LAB: Good Food, Bad Food

• Change food.rb so that it doesn't love every food.
• If it's a food you like (let's say, pizza), make it print "Yum! I love pizza."
• If it's a food you don't like (let's say, cabbage), make it print "Yuck! I hate cabbage."

times

Another loop in Ruby is called times, and it's a message you can send to a number.

Try this in IRB:

3.times do
  puts "Hip! Hip! Hooray!"
end

Let's unpack this:

• 3.times do means what it sounds like: "do this three times"
• end means this is the end of what I want you to do
• and of course, puts means "show this on the terminal"

Counting in a loop

Remember this poem?

1 potato
2 potato
3 potato
4
5 potato
6 potato
7 potato
More

We're going to examine a few different ways to code this.

times with a counter

Try this:

4.times do |count|
  puts count.to_s + " potato"
end

|count| means

"the count variable points to the current value of the counter"

Whoops! What's wrong?

times with a counter -- fixed

4.times do |count|
  puts (count+1).to_s + " potato"
end

loop with a counter

count = 1
loop do
  puts count.to_s + " potato"
  count = count + 1
end

Whoops! Hit Control-C and join me on the next slide...

loop with a counter -- fixed

count = 1
loop do
  puts count.to_s + " potato"
  count = count + 1
  break if count > 4
end

while

The magic word while combines loop and break.

count = 1
while count <= 4
  puts count.to_s + " potato"
  count = count + 1
end

• The while statement keeps checking the expression

  • if it's true then it loops back
  • if it's false then it stops looping and goes on to the rest of the program

This is fairly complicated, so let's stop here and make sure to understand everything that's happening in this little program.

while breakdown (pt.1)

count = 1

creates a variable named count and sets its value to 1.

while count <= 4

starts a loop and immediately compares count to 4.

1 is less than 4, so the expression is true, so we continue with the loop.

while breakdown (pt.2)

  puts count.to_s + " potato"

prints the current value of count (and the word "potato").

  count = count + 1

increments the count variable... it was 1, so now it's 2

end

goes back to the while line and checks again

while breakdown (pt.2)

while count <= 4

compares count to 4.

2 is less than 4, so the expression is true, so we continue with the loop.

Eventually, count becomes 5, and the while expression is false, and so we stop looping and go on.

LAB: One Potato

Write a program called potato.rb that prints the entire potato poem, accurately.

Lab: Adder

Write a program named adder.rb that keeps a running total.

For example:

ruby adder.rb
1
Total: 1
2
Total: 3
4
Total: 7
-5
Total: 2