Loops

Goals

  • Use loops to do operations for every element in an array.

  • Use puts to print strings to the screen.

  • Learn the two different syntaxes for creating blocks in Ruby.

Step 1

Type this in irb:
puts 'Hello World'

puts (put string) is a way of printing information to the user of your program.

Take some time to contemplate the output of puts in irb:

Expected result:
1.9.3p125 :006 > puts 'Hello World'
Hello World
 => nil 
1.9.3p125 :007 >

The method puts always has the return value of nil, which is what we see after the => in the output. Printing 'Hello World' to the screen is just a side-effect.

Step 2

Type this in irb:
fruits = ['peach', 'plum', 'pear']
fruits.each { |fruit| puts fruit }

The straight up-and-down | is called the 'pipe character', and is typically the shifted version of the \ (backslash) on your keyboard.

Loops are a way of doing something here multiple times. In this loop, we printed each fruit to the screen in order.

Step 3

Type this in irb:
numbers = [109, 10, 1001]
numbers.each { |n| puts n * 2 }

The curly braces here define a block, and whatever's in the pipes is a block variable.

each takes the first element in the array and sends it to the block, which temporarily stores it in the block variable and then runs the code after the pipes. It then goes back and does this again for each of the remaining items in the array.

Type this in irb:
ducks = ['huey', 'dewey', 'louie']
ducks.each { |duck| puts "#{duck} quacks!" }
ducks.each { |zombie| puts "#{zombie} quacks!" }

It doesn't matter what you call your block variable: the previous two statements are exactly equivalent to Ruby. But you should try to name your variables something useful so the code makes sense to you later!

Step 4

Type this in irb:
total = 256 ** 3
colors = ['red', 'blue', 'green']
colors.each do |color|
  puts "#{total} colors of paint on the wall..."
  puts "Take #{color} down, pass it around..."
  total = total - 1
  puts "#{total} colors of paint on the wall!"
end

The ** operator means 'to the power of', as in '256 to the third power'

There's more than one way to make a block in ruby. The do ... end syntax is typically used when a block needs to span multiple lines, while the { ... } syntax is for a single line block.

Explanation

As you build complex programs, you'll want to do something to many pieces of data without typing it all out. Loops help solve this problem.

Next Step:

Go on to Running Programs From A File