Ruby Loops & Iterators

Question 1

loop

Answer 1

loop takes a block, which is denoted by { … } or do … end. A loop will execute any code within the block (again, that’s just between the {} or do … end) until you manually intervene with Ctrl + c or insert a break statement inside the block, which will force the loop to stop and the execution will continue after the loop.

loop do
puts “This will keep printing until you hit Ctrl + c”
end

i = 0
loop do
  i += 1
  puts i
  break         # this will cause execution to exit the loop
end

Question 2

break

Answer 2

The break keyword allows us to exit a loop at any point, so any code after a break will not be executed. Note that break will not exit the program, but only exit the loop and execution will continue on from after the loop.

conditional_loop.rb

i = 0
loop do
  i += 2
  puts i
  if i == 10
    break       # this will cause execution to exit the loop
  end
end

Question 3

next

Answer 3

We can use the keyword next to skip the rest of the current iteration and start executing the next iteration.

next_loop.rb

i = 0
loop do
  i += 2
  if i == 4
    next        # skip rest of the code in this iteration
  end
  puts i
  if i == 10
    break
  end
end

Question 4

while loop

Answer 4

A while loop is given a parameter that evaluates to a boolean (remember, that’s just true or false). Once that boolean expression becomes false, the while loop is not executed again, and the program continues after the while loop.

countdown.rb

x = gets.chomp.to_i

while x >= 0
puts x
x = x - 1
end

puts “Done!”

Question 5

until loop

Answer 5

The until loop is simply the opposite of the while loop. You can substitute it in order to phrase the problem in a different way.

countdown.rb

x = gets.chomp.to_i

until x < 0
puts x
x -= 1
end

puts “Done!”

Question 6

do/while loop

Answer 6

A do/while loop is very similar to a while loop, with the only difference being that the code within the loop gets executed one time, prior to the conditional check to see if the code should be executed. In a “do/while” loop, the conditional check is placed at the end of the loop as opposed to the beginning.

loop do
  puts "Do you want to do that again?"
  answer = gets.chomp
  if answer != 'Y'
    break
  end
end

Question 7

for loops

Answer 7

for loops are used to loop over a collection of elements. Unlike a while loop where if we’re not careful we can cause an infinite loop, for loops have a definite end since it’s looping over a finite number of elements. It begins with the for reserved word, followed by a variable, then the in reserved word, and then a collection of elements.

countdown3.rb

x = gets.chomp.to_i

for i in 1..x do
puts i
end

puts “Done!”

countdown4.rb

x = [1, 2, 3, 4, 5]

for i in x do
puts i
end

puts “Done!”

Question 8

range

Answer 8

A range is a special type in Ruby that captures a range of elements. For example 1..3 is a range that captures the integers 1, 2, and 3.

Question 9

Iterators

Answer 9

Iterators are methods that naturally loop over a given set of data and allow you to operate on each element in the collection.

Question 10

each

Answer 10

each is a method used to iterate over each element.

practice_each.rb

names = [‘Bob’, ‘Joe’, ‘Steve’, ‘Janice’, ‘Susan’, ‘Helen’]

names.each { |name| puts name }

We have called the each method using the dot operator (.) on our array. What this method does is loop through each element in our array, in order, starting from ‘Bob’. Then it begins executing the code within the block. The block’s starting and ending points are defined by the curly braces {}. Each time we iterate over the array, we need to assign the value of the element to a variable. In this example we have named the variable name and placed it in between two pipes |. After that, we write the logic that we want to use to operate on the variable, which represents the current array element. In this case it is simply printing to the screen using puts.

Question 11

block

Answer 11

A block is just some lines of code ready to be executed. When working with blocks there are two styles you need to be aware of. By convention, we use the curly braces ({}) when everything can be contained in one line. We use the words do and end when we are performing multi-line operations.

practice_each.rb

names = ['Bob', 'Joe', 'Steve', 'Janice', 'Susan', 'Helen']
x = 1

names.each do |name|
puts “#{x}. #{name}”
x += 1
end

Question 12

Recursion

Answer 12

Recursion is another way to create a loop in Ruby. Recursion is the act of calling a method from within itself.

irb(main):001:0> def doubler(start)
irb(main):002:1>   puts start * 2
irb(main):003:1> end
=> :doubler
irb(main):004:0> doubler(2)
4
=> nil
irb(main):005:0> doubler(4)
8
=> nil
irb(main):006:0> doubler(8)
16
=> nil

You can do this much more simply using recursion. Take a look at this version of the method:

def doubler(start)
  puts start
  if start < 10
    doubler(start * 2)
  end
end