loops.md

Notes: Introduction to Loops

Loops are programming constructs that help you repeat a code action an arbitrary number of times without copying and pasting the code. Another term for a loop is iteration. All high-level programming languages provide various forms of loops, which can be used to execute one or more statements repeatedly.

For example, if we wanted to print out "hello" five hundred times, we could write:

puts "hello! "
puts "hello! "
puts "hello! "
puts "hello! "
puts "hello! "
# ... 495 more times

Usig a loop, we could re-write this as:

500.times do
  puts "hello! "
end

There are 2 categories of loops: counter-controlled and sentinel-controlled loops

Counter-controlled loops are used when the number of loops can be determined prior to loop execution. For example, a copying machine copies a paper a set number of times, and we know exactly how many times the copier will copy the paper.

Sentinel-controlled loops are used when the number of loops cannot be determined prior to loop execution. For example, if you do jumping jacks until you get tired, it is uncertain how many jumping jacks you will do before stopping.

There are four types of loops in Ruby that we will start with: times, range-each, while, and until.

Times

You use the times iterator to run a block of code N times.

You can use a times loops with or without an iteration variable. This variable is denoted between the vertical bars (pipe character) | and is used to store data associated with the iteration.

In the example below, N is equal to three. This example does not use an iteration variable.

3.times do
  puts "hello! "
end

output:

hello!
hello!
hello!

In the example below, N is equal to two. This example does use an iteration variable, i, which will correspond to value of the iteration.

2.times do |i|
  puts i # wheeee remember the slide?
end

output:

0
1

Ranges

The main use of ranges is to express a sequence. Sequences have a start point, an end point, and a way to produce successive values in the sequence.

Ruby creates these sequences using the .. and ... range operators. The two-dot form creates an inclusive range, while the three-dot form creates a range that excludes the specified high value. The each loop using a range uses an iteration variable (again passed to the block between | characters), num in this example.

# Inclusive range example
# note the ..
(5..9).each do |num|
  puts num
end

# output
5
6
7
8
9

# Exclusive range example
# note the ...
(5...9).each do |num|
  puts num
end

# output
5
6
7
8

While Loops

The while loop is useful when you want to continue doing an action while a certain condition is true but you may not know how many times you'll need to complete that action. It is an example of a sentinel-controlled loop. As soon as the condition stops being true, the loop will terminate.

i = 0

while i < 4
  puts i
  i += 1
end

The above code will output the values of i until i is no longer less than 4, resulting in the following output:

0
1
2
3

You can read the condition and execute of the while loop as while condition is true do...

A while loop example: This loop repeats while the user enters "yes".

again = "yes"

while again == "yes"
  puts "Let's play a game!"
  puts "..."
  puts "Would you like to play again? (yes/no) > "
  again = gets.chomp
end

Until Loops

The until loop acts as an inverse of the while loop. The loop will execute until the condition evaluates as true.

i = 0

until i == 4
   puts i
   i += 1
end

The above code will output the values of i until i is equal to 4, resulting in the following (equivalent to the while loop) output:

0
1
2
3

You can read the condition and execute of the until loop as until condition is true do...

An until loop example: This loop will repeat until the user guesses the number 6.

number = 0

until number == 6
  puts "Guess my number (1-10) > "
  number = gets.chomp.to_i
end

puts "You guessed it!"