A.13 Looping through Arrays

A.13.1 .each

Array#each do |Object|
  # ruby code here
end

⇒ Array

Given an array, the .each method will loop through each element of the array starting with the very first one.

Returns the Array the method was called on.

cities = ["Chicago", "LA", "NYC"]
cities.each do |city|
  p city
end

"Chicago"  
"LA"  
"NYC"

The block variable city holds the value of the elements in the array cities. It starts with the first element "Chicago" and then changes with each interation, holding the value of the next element ("LA") in the array and so on.

Full explanation

A.13.2 .each_with_index

Array#each_with_index do |Object, Integer|
  # ruby code here
end

⇒ Array

To keep a track of the iteration number while looping through an array, .each_with_index creates an additional block variable that starts of counting the iteration number starting at zero. After each execution of the code within the block, the block variable is incremented by 1.

cities.each_with_index do |city, count|
  p count.to_s + " " + city
end

"0 Chicago"  
"1 LA"  
"2 NYC"

city holds the value of elements in the array cities. count holds the index of the element that city currently holds.

Note:
Variables created as a block variables can only be used within that block (between do and end). Using that variable outside that block will throw an error.

Full explanation