Loops vs iterators: I don't get the difference

The difference is quite subtle indeed. Both loops and iterators are used to repeat a chunk of code. Loops are an ancient idea, they existed long before computers did. Almost every programming language has some kind of loops. Iterators are relatively new, and they only exist in a few languages such as Ruby.

Loops are both powerful and dangerous: it’s too easy to break your program by producing a so called infinite loop, if you fail to ensure that the termination condition (the one that must be true so the loop stops) really occurs. For instance, I was trying to count from a number (that the user entered) to 100 in steps of 10:

number = gets.chomp.to_i    
while number != 100
  puts number += 10
end

Do you see the problem? If the user types in 5, we have an infinite loop. Oops. OK, this is really easy to fix (just change the loop condition to be number < 100 instead), but you get the point. An infinite loop is impossible if we use an iterator instead:

(number..100).step(10).each {|n| puts n}

But that’s not the only good thing about iterators. They also allow you to write more expressive, clutter-free code. Because they abstract away from the nitty-gritty details of a loop. Compare by yourself:

# loop               vs.           iterator

i = 0                              5.times do
while i < 5                         
  puts "hello"                       puts "hello"
  i += 1
end                                end

Both pieces of code perform exactly the same task. But which one is easier to write, read, and to understand?

Similarly, with each, the perhaps most famous iterator of all:

words = ["one", "two", "three", "four"]

# loop               vs.           iterator

i = 0                              words.each do |w|
while i < words.length                         
  puts words[i]                      puts w
  i += 1
end                                end

Lastly, you can define custom iterators for your special looping needs, and Ruby provides some cool built-in ones:

list = "a b c d e f g h i j".split(" ")
# => ['a','b','c','d','e','f','g','h','i','j']

list.each_slice(3) do |group|
  puts group.join ""
end

# prints abc, def, ghi, j

Try to do the same with a while loop and you’ll see how great it is to have iterators.