Loops

Introduction

A common idea in computer science is that humans are good at performing tasks that are hard to describe but easy to accomplish, such as: "make a sandwich".

Computers, on the other hand, are good at performing tasks that are easy to describe, but hard to accomplish, such as: "write 'oo-la-la' 1000 times". That task might take you a while.. but introduce 5 lines of code:

i = 1
while i <= 1000
  puts "oo-la-la"
  i += 1
end

And you're done! Copy and paste the above loop into pry. Remember to use the clear command to clear your terminal afterwards!

The while loop

The while loop tells your computer to do something over and over again, but only while some condition is true. In the example above, we tell the computer to do something while i <= 1000. Meaning, as soon as i reaches 1001, we stop.

This might beg the question, what if i never reaches 1001? Or to phrase the question differently, what if our condition never becomes false?

while 1 == 1
  # ...
end

The above loop will run forever, until the end of time. This means we have to design our loops carefully!

All while loops should perform some operation that trends towards a falsey condition. In our first example, we increment i by one with each iteration.

The until loop

The until loop is to the while loop, what unless is to if. In other words, until is the same as while !(..). We can re-write the first example using an until loop like so:

i = 1
until !(i <= 1000)
  puts "oo-la-la"
  i += 1
end

or...

i = 1
until i > 1000
  puts "oo-la-la"
  i += 1
end

All three approaches are perfectly valid! You should use whichever makes more lexical sense -- whichever reads more like natural English.

break and next

break and next are two keywords that we can use inside of any ruby loop. So far, we only know while and until, but we'll learn more later.

break allows us to exit a loop early, similar to how return exits a method early. next will tell our loop to jump to the next iteration.

We could re-write our first while loop like so:

i = 1
while true
  break if i > 1000
  puts "oo-la-la"
  i += 1
end

The next command is good for skipping over irrelevant iterations. Let's say we have the following problem statement:

Define a method, sum_all_but_7(n), that accepts an integer, n, as an argument. Sum all the odd integers between 0 and n, but skip 7!

We can use a while loop to iterate over all the numbers between 0 and n. We can then use next to skip over 7.

def sum_all_but_7(n)
  i = 0
  sum = 0

  while i < n
    i += 1
    next if i == 7
    sum += i
  end

  sum
end

sum_all_but_7(6) == 21
sum_all_but_7(7) == 21
sum_all_but_7(8) == 29