Loops: while and until

A loop repeats code. A while loop keeps going as long as its condition is true.

Real-world analogy: "While the kettle isn't boiling, keep waiting."

Syntax

while condition
  # repeated code
end

Example

count = 1

while count <= 3
  puts "Count: #{count}"
  count += 1
end

puts "Done"

Expected output

Count: 1
Count: 2
Count: 3
Done

Each pass checks count <= 3. After the third pass, count becomes 4, the condition is false, and the loop ends.

until is the opposite — it loops while the condition is false:

balance = 0
balance += 40 until balance >= 100
puts balance

120

loop do ... end repeats forever until you stop it with break (see the break and next lesson). It's the clearest way to write "keep asking until the input is valid".

In everyday Ruby, while loops are less common than you might expect — iterators like times and each (next lesson) usually say the same thing more clearly.

Common mistakes

• Forgetting to change the variable in the condition. Without count += 1, the loop never ends — press Ctrl + C to stop it.

Practice

Use a while loop to print the numbers 10, 8, 6, 4, 2.

n = 10
while n > 0
  puts n
  n -= 2
end