How to Use Until Loop in Ruby: Syntax and Examples

In Ruby, the until loop runs code repeatedly until a condition becomes true. It is the opposite of a while loop, running as long as the condition is false. Use until condition do ... end to repeat code until the condition is met.

Syntax

The until loop runs the code inside it repeatedly until the given condition becomes true. It checks the condition before each iteration.

• until: keyword to start the loop
• condition: a test that returns true or false
• do ... end: block of code to repeat

The loop stops when the condition is true.

ruby

until condition do
  # code to repeat
end

Example

This example counts from 1 to 5 using an until loop. It repeats the code until the number is greater than 5.

ruby

count = 1
until count > 5 do
  puts count
  count += 1
end

Output

1
2
3
4
5

Common Pitfalls

Common mistakes include:

• Forgetting to change the variable inside the loop, causing an infinite loop.
• Using until when the condition is already true, so the loop never runs.

Always ensure the condition will eventually become true inside the loop.

ruby

count = 1
# Wrong: infinite loop because count never changes
# until count > 5 do
#   puts count
# end

# Correct:
until count > 5 do
  puts count
  count += 1
end

Output

1
2
3
4
5

Quick Reference

Keyword	Description
until	Starts the loop that runs until the condition is true
condition	The test that stops the loop when true
do ... end	Block of code to repeat
break	Optional keyword to exit the loop early

Key Takeaways

The until loop runs as long as the condition is false and stops when it becomes true.

Always update variables inside the loop to avoid infinite loops.

Use until when you want to repeat code until a condition is met, opposite of while.

The syntax is: until condition do ... end.

You can use break to exit the loop early if needed.