Ruby until Loop

Execute code until a condition becomes true

🔄 What is an until Loop?

The until loop is the opposite of while. It executes code repeatedly until a condition becomes true. It's more readable when you want to continue until something happens. 


# Basic until loop
count = 1
until count > 3
  puts "Count: #{count}"
  count += 1
end

Output: 

Count: 1
Count: 2
Count: 3

until vs while

🔄

until Syntax

Runs until condition is true 

until condition
  # code
end
⏰

while Syntax

Runs while condition is true 

while condition
  # code
end
✅

Readability

Use until for negative conditions 

until done
  work
end
🎯

Equivalence

until is opposite of while 

# Same result
until x >= 5
while x < 5

🔹 Basic until Loop

The until loop continues executing until the condition becomes true. It's perfect when you want to express "keep doing this until something happens." 

# Count up until reaching 5
number = 1
until number > 5
  puts "Number: #{number}"
  number += 1
end

puts "Reached 5!"

Output: 

Number: 1
Number: 2
Number: 3
Number: 4
Number: 5
Reached 5!

🔹 until vs while Comparison

Both loops can achieve the same result, but until is more readable when expressing negative conditions. Here's a side-by-side comparison: 

# Using until (more readable for this case)
attempts = 0
until attempts == 3
  puts "Attempt #{attempts + 1}"
  attempts += 1
end

# Using while (requires negative logic)
attempts = 0
while attempts != 3
  puts "Attempt #{attempts + 1}"
  attempts += 1
end

# Both produce the same output!

Output (both loops): 

Attempt 1
Attempt 2
Attempt 3

🔹 Practical until Loop Examples

Here are real-world scenarios where until loops make your code more readable and intuitive: 

# Wait until target is reached
score = 0
until score >= 10
  score += 3
  puts "Score: #{score}"
end
puts "Target reached!"

# Build a string until length is sufficient
text = ""
until text.length >= 10
  text += "Hi"
end
puts "Text: #{text}"
puts "Length: #{text.length}"

Output: 

Score: 3
Score: 6
Score: 9
Score: 12
Target reached!
Text: HiHiHiHiHi
Length: 10

🔹 One-line until Loop

Ruby allows you to write until loops in a single line for simple operations. This is called a modifier form and is very concise. 

# One-line until loop
x = 0
puts x += 1 until x == 3

# Traditional multi-line equivalent
y = 0
until y == 3
  puts y += 1
end

Output: 

1
2
3
1
2
3

🔹 until Loop with Multiple Conditions

You can combine multiple conditions using logical operators (&&, ||) to create more complex loop controls: 

# Loop until either condition is met
count = 0
sum = 0

until count >= 5 || sum >= 10
  count += 1
  sum += count
  puts "Count: #{count}, Sum: #{sum}"
end

puts "Loop ended: Count=#{count}, Sum=#{sum}"

Output: 

Count: 1, Sum: 1
Count: 2, Sum: 3
Count: 3, Sum: 6
Count: 4, Sum: 10
Loop ended: Count=4, Sum=10

🔹 When to Use until

Choose until over while when it makes your code more readable:

Use until when:

  • Waiting for completion: "Keep trying until successful"
  • Reaching a goal: "Continue until target is reached"
  • Negative conditions: "Run until NOT something"
  • Natural language: When "until" sounds more natural than "while not"

Use while when:

  • Positive conditions: "While something is true"
  • Active states: "While running" or "While available"
  • Conventional patterns: Standard counting loops 
# Good use of until (reads naturally)
until game_over
  play_turn
end

# Good use of while (reads naturally)
while player_alive
  continue_game
end

🧠 Test Your Knowledge

What is the main difference between until and while loops?