Booleans and nil

A boolean is a yes/no value: true or false. Comparisons produce booleans:

p 5 > 3
p 5 == 3
p "ruby" == "ruby"

true
false
true

nil means "no value at all". It's what you get when something is missing — for example, looking up a hash key that doesn't exist, or a method that has nothing to return.

scores = { ada: 90 }
p scores[:bob]
p nil.nil?
p 0.nil?

nil
true
false

Truthiness: only nil and false are falsy

When Ruby checks a condition, only false and nil count as false. Everything else — including 0, "" (an empty string), and [] (an empty array) — counts as true. This is different from Python, JavaScript, and PHP, where 0 and "" are falsy.

count = 0
name = ""
missing = nil

puts "0 is truthy" if count
puts "empty string is truthy" if name
puts "nil is falsy" unless missing

0 is truthy
empty string is truthy
nil is falsy

To check for "empty", ask explicitly: name.empty?, list.empty?, count.zero?.

Common mistakes

• Calling a method on nil by accident. nil.upcase raises undefined method 'upcase' for nil (NoMethodError) — the most common error in Ruby. It means a value you expected to exist was missing.
• Writing if count to mean "if count isn't zero". It's always true for any number; write if count.positive? or if count > 0.

Practice

Predict which of these lines print, then run them to check.

puts "A" if []
puts "B" if false
puts "C" if "false"

A
C