scores = [92, 78, 85, 41, 67]

scores.each do |score|
  grade = if score >= 90 then "A"
          elsif score >= 80 then "B"
          elsif score >= 60 then "C"
          else "F"
          end
  puts format("%3d -> %s", score, grade)
end

stock = 0
puts "out of stock" unless stock.positive?

def withdraw(balance, amount)
  return "amount must be positive" unless amount.positive?
  return "insufficient funds" if amount > balance
  "withdrew #{amount}, #{balance - amount} left"
end

puts withdraw(100, 30)
puts withdraw(100, 300)
puts withdraw(100, -5)

def describe(value)
  case value
  when 0 then "zero"
  when 1..9 then "single digit"
  when Integer then "a bigger integer"
  when /\A[a-z]+\z/ then "lowercase word"
  when String then "some other string"
  when nil then "nothing"
  else "a #{value.class}"
  end
end

[0, 5, 1000, "hello", "Hello!", nil, 3.5].each do |v|
  puts format("%-10s %s", v.inspect, describe(v))
end

3.times { |i| print i, " " }
puts

1.upto(3) { |i| print i, " " }
puts

(1..10).step(3) { |i| print i, " " }
puts

# while and until exist, and are the right tool when the end condition is not
# a collection running out.
countdown = 3
until countdown.zero?
  puts "t-minus #{countdown}"
  countdown -= 1
end

# loop rescues StopIteration for you, which makes it the partner of an
# external iterator.
numbers = [10, 20, 30].each
loop { puts numbers.next }
puts "done"

def area(width, height)
  width * height
end

# Default arguments, and keyword arguments that are required unless defaulted.
def introduce(name, greeting: "Hello", punctuation: "!")
  "#{greeting}, #{name}#{punctuation}"
end

puts area(3, 4)
puts introduce("Aisha")
puts introduce("Kenji", greeting: "Welcome", punctuation: ".")

# Keyword arguments are worth it the moment a call has a bare true in it:
# resize(image, 800, 600, true, false) tells the reader nothing.
def resize(width:, height:, preserve_ratio: true)
  "#{width}x#{height} ratio=#{preserve_ratio}"
end
puts resize(width: 800, height: 600, preserve_ratio: false)

# Splat collects the rest; double splat collects keywords.
def log(level, *messages, **context)
  "[#{level}] #{messages.join(' ')} #{context.inspect}"
end
puts log(:warn, "disk", "nearly full", device: "/dev/sda", pct: 91)

words = ["b", "a", "c"]

sorted = words.sort      # returns a new array
puts sorted.inspect
puts words.inspect       # unchanged

words.sort!              # ! means "this one mutates the receiver"
puts words.inspect

puts words.empty?        # ? means "this returns true or false"
puts words.frozen?

# A one-line method can be written with =, which reads well for short ones.
def double(n) = n * 2
puts double(21)