"elsif" and "else"

• If the condition of an if statement is false, the code in its body won't run.

if_condition = false
if if_condition
  puts "if clause"
end

• If you have some code that you want to run when the if statement doesn't run, you can add an else clause.
• You add the keyword else between if and end, followed by one or more lines of code that you want to run instead of the if statement's code.
• Ruby doesn't require it, but it's conventional to have the else keyword aligned with the if keyword, and the else code indented one level. Again, it just makes the code easier to read.

if_condition = false
if if_condition
  puts "if clause"
else
  puts "else clause"
end

• If you have other conditions that you want to test only when the if condition is false, you can add one or more elsif clauses between the if and else clauses.
• You add the keyword elsif, another condition expression, and some code you want to run if the condition is true.
• As with the else clause, the elsif keyword should be lined up with the if keyword, and the code following it should be indented one level.

if_condition = false
elsif_condition = true
if if_condition
  puts "if clause"
elsif elsif_condition
  puts "elsif clause"
else
  puts "else clause"
end

• If the if clause is true, then neither the elsif clauses nor the else clause will be run.
• If the if condition and all elsif conditions are false, then the else clause will be run.
• The else clause isn't required. If all conditions are false and there's no else clause, then no code will be run.

Widget store

Let's apply what we've learned to the widget store program. Previously, we were checking the conditions for all three if statements. We've tried to fix it so this won't happen, but if more than one condition were true, the price_per_unit variable would be set and then overwritten. No matter what, we only want price_per_unit set once. Let's use elsif and else clauses to clean this up.

# This is the same as before
def ask(question)
  print question + " "
  gets.chomp
end

def price(quantity)
  # Merge the three old "if" statements
  # into one if/elsif/else statement
  if quantity >= 100
    price_per_unit = 8
  elsif quantity >= 50 && quantity < 100
    price_per_unit = 9
  else
    price_per_unit = 10
  end
  quantity * price_per_unit
end

# This code is unchanged
puts "Welcome to the widget store!"
answer = ask("How many widgets are you ordering?")
number = answer.to_i
total = price(number)
puts "For #{number} widgets, your total is: $#{total}"

• But really, the elsif branch can't possibly run if the quantity is equal to or greater than 100, because then only the if branch will run. So we can get rid of && quantity < 100.

def price(quantity)
  if quantity >= 100
    price_per_unit = 8
  # Get rid of && operator:
  elsif quantity >= 50
    price_per_unit = 9
  else
    price_per_unit = 10
  end
  quantity * price_per_unit
end

After that, our bulk discount feature works great! Thanks to elsif and else clauses, we can be sure that a discount is applied only once.