Conditional structures/Ruby

if-then-else

if s == 'Hello World'
  foo
elsif s == 'Bye World'
  bar
else
  deus_ex
end

Note that if...end is an expression, so its return value can be captured in a variable:

s = 'yawn'
result = if s == 'Hello World'
           :foo
         elsif s == 'Bye World'
           :bar
         else
           :deus_ex
         end
# result now holds the symbol :deus_ex

ternary

 s == 'Hello World' ? foo : bar

case-when-else

A generic case statement

case
when Time.now.wday == 5 
  puts "TGIF"
when rand(3) == 2
  puts "had a 33% chance of being right"
else
  puts "nothing special here"
end

or, comparing to a specific object

case cartoon_character
when 'Tom'
  chase
when 'Jerry'
  flee
end

For the second case, the comparisions are preformed using the === "case equality" method like this: 'Tom' === cartoon_character. The default behaviour of === is simple Object#== but some classes define it differently. For example the Module class (parent class of Class) defines === to return true if the class of the target is the specified class or a descendant:

case some_object
when Numeric
  puts "I'm a number.  My absolute value is #{some_object.abs}"
when Array
  puts "I'm an array.  My length is #{some_object.length}"
when String
  puts "I'm a string.  When I'm down I look like this: #{some_object.downcase}"
else
  puts "I'm a #{some_object.class}"
end

The class Regexp aliases === to =~ so you can write a case block to match against some regexes

case astring
when /\A\Z/             then  puts "Empty"
when /\A[[:lower:]]+\Z/ then  puts "Lower case"
when /\A[[:upper:]]+\Z/ then  puts "Upper case"
else                    then  puts "Mixed case or not purely alphabetic"
end

The class Range aliases === to include?:

case 79
when 1..50   then   puts "low"
when 51..75  then   puts "medium"
when 76..100 then   puts "high"
end