How to write a switch statement in Ruby – Dev

The best answers to the question “How to write a switch statement in Ruby” in the category Dev.

QUESTION:

How do I write a switch statement in Ruby?

ANSWER:

case...when behaves a bit unexpectedly when handling classes. This is due to the fact that it uses the === operator.

That operator works as expected with literals, but not with classes:

1 === 1           # => true
Fixnum === Fixnum # => false

This means that if you want to do a case ... when over an object’s class, this will not work:

obj = 'hello'
case obj.class
when String
  print('It is a string')
when Fixnum
  print('It is a number')
else
  print('It is not a string or number')
end

Will print “It is not a string or number”.

Fortunately, this is easily solved. The === operator has been defined so that it returns true if you use it with a class and supply an instance of that class as the second operand:

Fixnum === 1 # => true

In short, the code above can be fixed by removing the .class from case obj.class:

obj = 'hello'
case obj  # was case obj.class
when String
  print('It is a string')
when Fixnum
  print('It is a number')
else
  print('It is not a string or number')
end

I hit this problem today while looking for an answer, and this was the first appearing page, so I figured it would be useful to others in my same situation.

ANSWER:

Ruby uses the case expression instead.

case x
when 1..5
  "It's between 1 and 5"
when 6
  "It's 6"
when "foo", "bar"
  "It's either foo or bar"
when String
  "You passed a string"
else
  "You gave me #{x} -- I have no idea what to do with that."
end

Ruby compares the object in the when clause with the object in the case clause using the === operator. For example, 1..5 === x, and not x === 1..5.

This allows for sophisticated when clauses as seen above. Ranges, classes and all sorts of things can be tested for rather than just equality.

Unlike switch statements in many other languages, Ruby’s case does not have fall-through, so there is no need to end each when with a break. You can also specify multiple matches in a single when clause like when "foo", "bar".

ANSWER:

case…when

To add more examples to Chuck’s answer:

With parameter:

case a
when 1
  puts "Single value"
when 2, 3
  puts "One of comma-separated values"
when 4..6
  puts "One of 4, 5, 6"
when 7...9
  puts "One of 7, 8, but not 9"
else
  puts "Any other thing"
end

Without parameter:

case
when b < 3
  puts "Little than 3"
when b == 3
  puts "Equal to 3"
when (1..10) === b
  puts "Something in closed range of [1..10]"
end

Please, be aware of “How to write a switch statement in Ruby” that kikito warns about.

ANSWER:

It is done using case in Ruby. Also see “Switch statement” on Wikipedia.

Quoted:

case n
when 0
  puts 'You typed zero'
when 1, 9
  puts 'n is a perfect square'
when 2
  puts 'n is a prime number'
  puts 'n is an even number'
when 3, 5, 7
  puts 'n is a prime number'
when 4, 6, 8
  puts 'n is an even number'
else
  puts 'Only single-digit numbers are allowed'
end

Another example:

score = 70

result = case score
   when 0..40 then "Fail"
   when 41..60 then "Pass"
   when 61..70 then "Pass with Merit"
   when 71..100 then "Pass with Distinction"
   else "Invalid Score"
end

puts result

On around page 123 of The Ruby Programming Language (1st Edition, O’Reilly) on my Kindle, it says the then keyword following the when clauses can be replaced with a newline or semicolon (just like in the if then else syntax). (Ruby 1.8 also allows a colon in place of then, but this syntax is no longer allowed in Ruby 1.9.)