How to write a switch statement in Ruby

Question

How do I write a switch statement in Ruby?

Answer 1

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 2

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:

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 3

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.)

Answer 4

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 5

In Ruby 2.0, you can also use lambdas in case statements, as follows:

is_even = ->(x) { x % 2 == 0 }

case number
when 0 then puts 'zero'
when is_even then puts 'even'
else puts 'odd'
end

You can also create your own comparators easily using a Struct with a custom ===

Moddable = Struct.new(:n) do
  def ===(numeric)
    numeric % n == 0
  end
end

mod4 = Moddable.new(4)
mod3 = Moddable.new(3)

case number
when mod4 then puts 'multiple of 4'
when mod3 then puts 'multiple of 3'
end

(Example taken from "Can procs be used with case statements in Ruby 2.0?".)

Or, with a complete class:

class Vehicle
  def ===(another_vehicle)
    self.number_of_wheels == another_vehicle.number_of_wheels
  end
end

four_wheeler = Vehicle.new 4
two_wheeler = Vehicle.new 2

case vehicle
when two_wheeler
  puts 'two wheeler'
when four_wheeler
  puts 'four wheeler'
end

(Example taken from "How A Ruby Case Statement Works And What You Can Do With It".)

Answer 6

Many programming languages, especially those derived from C, have support for the so-called Switch Fallthrough. I was searching for the best way to do the same in Ruby and thought it might be useful to others:

In C-like languages fallthrough typically looks like this:

switch (expression) {
    case 'a':
    case 'b':
    case 'c':
        // Do something for a, b or c
        break;
    case 'd':
    case 'e':
        // Do something else for d or e
        break;
}

In Ruby, the same can be achieved in the following way:

case expression
when 'a', 'b', 'c'
  # Do something for a, b or c
when 'd', 'e'
  # Do something else for d or e
end

This is not strictly equivalent, because it's not possible to let 'a' execute a block of code before falling through to 'b' or 'c', but for the most part I find it similar enough to be useful in the same way.

Answer 7

You can use regular expressions, such as finding a type of string:

case foo
when /^(true|false)$/
   puts "Given string is boolean"
when /^[0-9]+$/ 
   puts "Given string is integer"
when /^[0-9\.]+$/
   puts "Given string is float"
else
   puts "Given string is probably string"
end

Ruby's case will use the equality operand === for this (thanks @JimDeville). Additional information is available at "Ruby Operators". This also can be done using @mmdemirbas example (without parameter), only this approach is cleaner for these types of cases.

Answer 8

It's called case and it works like you would expect, plus lots more fun stuff courtesy of === which implements the tests.

case 5
  when 5
    puts 'yes'
  else
    puts 'else'
end

Now for some fun:

case 5 # every selector below would fire (if first)
  when 3..7    # OK, this is nice
  when 3,4,5,6 # also nice
  when Fixnum  # or
  when Integer # or
  when Numeric # or
  when Comparable # (?!) or
  when Object  # (duhh) or
  when Kernel  # (?!) or
  when BasicObject # (enough already)
    ...
end

And it turns out you can also replace an arbitrary if/else chain (that is, even if the tests don't involve a common variable) with case by leaving out the initial case parameter and just writing expressions where the first match is what you want.

case
  when x.nil?
    ...
  when (x.match /'^fn'/)
    ...
  when (x.include? 'substring')
    ...
  when x.gsub('o', 'z') == 'fnzrq'
    ...
  when Time.now.tuesday?
    ...
end

Answer 9

If you are eager to know how to use an OR condition in a Ruby switch case:

So, in a case statement, a , is the equivalent of || in an if statement.

case car
   when 'Maruti', 'Hyundai'
      # Code here
end

See "How A Ruby Case Statement Works And What You Can Do With It".

Answer 10

Ruby uses the case for writing switch statements.

As per the case documentation:

Case statements consist of an optional condition, which is in the position of an argument to case, and zero or more when clauses. The first when clause to match the condition (or to evaluate to Boolean truth, if the condition is null) “wins”, and its code stanza is executed. The value of the case statement is the value of the successful when clause, or nil if there is no such clause.

A case statement can end with an else clause. Each when a statement can have multiple candidate values, separated by commas.

Example:

case x
when 1,2,3
  puts "1, 2, or 3"
when 10
  puts "10"
else
  puts "Some other number"
end

Shorter version:

case x
when 1,2,3 then puts "1, 2, or 3"
when 10 then puts "10"
else puts "Some other number"
end

---

And as "Ruby's case statement - advanced techniques" describes Ruby case;

Can be used with Ranges:

case 5
when (1..10)
  puts "case statements match inclusion in a range"
end

## => "case statements match inclusion in a range"

Can be used with Regex:

case "FOOBAR"
when /BAR$/
  puts "they can match regular expressions!"
end

## => "they can match regular expressions!"

Can be used with Procs and Lambdas:

case 40
when -> (n) { n.to_s == "40" }
  puts "lambdas!"
end

## => "lambdas"

Also, can be used with your own match classes:

class Success
  def self.===(item)
    item.status >= 200 && item.status < 300
  end
end

class Empty
  def self.===(item)
    item.response_size == 0
  end
end

case http_response
when Empty
  puts "response was empty"
when Success
  puts "response was a success"
end

Answer 11

Depending on your case, you could prefer to use a hash of methods.

If there is a long list of whens and each of them has a concrete value to compare with (not an interval), it will be more effective to declare a hash of methods and then to call the relevant method from the hash like that.

# Define the hash
menu = {a: :menu1, b: :menu2, c: :menu2, d: :menu3}

# Define the methods
def menu1
  puts 'menu 1'
end

def menu2
  puts 'menu 2'
end

def menu3
  puts 'menu3'
end

# Let's say we case by selected_menu = :a
selected_menu = :a

# Then just call the relevant method from the hash
send(menu[selected_menu])

Answer 12

Since switch case always returns a single object, we can directly print its result:

puts case a
     when 0
        "It's zero"
     when 1
        "It's one"
     end

Answer 13

Multi-value when and no-value case:

print "Enter your grade: "
grade = gets.chomp
case grade
when "A", "B"
  puts 'You pretty smart!'
when "C", "D"
  puts 'You pretty dumb!!'
else
  puts "You can't even use a computer!"
end

And a regular expression solution here:

print "Enter a string: "
some_string = gets.chomp
case
when some_string.match(/\d/)
  puts 'String has numbers'
when some_string.match(/[a-zA-Z]/)
  puts 'String has letters'
else
  puts 'String has no numbers or letters'
end

Answer 14

You can write case expressions in two different ways in Ruby:

1. Similar to a series of if statements
2. Specify a target next to the case and each when clause is compared to the target.

age = 20
case 
when age >= 21
puts "display something"
when 1 == 0
puts "omg"
else
puts "default condition"
end

or:

case params[:unknown]
when /Something/ then 'Nothing'
when /Something else/ then 'I dont know'
end

Answer 15

You can do like this in more natural way,

case expression
when condtion1
   function
when condition2
   function
else
   function
end

Answer 16

Lots of great answers but I thought I would add one factoid.. If you are attempting to compare objects (Classes) make sure you have a space ship method (not a joke) or understand how they are being compared

"Ruby Equality And Object Comparison" is a good discussion on the topic.

Answer 17

As stated in many of the above answers, the === operator is used under the hood on case/when statements.

Here is additional information about that operator:

Case equality operator: ===

Many of Ruby's built-in classes, such as String, Range, and Regexp, provide their own implementations of the === operator, also known as "case-equality", "triple equals" or "threequals". Because it's implemented differently in each class, it will behave differently depending on the type of object it was called on. Generally, it returns true if the object on the right "belongs to" or "is a member of" the object on the left. For instance, it can be used to test if an object is an instance of a class (or one of its sub-classes).

String === "zen"  # Output: => true
Range === (1..2)   # Output: => true
Array === [1,2,3]   # Output: => true
Integer === 2   # Output: => true

The same result can be achieved with other methods which are probably best suited for the job, such as is_a? and instance_of?.

Range Implementation of ===

When the === operator is called on a range object, it returns true if the value on the right falls within the range on the left.

(1..4) === 3  # Output: => true
(1..4) === 2.345 # Output: => true
(1..4) === 6  # Output: => false

("a".."d") === "c" # Output: => true
("a".."d") === "e" # Output: => false

Remember that the === operator invokes the === method of the left-hand object. So (1..4) === 3 is equivalent to (1..4).=== 3. In other words, the class of the left-hand operand will define which implementation of the === method will be called, so the operand positions are not interchangeable.

Regexp Implementation of ===

Returns true if the string on the right matches the regular expression on the left.

/zen/ === "practice zazen today"  # Output: => true
# is similar to
"practice zazen today"=~ /zen/

The only relevant difference between the two examples above is that, when there is a match, === returns true and =~ returns an integer, which is a truthy value in Ruby. We will get back to this soon.

Answer 18

puts "Recommend me a language to learn?"
input = gets.chomp.downcase.to_s

case input
when 'ruby'
    puts "Learn Ruby"
when 'python'
    puts "Learn Python"
when 'java'
    puts "Learn Java"
when 'php'
    puts "Learn PHP"
else
    "Go to Sleep!"
end

Answer 19

$age =  5
case $age
when 0 .. 2
   puts "baby"
when 3 .. 6
   puts "little child"
when 7 .. 12
   puts "child"
when 13 .. 18
   puts "youth"
else
   puts "adult"
end

See "Ruby - if...else, case, unless" for more information.

Answer 20

I've started to use:

a = "secondcase"

var_name = case a
  when "firstcase" then "foo"
  when "secondcase" then "bar"
end

puts var_name
>> "bar"

It helps compact code in some cases.

Answer 21

It's critical to emphasize the comma (,) in a when clause. It acts as an || of an if statement, that is, it does an OR comparison and not an AND comparison between the delimited expressions of the when clause. See the following case statement:

x = 3
case x
  when 3, x < 2 then 'apple'
  when 3, x > 2 then 'orange'
end
 => "apple"

x is not less than 2, yet the return value is "apple". Why? Because x was 3 and since ',`` acts as an||, it did not bother to evaluate the expressionx < 2'.

You might think that to perform an AND, you can do something like this below, but it doesn't work:

case x
  when (3 && x < 2) then 'apple'
  when (3 && x > 2) then 'orange'
end
 => nil 

It doesn't work because (3 && x > 2) evaluates to true, and Ruby takes the True value and compares it to x with ===, which is not true, since x is 3.

To do an && comparison, you will have to treat case like an if/else block:

case
  when x == 3 && x < 2 then 'apple'
  when x == 3 && x > 2 then 'orange'
end

In the Ruby Programming Language book, Matz says this latter form is the simple (and infrequently used) form, which is nothing more than an alternative syntax for if/elsif/else. However, whether it is infrequently used or not, I do not see any other way to attach multiple && expressions for a given when clause.

Answer 22

No support for regular expressions in your environment? E.g. Shopify Script Editor (April, 2018):

[Error]: uninitialized constant RegExp

A workaround following a combination of methods already previously covered in here and here:

code = '!ADD-SUPER-BONUS!'

class StrContains
  def self.===(item)
    item.include? 'SUPER' or item.include? 'MEGA' or\
    item.include? 'MINI' or item.include? 'UBER'
  end
end

case code.upcase
when '12345PROMO', 'CODE-007', StrContains
  puts "Code #{code} is a discount code!"
when '!ADD-BONUS!'
  puts 'This is a bonus code!'
else
  puts 'Sorry, we can\'t do anything with the code you added...'
end

I used ors in the class method statement since || has higher precedence than .include?.

If you still prefer using ||, even though or is preferable in this case, you can do this instead: (item.include? 'A') || .... You can test it in this repl.it.

Answer 23

We can write switch statement for multiple conditions.

For Example,

x = 22

CASE x
  WHEN 0..14 THEN puts "#{x} is less than 15"    
  WHEN 15 THEN puts "#{x} equals 15" 
  WHEN 15 THEN puts "#{x} equals 15" 
  WHEN 15..20 THEN puts "#{x} is greater than 15" 
  ELSE puts "Not in the range, value #{x} " 
END

Answer 24

The case statement operator is like switch in the other languages.

This is the syntax of switch...case in C:

switch (expression)
​{
    case constant1:
      // statements
      break;
    case constant2:
      // statements
      break;
    .
    .
    .
    default:
      // default statements
}

This is the syntax of case...when in Ruby:

case expression
  when constant1, constant2 #Each when statement can have multiple candidate values, separated by commas.
     # statements 
     next # is like continue in other languages
  when constant3
     # statements 
     exit # exit is like break in other languages
  .
  .
  .
  else
     # statements
end

For example:

x = 10
case x
when 1,2,3
  puts "1, 2, or 3"
  exit
when 10
  puts "10" # it will stop here and execute that line
  exit # then it'll exit
else
  puts "Some other number"
end

For more information see the case documentation.

Answer 25

If you need "less than" or "greater than":

case x
when 1..5
  "It's between 1 and 5"
when 6
  "It's 6"
when 7..1.0/0
  "It's equal or greater than 7"
when -1.0/0..0
  "It's equal or less than 0"
end

1.0/0 is equal to Float::INFINITY, so you can use which you prefer.

Answer 26

I prefer to use case + than

number = 10

case number
when 1...8 then # ...
when 8...15 then # ...
when 15.. then # ...
end