How to write a switch statement in Ruby

Posted April 27, 2021 - tagged ruby  ‐  4 min read

In Ruby, the classic 'switch statement' is possible with the *case/when* keywords. Whenever if statements become too long and start looking messy you might want to consider using the case statement to write cleaner code.

So how do you switch to case/when ?

Case statements in Ruby are very powerful. To quote the popular ProgrammingRuby book, "The Ruby case expression is a powerful beast: a multiway if on steroids."

Let's begin by converting a small-sized if expression into a case/when statement. The following if expression

if name == "Peter Parker"
  hero_name = "Spiderman"
elsif name == "Bruce Wayne"
  hero_name = "Batman"
else
  hero_name = "unknown"
end

will look like this

case name
when "Peter Parker"
  hero_name = "Spiderman"
when name == "Bruce Wayne"
  hero_name = "Batman"
else
  hero_name = "unknown"
end

Important keywords

There are 3 important keywords that are used in the case statement:

  • case : It is similar to the switch keyword in other programming languages. It takes the variables that will be used by when keyword.

  • when : It is similar to the case keyword in other programming languages. It is used to match a single condition. There can be multiple when statements into a single case statement.

  • else : It is similar to the default keyword in other programming languages. It is optional and will execute when nothing matches.

case operates by comparing the target (the expression after the keyword case) with each of the comparison expressions after the when keywords. This test is done using comparison === target.

The key differences to note are that you no longer need to write the comparison operator or the then keyword. If you have any background with another programming language then you may have also noticed that with Ruby you don't need to end every individual case with a break clause.

Indentation of the case/when statement in Ruby

There is no official standard about this, however developers tend to put the when keyword on the same column as case (source : stackoverflow) :

case bar
when 4 then ...
when 2 then ...
else ...
end

Which is quite logical : think about the if/elsif indentation.

Case/When in Ruby : Advanced used... cases

Now it’s time to see more advanced use cases of the case expression. Inside your when clauses, you can do much more than single value comparisons. With case you can compare ranges, classes, regular expressions, multiple values, and much more. The following is a comprehensive example of the many ways you can use a case statement :

def switch_me(something)
  case something
  when /^1/ 
    "param starts with one" # return keyword is optional
  when 80..90
    "B+"
  when 50 then "param is 50" #then keyword allows return on same line
  when 10, 20
    "param is either 10 or 20"
  when String
    "You passed a String"
  end
end

In your Ruby console :

$> switch_me("111")
=> "param starts with one"

$> switch_me(82)
=> "B+"

$> switch_me(50)
=> "param is 50"

$> switch_me(10)
=> "param is either 10 or 20"

$> switch_me("20")
=> "You passed a String"

$> switch_me(42)
=> nil

Let's walk through each individual case:

  • The first when clause compares if var matches the regular expression like so /^1/ === something. As you can see, the comparison comes before the target of comparison.

  • In the second when clause var will be compared to the range 80..90.

  • In the third when clause we have done a simple comparison but instead of putting our return expression on the next line we have shortened it by using the then keyword.

  • In the fourth when clause there are multiple conditions. Each of these conditions will be checked sequentially. So first 10 === var and then 20 === var.

And finally, in our last when clause we check if the first param is a String object. You may have noticed that there is a puts keyword before the case statement. This is possible because as with if, case returns the value of the last expression executed so it can be input to a method call.

Another interesting way is to use the case statement like an if-else statement. You cannot give a value to case if using it like this :

def case_like_ifelse(param1)
  case
  when param1 == 1, param1 == 2
    "param1 is one or two"
  when param1 == 3
    "param1 is three"
  else
    "unknown"
  end
end

In your Ruby console :

$> case_like_ifelse(1)
=> "param1 is one or two"

$> case_like_ifelse(3)
=> "param1 is three"

$> case_like_ifelse(3)
=> "unknown"

Now you’re all set to write complex switch control expressions in Ruby!

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