Shortcut to make case/switch return a value

问题:

I'm pretty sure I saw someone do a shortcut technique like the code below (which doesn't work)

return case guess
  when guess > @answer then :high
  when guess < @answer then :low
  else :correct
end

Does anyone know the trick I'm referring to?

回答1:

This works:

return case
  when guess > @answer ; :high
  when guess < @answer ; :low
  else ; :correct
  end

回答2:

A case statement does return a value, you just have to use the right form of it to get the value you're expecting.

There are two forms of case in Ruby. The first one looks like this:

case expr
when expr1 then ...
when expr2 then ...
else ...
end

This will compare expr with each when expression using === (that's a triple BTW) and it will execute the first then where === gives a true value. For example:

case obj
when Array then do_array_things_to(obj)
when Hash  then do_hash_things_to(obj)
else raise 'nonsense!'
end

is the same as:

if(Array === obj)
  do_array_things_to(obj)
elsif(Hash === obj)
  do_hash_things_to(obj)
else
  raise 'nonsense!'
end

The other form of case is just a bunch of boolean conditions:

case
when expr1 then ...
when expr2 then ...
else ...
end

For example:

case
when guess > @answer then :high
when guess < @answer then :low
else :correct
end

is the same as:

if(guess > @answer)
  :high
elsif(guess < @answer)
  :low
else
  :correct
end

You're using the first form when you think you're using the second form so you end up doing strange (but syntactically valid) things like:

(guess > @answer) === guess
(guess < @answer) === guess

In either case, case is an expression and returns whatever the matched branch returns.

回答3:

You need to remove the guess from the case, because it's not valid ruby syntax.

For example:

def test value
  case 
  when value > 3
    :more_than_3
  when value < 0
    :negative
  else
    :other
  end
end

Then

test 2   #=> :other 
test 22  #=> :more_than_3 
test -2  #=> :negative 

The return is implicit.

Edit: you can use then if you like too, the same example would look like this:

def test value
  case 
    when value > 3 then :more_than_3
    when value < 0 then :negative
    else :other
  end
end

回答4:

Picking up on @mu's first comment on the question (an approach that looks fine to me), you could of course also write that as:

return case (guess <=> @answer)
       when -1 then :low
       when  0 then :correct
       when  1 then :high
       end

or

       ...
       else
         :high
       end