[ruby] How do I use the conditional operator (? :) in Ruby?

How is the conditional operator (? :) used in Ruby?

For example, is this correct?

<% question = question.size > 20 ? question.question.slice(0, 20)+"..." : question.question %>

Easiest way:

param_a = 1
param_b = 2

result = param_a === param_b ? 'Same!' : 'Not same!'

since param_a is not equal to param_b then the result's value will be Not same!

puts true ? "true" : "false"
=> "true"


puts false ? "true" : "false"
=> "false"

A simple example where the operator checks if player's id is 1 and sets enemy id depending on the result

player_id=1
....
player_id==1? enemy_id=2 : enemy_id=1
# => enemy=2

And I found a post about to the topic which seems pretty helpful.

@pst gave a great answer, but I'd like to mention that in Ruby the ternary operator is written on one line to be syntactically correct, unlike Perl and C where we can write it on multiple lines:

(true) ? 1 : 0

Normally Ruby will raise an error if you attempt to split it across multiple lines, but you can use the \ line-continuation symbol at the end of a line and Ruby will be happy:

(true)   \
  ? 1    \
  : 0

This is a simple example, but it can be very useful when dealing with longer lines as it keeps the code nicely laid out.

It's also possible to use the ternary without the line-continuation characters by putting the operators last on the line, but I don't like or recommend it:

(true) ?
  1 :
  0

I think that leads to really hard to read code as the conditional test and/or results get longer.

I've read comments saying not to use the ternary operator because it's confusing, but that is a bad reason to not use something. By the same logic we shouldn't use regular expressions, range operators ('..' and the seemingly unknown "flip-flop" variation). They're powerful when used correctly, so we should learn to use them correctly.

Why have you put brackets around true?

Consider the OP's example:

<% question = question.size > 20 ? question.question.slice(0, 20)+"..." : question.question %>

Wrapping the conditional test helps make it more readable because it visually separates the test:

<% question = (question.size > 20) ? question.question.slice(0, 20)+"..." : question.question %>

Of course, the whole example could be made a lot more readable by using some judicious additions of whitespace. This is untested but you'll get the idea:

<% question = (question.size > 20) ? question.question.slice(0, 20) + "..." \
                                   : question.question 
%>

Or, more written more idiomatically:

<% question = if (question.size > 20)
                question.question.slice(0, 20) + "..."
              else 
                question.question 
              end
%>

It'd be easy to argument that readability suffers badly from question.question too.

Your use of ERB suggests that you are in Rails. If so, then consider truncate, a built-in helper which will do the job for you:

<% question = truncate(question, :length=>30) %>

The code condition ? statement_A : statement_B is equivalent to

if condition == true
  statement_A
else
  statement_B
end