Ruby Block语法

Question

I was going through some code I found online and found the following

def change input 
   ('a'..'z').map { |letter| input.downcase.include?(letter) ? '1' : '0' }.join
end

I understand what this code is doing. It will take a string, check if the string contains each letter of the alphabet and return 1 if true and 0 if false.

However I am unfamiliar with this bit of syntax:

?(letter) ? '1' : '0' }

I know that a question mark is usually used to indicate that the method will return a boolean result. But I am insure why there is a second question mark after the argument.

Also, I understand that this will return 1 if true and 0 if false. Is that what this colon represents. Is it always ok to use a colon like this if the result of the method in the block will be a boolean?

Answer 1

The format boolean_expression ? option_a : option_b boolean_expression ? option_a : option_b is called a ternary operator. It is short for

if boolean_expression
  option_a
else
  option_b
end

Answer 2

The first question mark is part of the #include? method

The expession condition ? if_true : if_false condition ? if_true : if_false is called a ternary operator, which is shorthand for

if condition
  if_true
else
  if_false
end