Conditional Execution in Ruby

Booleans

There are three boolean values in Ruby: true, false & nil.

They are all built-in Ruby objects.

false & nil are the only falsy values in Ruby. All other expressions, including true, 0 (zero) and "" (empty string), are truthy. This disambiguation is important especially with regards to control flow in Ruby.

puts "I will be printed!" if true # => "I will be printed!"
puts "I won't be printed!" if false # => nil
puts "I won't be printed as well!" if nil # => nil

if

if expression controls whether a selected expression is executed or not.

if can be used as a multi-line expression:

if should_do_something?
 do_something
end

if can also be used as a single line expression:

do_something if should_do_something?

In both cases do_something is executed if should_do_something? evaluates to a truthy value.

if elsif

if expression can also be extended with an elsif keyword. The code following the elsif keyword is executed provided should_do_a? evaluates to a falsy value and should_do_b? evaluates to a truthy value.

if should_do_a?
  do_a
elsif should_do_b?
  do_b
end

if else

if expression can be extended with an else keyword. The code following the else keyword is executed provided should_do_a? evaluates to a falsy value.

if should_do_a?
  do_a
else
  do_b
end

if elsif else

if, elsif and else can be combined.

if expression can be extended with both elsif and else keywords. The code following the if expression is executed provided should_do_a? evaluates to a truthy value. The code following the elsif keyword is executed provided should_do_a? evaluates to a falsy value and should_do_b evaluates to a truthy value. The code following the else keyword is executed provided both should_do_a? and should_do_b evaluate to falsy values.

if should_do_a?
  do_a
elsif should_do_b?
  do_b
else
  do_c
end

case

case expression can be used to check for multiple conditions.

case foo
when condition_a
  do_a
when condition_b
  do_b
else
  do_c
end

The conditions are being evaluated using === method where the condition is the receiver and the expression following the case keyword is the argument. In other words when truthfulness of the condition_a is being evaluate it sends message === to condition_a with the argument of foo.

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