This is called ** early return ** and is often used to avoid ** rare cases or deep nesting of conditional statements. ** **

def greeting(country)
  #If country is nil, return a message and exit the method
  return 'Please enter country' if country.nil?
  
  #country'japan'If'Hello'And exit the method
  return 'Hello' if country == 'japan'
  #Otherwise'hello'Returns
  'Hello'
end
greeting(nil)     #=> "Please enter country"
greeting('japan') #=> "Hello"
greeting('us')    #=> "Hello"

3. Return value of an expression containing logical operators

Finally, I will explain the points that are often overlooked with expressions that use logical operators.

&&Or||Return value and timing of the end of evaluation

Logical operator&&Or||Care must be taken regarding the return value of the formula when using.

See the results below.

#Example 1
'apple' && 'banana' && 'grape' #=> "grape" 
#Example 2
'apple' && nil && 'grape'      #=> nil
'apple' && false && 'grape'    #=> false

#Example 3
nil || false                        #=> false
false || nil                        #=> nil
nil || false || 'apple' || 'banana' #=> 'apple'

As you can see, it is not necessarily ** true or false **.

** This is related to Ruby evaluating expressions from the left side until it is determined that the whole expression is true or false. ** **

Then, when the truth or falseness of the entire expression is confirmed, the evaluation of the expression is completed, and the value of the last evaluated expression is returned.

When the second nil was evaluated , it was decided that the boolean value of the entire expression was false , so the evaluation was finished at that point and ** < code> Returns the last nil **. The same is true for false

-** In case of Example 3 **

||The idea is the same in the case of, for example, in the case of the first line, since the truth value of the entire expression was fixed at the last point, it is the value of the last expression.**false**Is returned as a return value. The same applies to the second line.

Also, in the case of the third line, the return value is'apple' because the boolean value of the entire expression was determined when the expression 'apple' was reached.

As a further point to note, in the case of if statements, you do not have to be so conscious,In situations other than if statements&&Or||When intentionally usingThere is also.

Therefore, if you see these logical operators, be careful to see if they are used as described above.

Control flow using and and or

Finally above&&、||There are and and or as operators that work in the same way as, but I will briefly touch on how they are used.

and and or are basically&&When||When同じですが、いくつかの場面において違った結果を返すこWhenがあります。

t1 = true
t2 = false
f1 = false

t1 || t2 && f1 #=> true
t1 or t2 and f1 #=> false

# &&Is||Higher priority than
t1 || (t2 && f1)
#Since the priority of and and or is the same, they are evaluated in order from the left.
(t1 or t2) and f1

def greeting(country)
  #If country is nil or false, return a message and exit the method
  country or return 'Please enter country'

  return 'Hello' if country == 'japan'
  'hello'
end

greeting(nil)     #=> "Please enter country" 
greeting('japan') #=> "Hello"
greeting('us')    #=> "hello"