Ruby Examples > Conditionals 

#---If, Else, Unless and Case When---

In Ruby, conditionals are used to run code based on certain conditions. They allow your program to make decisions and execute different pieces of code depending on whether a condition is true or false.

Ruby provides several constructs for implementing conditionals, including if, unless, case-when, and ternary operators.

The if statement

if is the most common conditional. It executes a block of code if the supplied condition evaluates to true.

 
age = 25
if age >= 18
  p "You can drive now"
end 
## => "You can drive now"

You can add additional conditions using elsif and provide a default case with else.

 
age = 16
if age >= 18
  p "You can drink now"
elsif age >= 13
  p "You can't drink now"
else
  p "You can drink water"
end
## => "You can't drink now"

The unless statement

unless is the opposite of if. It executes a block of code if the condition is false.

 
raining = false
unless raining
  p "Let's sprint up the hill"
end
## =>  "Let's sprint up the hill"

If unless condition is hard to grok, you can totally replace it with if !condition, or if not condition (not is a ruby keyword).
It’s all the same.

 
day_time = true
if not day_time
  p "Let's train in the garage"
elsif !raining
  p "Let's sprint up the hill"
else
  p "Let's netflix and chill"
end
## =>  "Let's sprint up the hill"

The cognitive penalty of unless

Just like if-else, there’s also unless-else.

 
unless day_time
  p "Let's train in the garage"
else
  p "Let's sprint up the hill"
end

But don’t use this. It doesn’t read natural.
Instead, invert the condition and make it an if.

 
if day_time
  p "Let's sprint up the hill"
else
  p "Let's train in the garage"
end

And whether you’re using it with an else or not, don’t chain multiple conditions to it. It’s hard to make sense of the full condition.

 
run unless (x == y) || !(z != y) && friday? rescue nil

On second thought, it doesn’t make sense with if too. It’s better to extract these separate conditions into separate methods that are well-named.

 
run if friday? && joe_mad? rescue nil

Note: The rescue nil is only because the methods run , friday?, joe_mad? are all undefined.

All of this is not to say unless is bad. Use it like you would in plain English. Like, “Walk the dog unless it’s raining”.

Assigning the result to a variable

In Ruby every statement evaluates to a single expression. That means the whole if-else statement, despite spanning multiple lines, evaluates to a single value, noted by the last line of either of the blocks.
That value can be assigned to a variable.

So instead of writing this:

 
msg = ""
if day_time
  thumbs_up = '👍' * 2
  msg = "Hey, you're a cool person #{thumbs_up}"
else
  thumbs_up = '👍'
  msg = "Hey, you're a cool person #{thumbs_up}"
end

You can write this:

 
msg = if day_time
        thumbs_up = '👍' * 2
        "Hey, you're a cool person #{thumbs_up}"
      else
        thumbs_up = '👍'
        "Hey, you're a cool person #{thumbs_up}"
      end
p msg # => "Hey, you're a cool person 👍👍"

Inline if, unless

If the code you have to run based on a condition is just a single line, often you can write the whole thing as a one-liner.

Instead of this:

 
def greet(name)
  if name
    "You're stupid awesome #{name}!"
  else
    "You're stupid awesome!"
  end
end

You can write this:

 
def greet(name)
  return "You're stupid awesome!" unless name
  "You're stupid awesome #{name}!"
end

Ternary operator

Inside the if block, if you don’t have very many things to do except to just return a value for a variable, use the ternary operator instead to write simple one-line if-else statements.

It takes 3 operands: a condition, a value if true, and a value if false.

 
require 'date'
mood = Date.today.month <= 6 ? '😎' : '😬'
p mood # => "😎"
centurion = [99, 100, 98].include?(100) ? true : false
p centurion # => true

The Case When statement

This is Ruby’s “Switch” statement. Use it when you need multiple “if else” logic.

 
grade = 'B'
case grade
when 'A'
  p 'Nice'
when 'B'
  p 'Meh'
else
  p 'lol'
end
## => "Meh"

But you can do so much more:

 
def case_demo(grade)
  case grade
  when 'A'..'C'
    'within A and C'
  when /^-/
    'negative grade!'
  when Numeric
    'number grade'
  when 'X', 'Y'
    'grade is specifically X or Y'
  else
    'unknown grade'
  end
end
p case_demo('A')  # => "within A and C"
p case_demo(77)   # => "number grade"
p case_demo('-A') # => "negative grade!"
p case_demo('X')  # => "grade is specifically X or Y"
p case_demo('m')  # => "unknown grade"

To understand how it works, you should know about the === method that’s implemented on a handful of ruby classes.

This is how Ruby sees the above case statement:

 
if ('A'..'C').===(grade)
  'within A and C'
elsif /^-/.===(grade)
  'negative grade!'
elsif Numeric.===(grade)
  'number grade'
elsif 'X'.===(grade) || 'Y'.===(grade)
  'grade is specifically X or Y'
else
  'unknown grade'
end

As you can see, the === method does different things based on the class it’s implemented for.
  • for a range, it checks if the element is within the range specified
  • for a regex, it checks if element matches the pattern
  • for a proc, it calls the proc with the given argument
  • for all other simple types (int, float, string), it simply checks equality

Try adding one more codition to case_demo with a proc that returns a bool:

 
## A proc that returns a bool
a_proc = -> { it.is_a?(Numeric) && it.even? }
p a_proc.===(42) # => true

Also, the === thing is named confusingly. It’s usually called the “triple equals operator” or “case equality operator”.
But it’s neither an operator, nor about equality.
Jörg W Mittag calls it the “case subsumption operator” and I fully endorse it.

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