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Learning Exercise
Crystal has a type called Nil, which can only have one value: nil.
It is used to represent the absence of a value, and is similar to null or None in other languages.
Nil values can be returned from various methods for example String#[]?, which returns nil if the index is out of bounds.
"foo"[10]? # => nil
Crystal has what is known as NULL REFERENCE CHECKS, which means that all types are non-nilable by default.
This means you can not assign nil to a variable unless you explicitly declare it as a Nil type.
In turn this means that the compiler will automatically check for null references.
Certain approaches can be taken to handle nil values.
nil is a falsey value as well are false.
This means that statements and other places where a falsey or truthy value is expected it will be treated the same as false.
if nil
puts "nil is truthy"
else
puts "nil is falsey"
end
# => nil is falsey
This means that the truthy branch won't be taken if a nilable variable has a value of nil.
foo = "boo"[2]? # It is possible that foo is nil
if foo
p foo.class # => Char
end
Crystal has a method that can be used to check if a value is nil.
It is called nil? and it is available on all objects.
foo = "boo"[2]?
if foo.nil?
puts "foo is nil"
else
puts "foo is not nil"
end
This is a bit different when just using the value in an if statement, since in the last examples would both false and nil be treated as false.
Here only nil is treated as falsey, since if it was false it wouldn't have been nil thereby it would have been truthy.
The easiest way to deal with nil values is to ensure that they never become nil in the first place.
The or operator (||) is often used when dealing with Bools, but if understood correctly, it can also be used to deal with nil values.
The or operator checks if the first value is truthy, if not the second value is used.
This can be used to make if the value is nil it will be falsey and thereby the second value will be used.
"foo"[10]? || "bar" # => "bar"
Your majesty is hosting a dinner party in the Kalmar Unions capital Copenhagen. It is the biggest party of the year and all the nobles are invited. But rumors are spreading across the country that there is a plot to poison Your Majesty.
Her majesty has asked you, the royal poison finder, to find out if any food or drinks are poisoned. Your majesty is planning to serve only Mushroom pasties as food, but for drinks, there are a variety of drinks.
There is a high chance that the food is poisoned if it is not what her majesty ordered. If the food is not what her majesty ordered, it should be tossed out.
Implement the check_food? method, which takes the argument food which holds the name of the food as a String.
The method should return the food if it is Mushroom pasties; otherwise, it should return nil.
CastleDinner.check_food?("Mushroom pasties")
# => "Mushroom pasties"
CastleDinner.check_food?("Bread")
# => nil
It is possible that the drink is poisoned, but it is a bit more difficult to tell if it is.
The drink is poisoned if it does not include the letter i in the name, the casing doesn't matter.
The drink should be tossed out if it is poisoned.
Implement the check_drink? method, which takes the argument drink which holds the drink's name as a String.
The method should return the drink if it is not poisoned, otherwise it should return nil.
CastleDinner.check_drink?("Apple juice")
# => "Apple juice"
CastleDinner.check_drink?("Tea")
# => nil
Your majesty wouldn't want to make their guests worried about poison in their food. Therefore, if the drink is poisoned, it should be replaced with a new drink, specifically Apple juice.
Implement the replace_drink method, which takes the argument drink which holds the drink's name as a String.
The method should return the drink if it is not poisoned, otherwise it should return "Apple juice".
CastleDinner.replace_drink("Orange juice")
# => "Orange juice"
CastleDinner.replace_drink("Tea")
# => "Apple juice"
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