Here’s an example of a simple if-statement:
a_number = 5
if a_number < 10
puts "#{a_number} is less than 10"
end
This outputs:
PS C:\temp\irb> .\if-else.rb PS C:\temp\irb> ruby .\if-else.rb 5 is less than 10 PS C:\temp\irb>
Now here's how an if-else statement looks like:
a_number = 13
if a_number < 10
puts "#{a_number} is less than 10"
else
puts "#{a_number} is greater than 10"
end
This outputs:
PS C:\temp\irb> ruby .\if-else.rb 13 is greater than 10 PS C:\temp\irb>
Now here's an example of an elsif statment (note, it is spelt "elsif" and not "elseif"):
a_number = 13
if a_number < 10
puts "#{a_number} is less than 10"
elsif a_number == 13
puts "#{a_number} is an unlucky number"
else
puts "#{a_number} is greater than 10"
end
This outputs:
PS C:\temp\irb> ruby .\if-else.rb 13 is an unlucky number PS C:\temp\irb>
In ruby, only the keywords "nil" and "false" evaluates to be false. Everything else (including empty strings/arrays and "0") evaluates to be true. Here are some examples:
class Human
attr_reader :name
def initialize(surname)
@name = surname
end
def greeting
puts "hello everybody"
false
end
def hungry
puts "I am hungry"
true
end
end
john = Human.new(nil)
if john.name
puts "john has a surname which is '#{john.name}'"
else
puts "john has no surname"
end
if john.greeting
puts "this is a geniune greeting"
else
puts "this is a sarcastic greeting"
end
if john.hungry
puts "John is genuinely hungry"
else
puts "John is lying and isn't that hungry"
end
This outputs:
PS C:\temp\irb> ruby .\if-else.rb john has no surname hello everybody this is a sarcastic greeting I am hungry John is genuinely hungry PS C:\temp\irb>
One of the things commonly done, is to use an if-else statment to determine a value for a variable. Here is an example where we are trying to determine the value of the variable called "message":
a_number = 13
if a_number < 10
puts "#{a_number} is less than 10"
message = "Hence this is a small number."
else
puts "#{a_number} is greater than 10"
message = "Hence this is a big number."
end
puts message
This outputs:
PS C:\temp\irb> ruby .\if-else.rb 13 is greater than 10 Hence this is a big number. PS C:\temp\irb>
You can rewrite this in a way to show to show that you are capturing the output of the if statement, like this:
a_number = 13
message = if a_number < 10
puts "#{a_number} is less than 10"
"Hence this is a small number."
else
puts "#{a_number} is greater than 10"
"Hence this is a big number."
end
puts message
If you have a really simple if-else statement, e.g.:
age = 15 price = if age <= 16 "half price" else "full price" end puts price # outputs: "half price"
Then it might be worth writing this if-else statement on a single line without sacrificing readability:
age = 15 price = if age <= 16 then "half price" else "full price" end puts price # outputs: "half price"
Notice the inclusion of the "then" keyword here. You only use the "then" keyword when you squeeze an if-statement into one line, as we have done above.
If you just have a very simply if statement, and no else statement, then you can have even shorter single line if statements, here's an example:
def success_message puts "this is a success" end def result true end success_message if result # outputs "this is a success"