Conditionals

If this is true, then do that. Otherwise do something else.

Often we want to check for a certain condition, and then based on it, do either one thing or another. Or we want to do something only if a condition is true.

All practical programming languages have some way of expressing this, and in Ruby it looks like so:

number = 5
if number.between?(1, 10)
  puts "The number is between 1 and 10"
elsif number.between?(11, 20)
  puts "The number is between 11 and 20"
else
  puts "The number is bigger than 20"
end

You can probably guess what it does, and how it works, can’t you?

Let’s walk through it one by one:

• If you run this code it will print out The number is between 1 and 10, because the number assigned to the variable number on the first line is 5, and for this number the method call number.between?(1, 10) returns true. Ruby will therefore execute the code in the if branch: The if branch is the block of code that comes after the line with the if statement, and that is indented by two spaces. Once it is done executing the if branch Ruby will simply ignore the rest of the statement.

• If you change the number 5 on the first line to 15, and run the code again, then it will print out The number is between 11 and 20. In this case Ruby will, again, first check the first condition number.between?(1, 10), but this time this method call returns false. Therefore, Ruby will ignore the if branch, and check the next condition on the elsif line: number.between?(11, 20). Now, this method call returns true, because 5 is between 11 and 20. Ruby will therefore execute the elsif branch, and print out this message. Again, once it is done executing the elsif branch Ruby will ignore the rest of the statement.

• If you now change the number 15 to 25, and run the code again, then it will print out The number is bigger than 20. Again, Ruby will first check the first condition, and find it returns false. It will check the second condition, which now also returns false. Therefore Ruby will then execute the else branch, and print out that message.

The elsif and else statements and branches are optional: you don’t need to have them. You can have an if statement without elsif or else branches, an if statement only with an else, or you could have an if statement with one or more elsif statements. Or combine all of them together:

• There must be an if statement and branch.
• There can be many elsif statements and branches.
• There can be one else statement and branch.