Books / Ruby for Beginners / Chapter 11

Integer Class in Ruby

Methods Of Integer Class

There aren’t too many methods in Integer class, and it’s worth looking at documentation to get a better understanding of what’s there available for a programmer. However, we’ll take a close look to some of them.

even? and odd?

even? or odd? – two methods of Integer class, method names “come” with question mark at the end.

We can check any Integer if it’s even or odd using two methods above. Since question mark at the end of the method comes up for the first time in this book, let’s see why it’s there.

Question mark just indicates that method returns type Boolean. Well, technically there is no any Boolean in Ruby, we’ve just introduced it for the purpose of this book. There are two actual types you will see: TrueClass and FalseClass. In other words, there is true and false keywords in Ruby language, and you can use these keywords (see examples below).

Something unambiguous in real life can be represented by true or false, usually these methods start with prefix “is_”. For example:


There is no any other options possible when you’re using boolean, it’s either true or false. Question mark is optional in Ruby language, but expected by community. It’s highly recommended to have “?” and the end of methods that return boolean value.

Let’s look at example:

$ irb
> 1.even?
> 1.odd?
> 2.even?
> 2.odd?
> 10 % 2 == 0 # our own implementation of "even?" method

The last line from our REPL dialog has two parts:

  • 10 % 2 - divide by two, and returns remainder (which will be 0 or 1)
  • == 0 - comparison with zero, returns true or false

upto and downto

upto and downto methods of Integer class accept parameter and call provided block for certain amount of times. We’re already familiar with times method, which starts counting from zero. You can do the same with upto. For example:

> 3.times { |i| puts "I'm robot #{i}" }
I'm robot 0
I'm robot 1
I'm robot 2
> 0.upto(2) { |i| puts "I'm robot #{i}" }
I'm robot 0
I'm robot 1
I'm robot 2

The output is the same, but upto is more flexible, because it accepts parameter (2 in program above). With this parameter we can specify “from” and “to” values, like:

> 1000.upto(1002) { |i| puts "I'm robot #{i}" }
I'm robot 1000
I'm robot 1001
I'm robot 1002

downto is similar, but it counts backwards:

puts "Launching missiles..."
5.downto(1) { |i| puts "#{i} seconds left" }
puts "Boom!"


Launching missiles...
5 seconds left
4 seconds left
3 seconds left
2 seconds left
1 seconds left

Well, of course you can do block with do...end and result will be the same:

puts "Launching missles..."
5.downto(0) do |i|
  puts "#{i} seconds left"
puts "Boom!"

Exercise 1 Put all numbers to the screen starting from 50 to 100.

Exercise 2 Put all numbers to the screen starting from 50 to 100 with parity next to the number. If number is even, print “true”, if it’s odd, print “false”.

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