Gain insights into Ruby development with 78 tasks tailored for beginners and experienced coders. Explore object-oriented concepts, debug programs, and advance your freelance coding career.

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This self-study course written as a book is your first step towards making money by coding and programming from home, and having fun doing it. You will understand how the Ruby language works, and establish a foundation for being a successful freelance developer. This course gives you not only technical detail, but presents knowledge in a friendly format, understandable by all ages, providing a thorough explanation of modern concepts for absolute beginners.

In this course you will find 78 easy to follow tasks, so even people without any programming skill can solve them. Beginners will learn how to create, debug, and understand programs in Ruby. Those who are already familiar with the language will rediscover object-oriented concepts, memory organization, find career advice, and save the planet from a nuclear apocalypse.

Ready to dive into one of the most influential and demanding third generation programming languages? Let's tune in!

The Handbook for Ruby Developers

## Iteration over nested data structure
Let's look at the following nested data structure:

```ruby
users = [
    { first: 'John', last: 'Smith', address: { city: 'San Francisco', country: 'US' } },
    { first: 'Pat', last: 'Roberts', address: { country: 'US' } },
    { first: 'Sam', last: 'Schwartzman' }
]
```

The structure above has its data scheme. The format is the same for every record, and there are three total records in this array, but the two last records are missing something. For example, the second one is missing the `city`. The third record doesn’t have an `address`. What we want is to print all the cities from all the records, and in the future we might have more than three.

The first thing that comes to our mind is iteration over the array of elements and using standard hash access:

```ruby
users.each do |user|
  puts user[:address][:city]
end
```

Let's give it a try:

```ruby
San Francisco

-:8:in `block in <main>': undefined method `[]' for nil:NilClass (NoMethodError).
```

As we can see, it produces an error. But why? Well, let’s try to access every element manually

```ruby
$ pry
> users[0][:address][:city]
=> "San Francisco"
> users[1][:address][:city]
=> nil
> users[2][:address][:city]
NoMethodError: undefined method `[]' for nil:NilClass
```

Here we go. It works for the first element. There is also no error for the second element, and the result is just `nil`. However, for the third user, with index 2, the expression `users[2][:address]` gives `nil` because there is no `address` field for Sam. Then, we execute `nil[:city]`, which always produces an error because we can’t access nil like that because there is nothing inside`nil`.


## Fixing the error
So, how do we fix this program? We can use an `if` statement:

```ruby
users.each do |user|
  if user[:address]
    puts user[:address][:city]
  end
end
```

It works now, and there is no error; we did a great job! But what if we add one more nested object to `address` so that we always have two lines of street address for the `address` node?

```ruby
street: { line1: '...', line2: '...' }
```

Here's how it looks:

```ruby
users = [
    {
        first: 'John',
        last: 'Smith',
        address: {
            city: 'San Francisco',
            country: 'US',
            street: {
                line1: '555 Market Street',
                line2: 'apt 123'
            }
        }
    },
    { first: 'Pat', last: 'Roberts', address: { country: 'US' } },
    { first: 'Sam', last: 'Schwartzman' }
]
```

Now, we want to print all `line1` addresses for all the records. The first thing we will do is to improve the already existing program by adding `[:line1]` navigation:

```ruby
users.each do |user|
  if user[:address]
    puts user[:address][:street][:line1]
  end
end
```

However, the code above will not work on the second record, because for the second record,`user[:address][:street]` is `nil`. If that’s not clear, we should try it in our `pry` or `irb` console.

We can add another check for `nil`:

```ruby
users.each do |user|
  if user[:address] && user[:address][:street]
    puts user[:address][:street][:line1]
  end
end
```

It works great with the second condition. Now, we will check if `address` is not `nil` and if the following `street` is not `nil`:

```ruby
if user[:address] && user[:address][:street]
```

In other words, for every level of nesting, we will need to add one more check, so our program won’t raise any errors on `nil`. It wasn’t very convenient and programmer-friendly, so Ruby starting from 2.3.0—you can check your version by running `ruby -v`—has the `dig` method:

```ruby
users.each do |user|
  puts user.dig(:address, :street, :line1)
end
```

# Iteration over nested data structure
Let's look at the following nested data structure:

```ruby
users = [
    { first: 'John', last: 'Smith', address: { city: 'San Francisco', country: 'US' } },
    { first: 'Pat', last: 'Roberts', address: { country: 'US' } },
    { first: 'Sam', last: 'Schwartzman' }
]
```

The structure above has its data scheme. The format is the same for every record, and there are three total records in this array, but the two last records are missing something. For example, the second one is missing the `city`. The third record doesn’t have an `address`. What we want is to print all the cities from all the records, and in the future we might have more than three.

The first thing that comes to our mind is iteration over the array of elements and using standard hash access:

```ruby
users.each do |user|
  puts user[:address][:city]
end
```

Let's give it a try:

```ruby
San Francisco

-:8:in `block in <main>': undefined method `[]' for nil:NilClass (NoMethodError).
```

As we can see, it produces an error. But why? Well, let’s try to access every element manually

```ruby
$ pry
> users[0][:address][:city]
=> "San Francisco"
> users[1][:address][:city]
=> nil
> users[2][:address][:city]
NoMethodError: undefined method `[]' for nil:NilClass
```

Here we go. It works for the first element. There is also no error for the second element, and the result is just `nil`. However, for the third user, with index 2, the expression `users[2][:address]` gives `nil` because there is no `address` field for Sam. Then, we execute `nil[:city]`, which always produces an error because we can’t access nil like that because there is nothing inside`nil`.


# Fixing the error
So, how do we fix this program? We can use an `if` statement:

```ruby
users.each do |user|
  if user[:address]
    puts user[:address][:city]
  end
end
```

It works now, and there is no error; we did a great job! But what if we add one more nested object to `address` so that we always have two lines of street address for the `address` node?

```ruby
street: { line1: '...', line2: '...' }
```

Here's how it looks:

```ruby
users = [
    {
        first: 'John',
        last: 'Smith',
        address: {
            city: 'San Francisco',
            country: 'US',
            street: {
                line1: '555 Market Street',
                line2: 'apt 123'
            }
        }
    },
    { first: 'Pat', last: 'Roberts', address: { country: 'US' } },
    { first: 'Sam', last: 'Schwartzman' }
]
```

Now, we want to print all `line1` addresses for all the records. The first thing we will do is to improve the already existing program by adding `[:line1]` navigation:

```ruby
users.each do |user|
  if user[:address]
    puts user[:address][:street][:line1]
  end
end
```

However, the code above will not work on the second record, because for the second record,`user[:address][:street]` is `nil`. If that’s not clear, we should try it in our `pry` or `irb` console.

We can add another check for `nil`:

```ruby
users.each do |user|
  if user[:address] && user[:address][:street]
    puts user[:address][:street][:line1]
  end
end
```

It works great with the second condition. Now, we will check if `address` is not `nil` and if the following `street` is not `nil`:

```ruby
if user[:address] && user[:address][:street]
```

In other words, for every level of nesting, we will need to add one more check, so our program won’t raise any errors on `nil`. It wasn’t very convenient and programmer-friendly, so Ruby starting from 2.3.0—you can check your version by running `ruby -v`—has the `dig` method:

```ruby
users.each do |user|
  puts user.dig(:address, :street, :line1)
end
```

Learn what the dig method is and how to use it in Ruby.

Before We Begin

First Steps

Fundamentals of Ruby

Conditional Statements

Operators and Methods

Arrays

Hash

Fun with Object Oriented Programming

Debugging in Ruby

Ruby Version Manager

Testing Ruby Programs

Wrap Up!

Appendix

The “dig” method

Iteration over nested data structure