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Iterators

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Iterators are a fundamental concept in Ruby, allowing you to traverse collections such as arrays and hashes. They provide a way to access each element in a collection without needing to use explicit indexing. This section will cover the basics of iterators, common iterator methods, and practical examples to help you understand how to use them effectively.

Key Concepts

  1. Definition of Iterators: Methods that allow you to loop through a collection.
  2. Common Iterator Methods: each, map, select, reject, find, inject, etc.
  3. Blocks: Code enclosed in {} or do...end that is passed to iterators.
  4. Yielding: The process of passing control from the iterator method to the block.

Common Iterator Methods

each

The each method is the most basic iterator. It loops through each element in a collection and executes the block of code provided.

# Example using an array
[1, 2, 3, 4, 5].each do |number|
  puts number
end

# Example using a hash
{a: 1, b: 2, c: 3}.each do |key, value|
  puts "#{key}: #{value}"
end

map

The map method creates a new array containing the values returned by the block.

# Example using an array
squared_numbers = [1, 2, 3, 4, 5].map do |number|
  number ** 2
end
puts squared_numbers.inspect  # Output: [1, 4, 9, 16, 25]

select

The select method returns a new array containing all elements for which the block returns a true value.

# Example using an array
even_numbers = [1, 2, 3, 4, 5].select do |number|
  number.even?
end
puts even_numbers.inspect  # Output: [2, 4]

reject

The reject method is the opposite of select. It returns a new array containing elements for which the block returns a false value.

# Example using an array
odd_numbers = [1, 2, 3, 4, 5].reject do |number|
  number.even?
end
puts odd_numbers.inspect  # Output: [1, 3, 5]

find

The find method returns the first element for which the block returns a true value.

# Example using an array
first_even = [1, 2, 3, 4, 5].find do |number|
  number.even?
end
puts first_even  # Output: 2

inject

The inject method combines all elements of an array by applying a binary operation, specified by a block or a symbol.

# Example using an array
sum = [1, 2, 3, 4, 5].inject(0) do |accumulator, number|
  accumulator + number
end
puts sum  # Output: 15

Practical Examples

Example 1: Iterating Over an Array

numbers = [1, 2, 3, 4, 5]

# Using each to print each number
numbers.each do |number|
  puts number
end

Example 2: Transforming an Array with map

numbers = [1, 2, 3, 4, 5]

# Using map to create a new array of squared numbers
squared_numbers = numbers.map do |number|
  number ** 2
end

puts squared_numbers.inspect  # Output: [1, 4, 9, 16, 25]

Example 3: Filtering an Array with select

numbers = [1, 2, 3, 4, 5]

# Using select to create a new array of even numbers
even_numbers = numbers.select do |number|
  number.even?
end

puts even_numbers.inspect  # Output: [2, 4]

Exercises

Exercise 1: Using each

Write a Ruby program that iterates over an array of names and prints each name in uppercase.

names = ["Alice", "Bob", "Charlie"]

# Your code here

Solution:

names = ["Alice", "Bob", "Charlie"]

names.each do |name|
  puts name.upcase
end

Exercise 2: Using map

Write a Ruby program that takes an array of numbers and returns a new array with each number doubled.

numbers = [1, 2, 3, 4, 5]

# Your code here

Solution:

numbers = [1, 2, 3, 4, 5]

doubled_numbers = numbers.map do |number|
  number * 2
end

puts doubled_numbers.inspect  # Output: [2, 4, 6, 8, 10]

Exercise 3: Using select

Write a Ruby program that filters out all the odd numbers from an array.

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

# Your code here

Solution:

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

even_numbers = numbers.select do |number|
  number.even?
end

puts even_numbers.inspect  # Output: [2, 4, 6, 8, 10]

Common Mistakes and Tips

  • Forgetting to use a block: Iterators require a block to be passed. Ensure you use {} or do...end to define the block.
  • Misunderstanding map vs each: Remember that map returns a new array, while each simply iterates over the collection without returning a new array.
  • Using the wrong iterator: Choose the iterator that best fits your needs. For example, use select for filtering and map for transforming elements.

Conclusion

Iterators are a powerful feature in Ruby that allow you to work with collections in a clean and efficient manner. By mastering iterators like each, map, select, and others, you can write more readable and maintainable code. Practice using these iterators with different collections to become more comfortable with their usage. In the next section, we will explore the Enumerable module, which provides additional methods for working with collections.

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