How can I efficiently iterate over multiple nodes with Nokogiri?

When working with Nokogiri, an efficient way to iterate over multiple nodes is to use the .each method on a nodeset. A nodeset is a collection of XML/HTML elements that Nokogiri returns when you perform a search using methods like .xpath or .css.

Here's an example in Ruby that demonstrates how to iterate over multiple nodes using Nokogiri:

require 'nokogiri'
require 'open-uri'

# Fetch and parse HTML document
doc = Nokogiri::HTML(URI.open('http://www.example.com'))

# Select nodes using CSS selectors
nodeset = doc.css('div.some-class')

# Iterate over each node in the nodeset
nodeset.each do |node|
  # Perform operations with each node
  puts node.text.strip
end

Alternatively, if you're using XPath to select nodes, it would look like this:

# Select nodes using XPath selectors
nodeset = doc.xpath('//div[@class="some-class"]')

# Iterate over each node in the nodeset
nodeset.each do |node|
  # Perform operations with each node
  puts node.text.strip
end

Both .css and .xpath methods return a Nokogiri::XML::NodeSet object, which is an array-like object containing Nokogiri::XML::Element nodes. You can iterate over this object with .each, as shown, or you can use other Enumerable methods like .map, .select, .find, etc.

Here are some tips to ensure you're iterating efficiently:

1. Minimize Searches: Try to minimize the number of searches you perform. If you can retrieve all the nodes you need in a single search and then iterate over them, this is preferable to performing multiple searches within a loop.

2. Use Specific Selectors: When selecting nodes, be as specific as possible. Narrowing down your search to the most specific selector or XPath query will reduce the number of nodes Nokogiri has to process.

3. Avoid Repeated Computation: If you need to perform complex calculations or transformations on each node, consider whether you can compute certain values once before the iteration and then reuse them inside the loop.

4. Modify in Place: If you're modifying the nodeset, do it in place to avoid creating additional copies of large data structures.

Here's an example that incorporates some of these efficiency tips:

# Fetch and parse HTML document
doc = Nokogiri::HTML(URI.open('http://www.example.com'))

# Perform a specific search to minimize the number of nodes returned
specific_nodes = doc.css('div.some-class > p.special-paragraph')

# Pre-compute any values that can be computed once
some_precomputed_value = compute_something_useful()

# Iterate and modify in place
specific_nodes.each do |paragraph|
  # Use precomputed values to avoid redundant computation
  modify_paragraph(paragraph, some_precomputed_value)
end

By following these guidelines, you can ensure that your node iteration with Nokogiri is as efficient as possible.