Getting Started with Ruby: Iterators - Continued

Common Shortcut

A common pattern is to use an iterator to apply the exact same method to each object in a collection. For example:

['THE', 'RAIN'].map{|x| x.downcase } # => ["the", "rain"]

This pattern is so common you will often see a shortcut used. It is possible to replace the block {|x| x.downcase } with (&:downcase) to save a few characters:

['THE', 'RAIN'].map(&:downcase) # => ["the", "rain"]

However, this shortcut does not work everywhere, but it does work in Ruby on Rails. Whether or not to use this shortcut is a matter of personal taste, but you will see it used frequently.

Calculations

sum = 0
[0,1,2,3,5].each do |i|
    sum += i
end
sum # => 11

This block can be implemented in one line of Ruby code using the inject method, borrowed from Smalltalk.

[0,1,2,3,5].inject(0) {|s,i| s+i} # => 11

Jim Weirich provides the following tip to help remember the name inject: think of the inject method as injecting the provided operator + into the collection. The inject method is extremely powerful and can used to implement the results of many common loops.

Disadvantages

By chaining iterators it is often possible to produce complex behavior in a single line of Ruby code. In addition, the resulting code is usually expressive and readable. There is a disadvantage to this style of programming: suboptimal performance.

For example, to find the least positive element of [-1,2,-3,4,7,1] we used:

[-1,2,-3,4,7,1].select {|x| x > 0}.sort.first # => 1

In this example, the select method will visit every element of the array [-1,2,- 3,4,7,1] and so will the sort method, resulting in two loops over the entire collection. However, it is possible to compute the same result by iterating over the array only once. In most cases this performance penalty is not worth the resulting reduction in code clarity, but it is up to the developer to make this decision.

Further Reading

The common Ruby iterator methods used in practice are implemented in the Enumerable module. Arrays include the Enumerable module as do other collections such as Hash and Set. Documentation for the Enumerable` module can found at: http://www.ruby- doc.org/core/classes/Enumerable.html.

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Ruby Iterators
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