Example code:
numlist.rb - defining a class with an each method
findmin.rb - using inject to find the minimum value in a sequence
Ruby Arrays
Ruby arrays - instances of the Array class. Documentation:
Arrays have a literal notation - a series of comma-separated values within square brackets:
arr = [1, 2, 3]Elements may be accessed using the subscript operator
puts arr[0] # prints "1"The each method is the usual way to iterate over the values in an array:
puts.each {|n| puts n } # prints "1", "2", "3" on separate linesRuby Hashes
Ruby hashes: instances of the Hash class. Documentation:
Hashes associate a set of keys with corresponding values. As with arrays, hashes have a literal notation - a comma-separated list of key/value pairs in curly braces:
hash = { 'oranges' => 12, 'bananas' => 7, 'blueberries' => 45 }To access the value associated with a key, use the subscript operator:
puts hash['oranges'] # prints "12"Iterating over the keys and/or values can be done with the each_key, each_pair, and each_value methods. Note that because a Hash object is implemented as a hash table, there is no guaranteed ordering of keys:
hash.each_key {|key| puts key }
# prints "blueberries", "bananas", "oranges"
# on separate linesMixins
A mixin is a module (collection of methods) that may be added ("mixed in") to any class or object.
One important Ruby mixin is Enumerable. Many of the useful methods in the built-in Array class are actually provided by Enumerable.
The Enumerable mixin provides a number of methods for operating on sequences of values. This mixin requires the class or object to support the each method, which given a block generates all of the members of the sequence.
Example: the built-in Array class uses the Enumerable mixin to provide a number of methods. For example, the inject method invokes a block for each element in the collection. The value computed by the block is then passed as an argument to the invocation of the block on the next element, until the end of the sequence is reached, at which point the result of the last invocation of the block is the overall result.
For example, we can use inject to compute the sum of an array of numbers:
arr = [9, 6, 8, 1, 5]
sum = arr.inject(0) {|sum, i| sum + i}
puts sum # prints 29Note that the initial value of sum (0), which is the variable that is "carried" by the computation, is passed explicitly to the inject method. If we call the inject method without an argument, then it would use the first element of the sequence for this value, and start by invoking the block on the second element of the sequence. So, computing the sum could also be done as
arr.inject {|sum, i| sum + i}Another insanely useful method added by the Enumerable mixin is map, which returns an array formed by invoking a block on each element of the sequence and appending the result to the array. For example:
arr = [1, 2, 3]
arr2 = arr.map {|n| n * 2}
arr2.each {|n| puts n} # prints "2", "4", "6" on separate lines