Iterator
定义

A Ruby iterator is simple a method that can invoke a block of code.

•         Block 一般是跟着 method 出现的, 并且 block 中的代码不一定会执行
•         如果 method 中有 yield, 那么它的block 中的代码会被执行
•         Block 可以接收参数，和返回 value

def two_times
  yield
  yield
end
two_times { puts "Hello" }
# Hello
# Hello

def fib_up_to(max)
 i1, i2 = 1. 1
 while i1 = max
   yield i1
   i1, i2 = i2, i1 + i2
 end
end

fib_up_to(1000) { |f| print f, " " }

# 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987

    上面代码中的 yield 之后的 i1 会作为 parameter 传入到 block 中， 赋值给 block 的 argument f。
    Block 中可以有多个 arguments.

常见的 iterator
each

each is probable the simplest iterator - all it does is yield successive elements of its collection.

[1, 3, 5, 7, 9].each { |i| puts i }

# 1 
# 3
# 5
# 7
# 9

find

A blocl may also return a value to the method. The value of the last expression evaluated in the block is passed back to the method as the value of the yield.

class Array
 def find
  each do |value|
    return value if yield(value)
  end
 end
end

[1,3,4,7,9].find { |v| V*V > 30 } # => 7

collect (also known as map)

Which takes each element from the collection and passes it to the block. The results returned by the block are used to construct a new array

["H", "A", "L"].collect { |x| x.succ } # => ["I", "B", "M"]

inject

The inject method lets you accumulate a value across the members of a collection.

[1,3,5,7].inject { |sum, element| sum + element } # => 16

# sum = 1, element = 3
# sum = 4, element = 5
# sum = 9, element = 7
# sum = 16

[1,3,5,6].inject { |product, element| product*element } # => 105

If inject is called with no parameter, it uses the first element of the collections as the initial value and starts the iteration with the second value.

上面代码的另一种简便写法：

[1,3,5,7].inject(:+) # => 16
[1,3,5,7]/inject(:*) # => 105

Iterator 和 I/O 系统的交互

Iterators 不仅仅能够访问 Array 和 Hash 中的数据， 和可以和 I/O 系统交互

f = File.open("testfile")
f.each do |line|
 puts "The line is: #{line}"
end
f.close

produces:
The line is: This is line one
The line is: This is line two
The line is: This is line three