Iterator
定義

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

•         Block 一般是跟著 method 出現的, 并且 block 中的代碼不一定會執行
•         如果 method 中有 yield, 那么它的block 中的代碼會被執行
•         Block 可以接收參數，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

    上面代碼中的 yield 之后的 i1 會作為 parameter 傳入到 block 中， 賦值給 block 的 argument f。
    Block 中可以有多個 arguments.

常見的 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.

上面代碼的另一種簡便寫法：

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

Iterator 和 I/O 系統的交互

Iterators 不僅僅能夠訪問 Array 和 Hash 中的數據， 和可以和 I/O 系統交互

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