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
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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
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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.
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[1, 3, 5, 7, 9].each { |i| puts i }
# 1
# 3
# 5
# 7
# 9
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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.
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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
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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
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["H", "A", "L"].collect { |x| x.succ } # => ["I", "B", "M"]
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inject
The inject method lets you accumulate a value across the members of a collection.
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[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
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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：
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[1,3,5,7].inject(:+) # => 16
[1,3,5,7]/inject(:*) # => 105
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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
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f = File.open("testfile")
f.each do |line|
puts "The line is: #{line}"
end
f.close
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produces:
The line is: This is line one
The line is: This is line two
The line is: This is line three