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(xiàn)的, 并且 block 中的代碼不一定會執(zhí)行
- 如果 method 中有 yield, 那么它的block 中的代碼會被執(zhí)行
- Block 可以接收參數(shù)，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(tǒng)的交互
Iterators 不僅僅能夠訪問 Array 和 Hash 中的數(shù)據(jù)， 和可以和 I/O 系統(tǒng)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
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