lua自定義迭代器

迭代器

http://www.tutorialspoint.com/lua/lua_iterators.htm

迭代器能夠讓你遍歷某個集合或者容器中的每一個元素。 對于lua來說， 集合通常指代 table， 用于創建變化的數據結構， 類似數組。

Iterator is a construct that enables you to traverse through the elements of the so called collection or container. In Lua, these collections often refer to tables, which are used to create various data structures like array.

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通用For迭代器

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通常使用的for循環， 配合in使用， in后的參數 就是一個迭代器函數。

A generic for iterator provides the key value pairs of each element in the collection. A simple example is given below.

array = {"Lua", "Tutorial"} for key,value in ipairs(array) do print(key, value) end

對于迭代器函數， 根據迭代器函數中狀態的維護， 可以分為如下兩種類型， 有狀態迭代器， 和 無狀態迭代器。

In Lua we use functions to represent iterators. Based on the state maintenance in these iterator functions, we have two main types ?

• Stateless Iterators
• Stateful Iterators

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無狀態迭代器

迭代器函數中不維護任何狀態。

By the name itself we can understand that this type of iterator function does not retain any state.

Let us now see an example of creating our own iterator using a simple function that prints the squares of n numbers.

如下例子， 迭代狀態， 由for的參數 i 記錄。

function square(iteratorMaxCount,currentNumber) if currentNumber<iteratorMaxCount then currentNumber = currentNumber+1 return currentNumber, currentNumber*currentNumber end end for i,n in square,3,0 do print(i,n) end

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改進版：

function square(iteratorMaxCount,currentNumber) if currentNumber<iteratorMaxCount then currentNumber = currentNumber+1 return currentNumber, currentNumber*currentNumber end end function squares(iteratorMaxCount) return square,iteratorMaxCount,0 end for i,n in squares(3) do print(i,n) end

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有狀態迭代器

利用閉包，將狀態管理在閉包類， 迭代器函數為閉包。

The previous example of iteration using function does not retain the state. Each time the function is called, it returns the next element of the collection based on a second variable sent to the function. To hold the state of the current element, closures are used. Closure retain variables values across functions calls. To create a new closure, we create two functions including the closure itself and a factory, the function that creates the closure.

Let us now see an example of creating our own iterator in which we will be using closures.

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如下例子，推薦使用如下寫法，減少信息暴漏：

array = {"Lua", "Tutorial"} function elementIterator (collection) local index = 0 local count = #collection -- The closure function is returned return function () index = index + 1 if index <= count then -- return the current element of the iterator return collection[index] end end end for element in elementIterator(array) do print(element) end

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自定義例子

使用有狀態迭代器， 實現字符串拆分為固定長度的字符串：

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local instr = "2334t545dfgjkkkk"function StrSegIterator (str, segSize)local strIndex = 1-- The closure function is returnedreturn function ()local segStart = strIndexlocal segEnd = strIndex + segSize - 1local strseg = string.sub(str, segStart, segEnd)if #strseg > 0 thenstrIndex = strIndex + segSize-- return the current element of the iteratorreturn strsegendendendfor element in StrSegIterator(instr, 2)
doprint(element)
end

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