Matlab迭代器模式

迭代器（Iterator）模式，又叫做游标（Cursor）模式。提供一种方法访问一个容器（container）或者聚集（Aggregator）对象中各个元素，而又不需暴露该对象的内部细节。在采用不同的方式迭代时，只需要替换相应Iterator类即可。本文采用Matlab语言实现对元胞数组和string数组的遍历。

Aggregator.m

classdef Aggregator < handle
    methods(Abstract)
        iterObj = createIterator(~);
    end
end

CellAggregator.m

classdef CellAggregator < Aggregator
    properties
        cell
    end
    methods
        function obj = CellAggregator(cell)
            obj.cell = cell;
        end
        function iterObj = createIterator(obj)
            iterObj = CellIterator(obj);
        end
    end
end

StringAggregator.m

classdef StringAggregator < Aggregator
    properties
        string_arr
    end
    methods
        function obj = StringAggregator(string_arr)
            obj.string_arr = string_arr;
        end
        function iterObj = createIterator(obj)
            iterObj = StringIterator(obj);
        end
    end
end

Iterator.m

classdef Iterator < handle
    methods(Abstract)
        hasNext(~);
        next(~);
    end
end

CellIterator.m

classdef CellIterator < Iterator
    properties
        index = 1;
        aggHandle;
    end
    methods
        function obj = CellIterator(agg)
            obj.aggHandle = agg;
        end
        function res = hasNext(obj)
            if(obj.index <= length(obj.aggHandle.cell))
                res = true;
            else
                res = false;
            end
        end
        function ele = next(obj)
            if(obj.hasNext())
                ele = obj.aggHandle.cell{obj.index};
                obj.index = obj.index + 1;
            else
                ele = [];
            end
        end
    end
end

StringIterator.m

classdef StringIterator < Iterator
    properties
        index = 1;
        aggHandle;
    end
    methods
        function obj = StringIterator(agg)
            obj.aggHandle = agg;
        end
        function res = hasNext(obj)
            if(obj.index <= obj.aggHandle.string_arr.length)
                res = true;
            else
                res = false;
            end
        end
        function ele = next(obj)
            if(obj.hasNext())
                ele = obj.aggHandle.string_arr(obj.index);
                obj.index = obj.index + 1;
            else
                ele = string.empty();
            end
        end
    end
end

　测试代码：

cell = CellAggregator({'matlab','cell','iter'});
iterObj = cell.createIterator();
while iterObj.hasNext()
    disp(iterObj.next());
end

str_arr = StringAggregator(["matlab","string","iter"]);
iterObj = str_arr.createIterator();
while iterObj.hasNext()
    disp(iterObj.next());
end

迭代器模式不仅可以对集合进行顺序遍历，而且可以以用户想要的条件进行遍历，如果想要统计列表中所有偶数的和，可以写成循环语句，如下所示：

test_for.m

list = [13,8,17,11,8,19,18,12,13,12];

sum = 0;
for i=1:length(list)
    if mod(list(i),2) == 0
        sum = sum + list(i);
    end
end

disp("列表中所有偶数的和:" + sum);

sum = 0;
for i=1:length(list)
    if mod(list(i),2) == 1
        sum = sum + list(i);
    end
end

disp("列表中所有奇数的和:" + sum);

如果要统计列表中所有奇数的和，这就需要对循环语句块进行修改，如上所示。这就违反了对修改关闭的原则。而迭代器模式可以将循环语句块和业务语句块（sum求和）分离出去，这就需要对Aggregator和Iterator类进行继承，如下所示：

ListAggregator.m

classdef ListAggregator < Aggregator
    properties(Constant)
        EVEN_ITER = 0
        ODD_ITER = 1
    end
    properties
        list
    end
    methods
        function obj = ListAggregator(list)
            obj.list = list;
        end
        function iterObj = createIterator(obj,type_iter)
            if(type_iter == ListAggregator.EVEN_ITER)
                iterObj = EvenIterator(obj);
            else
                iterObj = OddIterator(obj);
            end
        end
    end
end

OddIterator.m

classdef OddIterator < Iterator
    properties
        index = 1;
        aggHandle;
    end
    methods
        function obj = OddIterator(agg)
            obj.aggHandle = agg;
        end
        function res = hasNext(obj)
            res = false;
            while(obj.index <= length(obj.aggHandle.list))
                if mod(obj.aggHandle.list(obj.index),2) == 1
                    res = true;
                    break;
                end
                obj.index = obj.index + 1;
            end
        end
        function ele = next(obj)
            if(obj.hasNext())
                ele = obj.aggHandle.list(obj.index);
                obj.index = obj.index + 1;
            else
                ele = [];
            end
        end
    end
end

EvenIterator.m

classdef EvenIterator < Iterator
    properties
        index = 1;
        aggHandle;
    end
    methods
        function obj = EvenIterator(agg)
            obj.aggHandle = agg;
        end
        function res = hasNext(obj)
            res = false;
            while(obj.index <= length(obj.aggHandle.list))
                if mod(obj.aggHandle.list(obj.index),2) == 0
                    res = true;
                    break;
                end
                obj.index = obj.index + 1;
            end
        end
        function ele = next(obj)
            if(obj.hasNext())
                ele = obj.aggHandle.list(obj.index);
                obj.index = obj.index + 1;
            else
                ele = [];
            end
        end
    end
end

test_iter.m

list_agg = ListAggregator([13,8,17,11,8,19,18,12,13,12]);
iterObj = list_agg.createIterator(ListAggregator.EVEN_ITER);
sum = 0;
while iterObj.hasNext()
    sum = sum + iterObj.next();
end
disp("列表中所有偶数的和:" + sum);

iterObj = list_agg.createIterator(ListAggregator.ODD_ITER);
sum = 0;
while iterObj.hasNext()
    sum = sum + iterObj.next();
end
disp("列表中所有奇数的和:" + sum);

如果需要返回列表标，可以将next函数的返回值定义成[ele,index] = next(obj)，python里面有yield关键字，可以让迭代器模式更加简洁，见参考资料。

迭代器模式还可以扁平化多层嵌套循环，如果找到所有水仙花数，可以使用三层循环，代码如下：

test_for.m

a = 1:9;
b = 0:9;
c = 0:9;

for i=1:length(a)
    for j = 1:length(b)
        for k = 1:length(c)
            num = a(i)*100 + b(j)*10 + c(k);
            if a(i)^3 + b(j)^3 + c(k)^3 == a(i)*100 + b(j)*10 + c(k)
                disp("" + num + "是水仙花数");
            end
        end
    end
end

如果使用迭代器模式来简化三层循环，只需要对Aggregator和Iterator类进行继承，代码如下：

MutiListAggregator.m

classdef MutiListAggregator < Aggregator
    properties
        lists
    end
    methods
        function obj = MutiListAggregator(varargin)
            obj.lists = varargin;
        end
        function iterObj = createIterator(obj)
            iterObj = MutiListIterator(obj);
        end
    end
end

MutiListIterator.m

classdef MutiListIterator < Iterator
    properties
        indexs = [];
        lengths = [];
        aggHandle;
    end
    methods
        function obj = MutiListIterator(agg)
            obj.aggHandle = agg;
            for i= 1:length(obj.aggHandle.lists)
                obj.lengths(i) = length(obj.aggHandle.lists{i});
            end
            obj.indexs = ones(1,length(obj.aggHandle.lists));
        end
        function res = hasNext(obj)
            if(obj.indexs(1) > obj.lengths(1))
                res = false;
            else
                res = true;
            end
        end
        function varargout = next(obj)
            if(obj.hasNext())
                for i = 1:length(obj.aggHandle.lists)
                    temp_list = obj.aggHandle.lists{i};
                    varargout{i} = temp_list(obj.indexs(i));
                end
                obj.indexs(length(obj.aggHandle.lists)) = obj.indexs(length(obj.aggHandle.lists)) + 1;
                for i = length(obj.aggHandle.lists):-1:2
                    if(obj.indexs(i) > obj.lengths(i))
                        obj.indexs(i) = mod(obj.indexs(i), obj.lengths(i));
                        obj.indexs(i - 1) = obj.indexs(i - 1) + 1;
                    end
                end
            else
                for i = 1:length(obj.aggHandle.lists)
                    varargout{i} = [];
                end
            end
        end
    end
end

test_iter.m

list_agg = MutiListAggregator(1:9,0:9,0:9);
iterObj = list_agg.createIterator();
while iterObj.hasNext()
    [a,b,c] = iterObj.next();
    num = a * 100 + b * 10 + c;
    if a^3 + b^3 + c^3 == a * 100 + b * 10 + c
        disp("" + num + "是水仙花数");
    end
end

参考资料：

https://mp.weixin.qq.com/s/tghaVkTPUijBJO0qxSXdZQ