Matlab解釋器模式

解釋器模式(Interperter)，給定一個語言，定義一個語言的文法，而且創建一個解釋器來解釋該語言中的句子，實際開發中EL表達式或者正則表達式的解釋器就是採用這種設計模式。其模式結構以下圖。本文使用matlab語言，利用解釋器模式來實現後綴表達式的解析。

Context.m （環境類，包含解釋器以外的一些全局信息）

classdef Context < handle
    properties
        variables = containers.Map();
    end  
    methods
        function put(obj,var,expr)
            obj.variables(char(var)) = expr;
        end
        function expr = lookup(obj,var)
            if(obj.variables.isKey(char(var)))
                expr = obj.variables(char(var));
            else
                expr = Expression.empty();
            end
        end
    end
end

Expression.m (抽象表達式)

classdef Expression < handle & matlab.mixin.Heterogeneous 
    methods(Abstract)
        interpret(obj,ctx);
    end
end

Plus.m （非終結表達式，加法類）

classdef Plus < Expression
    properties
        left;
        right;
    end
    methods
        function obj = Plus(left,right)
            obj.left = left;
            obj.right = right;
        end
        function res = interpret(obj,ctx)
           res = obj.left.interpret(ctx) + obj.right.interpret(ctx);
        end
    end
end

Minus.m (非終結表達式，減法類)

classdef Minus < Expression
    properties
        left;
        right;
    end
    methods
        function obj = Minus(left,right)
            obj.left = left;
            obj.right = right;
        end
        function res = interpret(obj,ctx)
           res = obj.left.interpret(ctx) - obj.right.interpret(ctx);
        end
    end
end

Variable.m (非終結表達式，變量類)

classdef Variable < Expression
    properties
        name
    end
    methods
        function obj = Variable(name)
            obj.name = name;
        end
        function res = interpret(obj,ctx)
            if(isempty(ctx.lookup(obj.name)))
                res  = 0;
            else
                res = ctx.lookup(obj.name).interpret(ctx);
            end
        end
    end
end

Number.m (終結表達式，數字類)

classdef Number < Expression
    properties
        number
    end
    methods
        function obj = Number(number)
            obj.number = number;
        end
        function number = interpret(obj,~)
            number = obj.number;
        end
    end
end

Evaluator.m (非終結表達式，解析入口類) 

classdef Evaluator < Expression
    properties
        syntaxTree
    end
    methods
        function obj = Evaluator(expr)
            exprs = Expression.empty();
            tokens = expr.split(" ");
            for i=1:length(tokens)
                switch(tokens(i))
                    case "+"
                        subexpr = Plus(exprs(end-1),exprs(end));
                        exprs = exprs(1:end-2);
                        exprs(end + 1) = subexpr;
                    case "-"
                        subexpr = Minus(exprs(end-1),exprs(end));
                        exprs = exprs(1:end-2);
                        exprs(end + 1) = subexpr;
                    otherwise
                        exprs(end + 1) = Variable(tokens(i)); 
                end    
            end
            obj.syntaxTree = exprs(end);
        end        
        function res = interpret(obj,ctx)
            res = obj.syntaxTree.interpret(ctx);
        end
    end
end

test.m (測試代碼)

expr = "w x z + -";
ctx = Context();
ctx.put("w",Number(5));
ctx.put("x",Number(6));
ctx.put("z",Number(4));
eva = Evaluator(expr);
res = eva.interpret(ctx);
disp(res);

參考資料：

https://www.jianshu.com/p/0e7e26bbe204

http://www.javashuo.com/article/p-tpfbpzmq-dk.html