解釋器模式(Interperter),給定一個語言,定義一個語言的文法,並且建立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。其模式結構如下圖。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,包含解釋器之外的一些全局信息)
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);
結果展示:
參考資料: