Nutty Coder

上一篇文章提到了在Lua中实现类似于JavaScript中的eval函数，遗憾是该eval函数不支持赋值语句，原因是Lua的赋值运算符是不支持返回值。所以如果要让该eval函数也支持赋值语句，就需要一个额外的工作，让它鉴别一个语句是不是赋值语句，如果是，则return的是被赋值后变量的值。为此，我写了一个isAssignmentExpression函数，比较粗糙，不过够用了，基本思想是检测语句中第一个出现的”=“操作符，且该”=“不能在一对引号当中。

-- Lua code
function isAssignmentExpression( str )
	local i = 1;
	local curChar;
	local quotesType = "none" -- none, single or double
	local isEscaping = false

	curChar = string.sub( str, 1, 1 )
	while ( curChar ~= "" ) do
		if ( curChar == "'" and
			 isEscaping == false and
			 quotesType ~= "double" )
		then
			if ( quotesType == "single" )
			then quotesType = "none"
			elseif ( quotesType == "none" )
			then quotesType = "single"
			end
		end

		if ( curChar == "\"" and
			 isEscaping == false and
			 quotesType ~= "single" )
		then
			if ( quotesType == "double" )
			then quotesType = "none"
			elseif ( quotesType == "none" )
			then quotesType = "double"
			end
		end

		if ( curChar == "\\" and isEscaping == false )
		then isEscaping = true
		else isEscaping = false
		end

		if ( curChar == "=" and quotesType == "none" )
		then
			if ( string.sub( str, i+1, i+1 ) ~= "=" )
			then
				return true, string.sub( str, 1, i - 1 )
			else
				return false
			end
		end

		i = i + 1
		curChar = string.sub( str, i, i )
	end

	return false
end

function eval( str )
	local bAssign
	local var
	bAssign, var = isAssignmentExpression( str )
	if ( bAssign )
	then
		print( "Assignment, var=" .. var )
		loadstring( str )()
		return loadstring( "return " .. var )()
	else
		return loadstring( "return " .. str )()
	end
end

-- 以下是一组测试
print( eval( "3+4" ) )
-- 7
function Multiply( a, b )
	return a*b
end
print( eval( "Multiply( 3, 4 )" ) ) 
-- 12
print( eval( "i" ) ) 
-- nil
print( eval( "i = 1" ) ) 
-- Assignment, var=i    
-- 1
print( eval( "i = i + 1" ) ) 
-- Assignment, var=i    
-- 2
print( eval( "i" ) ) 
-- 2
print( eval( "i+1" ) ) 
-- 3
print( eval( "i, j = 4, 5" ) ) 
-- Assignment, var=i, j    
-- 4	   5
print( eval( "i = {}" ) ) 
-- Assignment, var=i    
-- table: 003CD818
print( eval( "i[ \"0\" ] = 0" ) ) 
-- Assignment, var=i[ "0" ]     
-- 0
print( eval( "i[ \"\\\"0=\" ] = 1" ) ) 
-- Assignment, var=i[ "\"0=" ]     
-- 1
print( eval( "i.name=\"hello\"" ) ) 
-- Assignment, var=i.name     
-- hello
print( eval( "i[0], i.name = 4" ) ) 
-- Assignment, var=i[0], i.name     
-- 4	  nil
print( eval( "i == 10" ) ) 
-- false

10 Responses to “Lua相关问题整理（4） – 让Lua的eval函数支持赋值语句”

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Reply

1q6u4i says:

December 14, 2015 at 11:02 am

我来看看，欢迎不欢迎？

Reply

zengda says:

April 6, 2016 at 11:15 am

不错，不错，看看了！

Reply

420451338 says:

April 19, 2016 at 3:50 pm

看看！

Reply

229966526 says:

June 17, 2016 at 9:17 am

年中快乐！

Reply

QQ676256603 says:

August 11, 2016 at 4:50 pm

博客不错，嘎嘎！

Reply

增达网 says:

May 14, 2017 at 5:00 pm

受教了！呵呵！

Reply

me writing essay says:

February 12, 2018 at 7:55 am

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Reply