Programming in Lua(Thrid Edition)笔记---4 Statements

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4 Statements

  • 多重赋值：先求出所有值，再赋值

  • 交换两值

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    x, y = y, x

  • 多余的变量赋nil，多余的值丢弃

  • 用局部变量保护全局变量

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    local foo = foo

  • if-then-elseif-else-end充当switch

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    if op == "+" then
    	r = a + b
    elseif op == "-" then
    	r = a - b
    elseif op == "*" then
    	r = a * b
    elseif op == "/" then
    	r = a / b
    else
    	error("invalid operation")
    end

  • numeric for

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    for var = exp1, exp2, exp3 do
    	<somethin>
    end

exp3可选，为变量的增量

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for i= 10， 1， -1 do print(i) end

  • math.huge用作无穷大

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    for i = 1, math.huge do
    	if(0.3*i^3 - 20*i^2 - 500 >= 0) then
    		print(i)
    		break
    	end
    end

  • for循环的表达式只求值一次，变量为局部变量

  • generic for

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    for k, v in pairs(t) do print(k, v) end

  • 用generic for和迭代器建立反向表

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    days = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday"， "Friday", "Saturday"}
    revDays = {}
    for k, v in pairs(days) do
    	revDays[v] = k
    end

  • goto实现redo和continue

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    while some_condition do
    	::redo::
    	if some_other_condition then goto continue
    	else if yet_another_condition then goto redo
    	end
    	<some code>
    	::continue::
    end

  • goto的域

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    while some_condition do
    	if some_other_condition then goto continue end
    	local var = something
    	<some code>
    	::continue::
    end

局部变量的域结束在最后一个非空语句，continue标签出现在最后一个非空语句之后，所以goto并没有跳入var的域。

  • goto不能从一个函数内跳到函数外