Programming in Lua(Thrid Edition)笔记

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3 Expressions

  • a % b == a - math.floor(a / b) * b，可以用于浮点数，x % 1为x的小数部分，x - x % 1为x的整数部分，x-x%0.01可以保留x两位小数，也可以用于角度对360取模和弧度对2PI取模angle%(2*math.pi)

  • ~=与==作用相反

  • Lua可以根据本地字符编码比较字符串大小

  • 逻辑操作符and，or，not

  • x = x or v相当于

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    if not x then x = v end

可以在x未被赋值时对其赋值

  • (a and b) or c与C的a ? b : c等价

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    max = (x > y) and x or y

  • ..用于连接字符串

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    print("Hello " .. "world") 
    print(0 .. 1)              --> 01
    print(000 .. 01)           --> 01

print(000 .. 01)中会先求值，再转换为字符串，再连接

  • a[#a + ] = v把v加到列表a的末尾

  • 关于有洞的列表的长度

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    c = {1, 2, 3}
    print(#c) --> 3
    c[2] = nil
    print(#c) --> 3
    c[3] = nil
    print(#c) --> 1

  • a = {1, 2, 3, nil, nil}与a = {1, 2, 3}相同，长度均为3

  • 优先级：

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    ^
    not # -(unary)
    * / %
    + -
    ..
    < > <= >= ~= ==
    and
    or

  • a = {x = 10, y = 20}比a = {}; a.x = 10; a.y = 20快一些

  • 混用list风格和record风格构造table

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    polyline = {
    color = "blue",
    thickness = 2,
    npoints = 4,
    {x = 0, y = 0},   -- polyline[1]
    {x = -10, y = 0}, -- polyline[2]
    {x = -10, y = 1}, -- polyline[3]
    {x = 0, y = 1},   -- polyline[4]
    }

  • 几种特殊的table构造方式：

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    a = {["+"] = "add", ["-"] = "sub",
         ["*"] = "mul", ["/"] = "div"}
    b = {[1]="red", [2]="green", [3]="blu",}

最后一个entry后的逗号可选

  • 在table的构造中，可以用分号代替逗号来分隔不同部分，例如list和record