This first edition was written for Lua 5.0. While still largely relevant for later versions, there are some differences.
The fourth edition targets Lua 5.3 and is available at Amazon and other bookstores.
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4.3.1 – if then else

An if statement tests its condition and executes its then-part or its else-part accordingly. The else-part is optional.

    if a<0 then a = 0 end
    if a<b then return a else return b end
    if line > MAXLINES then
      line = 0
When you write nested ifs, you can use elseif. It is similar to an else followed by an if, but it avoids the need for multiple ends:
    if op == "+" then
      r = a + b
    elseif op == "-" then
      r = a - b
    elseif op == "*" then
      r = a*b
    elseif op == "/" then
      r = a/b
      error("invalid operation")