Programming in Lua : 4.3.1

This first edition was written for Lua 5.0. While still largely relevant for later versions, there are some differences.
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Programming in Lua

Part I. The Language Chapter 4. Statements

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
      showpage()
      line = 0
    end

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
    else
      error("invalid operation")
    end