Scripting with Lua - Game Coding Complete - page 341

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The nil type is a special type that is equivalent in concept to C++ s NULL value,

although it behaves a bit differently. Any name that has never been used before is
considered to have a value of nil . This is also how you can delete objects from Lua.

print(type(y)) -- this will print “ nil ”

y=20
print(type(y)) -- this will print ”

y = nil -- This effectively deletes y, causing the memory to be marked
-- for garbage collection

“ number

Attempting to Use a nil Value

C++ s nil are conceptually the same, but they behave very

differently. Most C++ compilers define NULL as 0. In Lua, nil has no value, it
only has a type. Be careful attempting to use nil as a valid value.

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Like most other programming languages, variables in Lua have scope. Unlike most
other programming languages, the default scope for Lua variables is global. That
means that even if you declare a variable inside of a function or if statement, it is
still considered a global variable. In order to make a variable local, you must explic-
itly use the local keyword:

local x = 10 -- this is a local variable

Variable Scoping in Lua
These scoping issues can really bite you if you re not careful. You should always

declare a variable as local unless there is a real reason not to; otherwise, you
will find yourself with hard-to-fix bugs.

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Variable Naming Conventions

Lua is a dynamically typed language, which means that it s not always clear

what type a variable is intended to be. If you see a variable named
position , what do you think it is? Maybe it

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' s a 3D vector or a matrix or

even a custom table. It s impossible to know without running the code and

inspecting the value. For these types of ambiguous variables, it

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s often a good
idea to bake the type into the name. For example, the above variable could be
named positionVec3 or positionMat instead. This will save you a lot
more time than you might think.

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