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LUA

A for embedded use in applications. It has a C API and so interfaces with other languages easily. Lua is cross-platform, since the interpreter is written in ANSI C.

Comments start with -- rather than the more common //. Multi-line comments... or strings... are delineated with double square brackets. e.g. [[ this is a comment/string ]]

Each statement must be on it's own line. There are no ';' or other terminators. Intenting is not required (not Python)

Tables / Arrays

Tables make up ever data structure in LUA. The {} constructor makes a table. e.g. T = {} They contain key, value pairs. {x=10,y=20} creates a table with two entries, one mapping the string "x" to the integer 10, the other mapping "y" to 20. Lua supports records with syntactic sugar, translating a field reference like t.x to a table-indexing operation t["x"]. the expression {10, 20, 30} creates a table with three entries, mapping 1 to 10, 2 to 20, and 3 to 30. Tables also implement objects, functions, scopes and just about everything else. +

Functions

All functions are anonymous. The syntax:

function add (x, y)
    return x + y
end

is just syntactic sugar for an assignment of an anonymous function to a variable

add = function (x, y)
    return x + y
end

Lua's treatment of functions as first-class values is shown in the following example, where the print function's behavior is modified:

do
  local oldprint = print
  -- Store current print function as oldprint
  function print(s)
    --[[ Redefine print function. The usual print function can still be used
         through oldprint. The new one has only one argument.]]
    oldprint(s == "foo" and "bar" or s)
  end
end

Any future calls to print will now be routed through the new function, and because of Lua's lexical scoping, the old print function will only be accessible by the new, modified print. Lua also supports closures:

function addto(x)
  -- Return a new function that adds x to the argument
  return function(y)
    --[=[ When we refer to the variable x, which is outside the current
         scope and whose lifetime would be shorter than that of this anonymous
         function, Lua creates a closure.]=]
    return x + y
  end
end
fourplus = addto(4)
print(fourplus(3))  -- Prints 7

--This can also be achieved by calling the function in the following way:
print(addto(4)(3))
--[[ This is because we are calling the returned function from 'addto(4)' with the argument '3' directly.
     This also helps to reduce data cost and up performance if being called iteratively.
]]

A new closure for the variable x is created every time addto is called, so that each new anonymous function returned will always access its own x parameter. The closure is managed by Lua's garbage collector, just like any other object.

Loops

Lua has four types of loops: the while loop, the repeat loop (similar to a do while loop), the numeric for loop, and the generic for loop.

--condition = true

while condition do
  --statements
end

repeat
  --statements
until condition

for i = first, last, delta do  --delta may be negative, allowing the for loop to count down or up
  --statements
  --example: print(i)
end

The generic for loop:

for key, value in pairs(_G) do
  print(key, value)
end

would iterate over the table _G using the standard iterator function pairs, until it returns nil.

You can also do a nested loop, which is a loop inside of another loop.

local objects = { "hello", "hi" }

for i,v in pairs(_G) do
    for index,value in pairs(object) do
        print(objects[i])
        print(value)
    end
end

See also:


file: /Techref/language/luas.htm, 4KB, , updated: 2018/11/26 12:08, local time: 2019/4/24 18:11, owner: JMN-EFP-786,
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