Difference between revisions of "Lua: Tables"

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{{Lua}}
 
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'''Tables''' are the only real complex data structure provided in [[Lua]]. Tables are basically arrays, specifically associative arrays, associating a ''key'' with a corresponding ''value''. Unlike most languages, Lua's tables start at 1 instead of 0. Lua tables can store every other non-nil data type, and even mixed types. The following is a valid table:
 
  
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<source lang="lua" enclose="div">example_table = {
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'''Tables''' compose [[Lua]]'s only complex data structure. As such, they cannot be added together, unless supplied with a [[Lua: Metatables|metatable]].
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Tables are associative arrays that begin the key index from 1. Values in a table may be of any other non-nil or mixed type, including methods / functions.
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When a variable is assigned to a table, it may be accessed by the following terms:
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* ''table.varname'' and ''table["varname"]'', where ''varname'' is the name of the assigned variable. The latter must be used if the variable name contains otherwise invalid characters, for syntax parsing purposes.
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* ''table[x]'', where x is the key's index.
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* ''table.function()'', ''table.["function"]()'', or ''table:function()'', to call a function or method (which is simply another type of variable.) Any values added into parenthesis are parameters.
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* Tables themselves may be called as a function if they contain the [[Lua: Metamethods|metamethod]] ''__call''.
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'' ''
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== Example code ==
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<div style='width: 60%; margin-left: 5%; background-color: #f0f0f0; border: 1px solid #444; padding: 8px;'><source lang="lua" enclose='div'>example_table = {
 
     [1] = "one", -- assigns "one" to first index, regular enumerated tables don't need the index explicitly stated
 
     [1] = "one", -- assigns "one" to first index, regular enumerated tables don't need the index explicitly stated
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     [0] = "can't touch this", -- index 0 is legal but Lua's table-based functions will ignore it
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     [0] = "can't touch this", -- index 0 is legal but table-based functions will ignore it
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     [5000] = "huge jump", -- Lua tables' array indexes are sparse, meaning this won't allocate memory for the unused 4999 indices
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     [5000] = "huge jump", -- Won't allocate memory for unused 4999 indices
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     test = "variable key", -- variables may also be used within tables, accessed via 'table.var' or 'table["var"]'
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     var = "variable key", -- Variables are accessed with 'table.var' or 'table["var"]'
 
     ["$"] = "string key", -- using an explicit string key allows characters that can't be used in variable names
 
     ["$"] = "string key", -- using an explicit string key allows characters that can't be used in variable names
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     [true] = false, -- other non-number and non-string datatypes can be used as keys (except nil)
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     [true] = false, -- other non-number and non-string datatypes can be used as keys, except nil
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     subtable = { -- tables can be arbitrarily nested
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     subtable = { -- Tables may be nested freely
 
         subtable2 = { },
 
         subtable2 = { },
 
         subtable3 = {  
 
         subtable3 = {  
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     },
 
     },
 
     func = function (arg) return arg end, -- functions/methods must be declared in this manner within tables   
 
     func = function (arg) return arg end, -- functions/methods must be declared in this manner within tables   
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}</source>
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}</source></div>
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Values in tables can be accessed in a few ways. One way is via dot syntax: '''table1.key''' will return ''test''. Another is by specifying an integer index: '''table1[3]''' will return ''5'', not ''true'', as ''5'' was explicitly stored in '[3]'. Strings can also used within square brackets: '''table1["what"]''' is exactly the same as '''table1.what'''. However, bracket syntax must be used instead of dot syntax in some cases; this is generally the case if you need an evaluated index, or if your index is not a valid identifier name (if it starts with a number or contains spaces, etc)
 
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Functions inside tables can called in various ways; remember, they are variables, after all. '''table1.testfunc(arg)''' and '''table1["testfunc"](arg)''' are two valid ways to call a function. Functions are also able to use colon syntax, which effectively gives Lua [[Lua: Basic OO|OO]] capabilities. '''table1:testfunc(arg)''' is the same as '''table1.testfunc(table1, arg)'''. Tables can be called like functions if they contain a [[Lua: Metamethods|'''__call''' metamethod]].
 
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''[[Lua: Metatables|Metatables]]'' are normal Lua tables that can be "attached" to other tables. They allow you to define otherwise undefined behavior: normally, adding two tables would result in an error. However, metatables allow you to define exactly how tables should be added, which would then make adding tables a valid operation. Tables can only have one metatable, but that metatable itself can have a metatable, and so on. Tables can even be their own metatables, which is useful for methods.
 

Revision as of 01:13, 6 October 2009

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Tables compose Lua's only complex data structure. As such, they cannot be added together, unless supplied with a metatable.

Tables are associative arrays that begin the key index from 1. Values in a table may be of any other non-nil or mixed type, including methods / functions.

When a variable is assigned to a table, it may be accessed by the following terms:

  • table.varname and table["varname"], where varname is the name of the assigned variable. The latter must be used if the variable name contains otherwise invalid characters, for syntax parsing purposes.
  • table[x], where x is the key's index.
  • table.function(), table.["function"](), or table:function(), to call a function or method (which is simply another type of variable.) Any values added into parenthesis are parameters.
  • Tables themselves may be called as a function if they contain the metamethod __call.

Example code

example_table = {
    [1] = "one", -- assigns "one" to first index, regular enumerated tables don't need the index explicitly stated
    [0] = "can't touch this", -- index 0 is legal but table-based functions will ignore it
    [5000] = "huge jump", -- Won't allocate memory for unused 4999 indices
    var = "variable key", -- Variables are accessed with 'table.var' or 'table["var"]'
    ["$"] = "string key", -- using an explicit string key allows characters that can't be used in variable names
    [true] = false, -- other non-number and non-string datatypes can be used as keys, except nil
    subtable = { -- Tables may be nested freely
        subtable2 = { },
        subtable3 = { 
            subtable4 = { },
        },
    },
    func = function (arg) return arg end, -- functions/methods must be declared in this manner within tables  
}
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