table.sort
Lua function

table.sort

Summary

Sorts a table

Prototype

table.sort (t, f)


Description

Sorts the table using the supplied function f as the comparison function for each element.

Function f should return true if the first element is < the second element. If the function omitted it defaults to the operator <.

Sorting is not stable, that is, the sequence of equal keys is not necessarily preserved.


t = { "the", "quick", "brown", "fox" }
table.sort (t)
table.foreachi (t, print)

 -->
 
1 brown
2 fox
3 quick
4 the


Sorting is really only relevant for numerically keyed tables. If you want to sort the keys for other types of tables you need to make a copy of the keys and sort that, like this:


t = { str = 42, dex = 10, wis = 100 }
ts = {} -- table to hold the keys
table.foreach (t, function (k, v) table.insert (ts, k) end )
table.sort (ts) -- sort keys
table.foreachi (ts, print) -- print sorted keys

 -->
 
1 dex
2 str
3 wis


Here is an example of a custom sort function. This is needed here because we are sorting tables, which do not have a natural "less than" operator:


t = {
    { str = 42, dex = 10, wis = 100 },
    { str = 18, dex = 30, wis = 5 }
    }

table.sort (t, function (k1, k2) return k1.str < k2.str end )

table.foreachi (t, function (k, v) table.foreach (v, print) end )

 -->
 
str 18
dex 30
wis 5
str 42
dex 10
wis 100


We can see from the results that the two tables were sorted into "str" order.

An alternative approach to supplying a comparison function for the sort would be to set up a metatable for the individual table items (not the container table) which specifies a __lt (less than) operator. Here is an example:


t = {
    { str = 42, dex = 10, wis = 100 },
    { str = 18, dex = 30, wis = 5 }
    }

mt =  { __lt = function (k1, k2) return k1.wis < k2.wis end }

-- apply metatable to all tables inside our table
for _, v in ipairs (t) do
  setmetatable (v, mt)
end -- for

table.sort (t)

table.foreachi (t, function (k, v) table.foreach (v, print) end )

 -->
 
str 18
dex 30
wis 5
str 42
dex 10
wis 100


In this case I have made a metatable "mt" which is then applied to each table item. It compares the "wis" field in this case. With this in place the sort can be called without a helper function. Of course, for speed purposes you would do this once (perhaps when creating the individual table entries) rather than every time you wanted to sort it.

See Also ...

Topics

DOC_lua_base Lua base functions
DOC_lua_bc Lua bc (big number) functions
DOC_lua_bit Lua bit manipulation functions
DOC_lua_coroutines Lua coroutine functions
DOC_lua_debug Lua debug functions
DOC_lua_io Lua io functions
DOC_lua_math Lua math functions
DOC_lua_os Lua os functions
DOC_lua_package Lua package functions
DOC_lua_rex Lua PCRE regular expression functions
DOC_lua Lua script extensions
DOC_lua_string Lua string functions
DOC_lua_tables Lua table functions
DOC_lua_utils Lua utilities
DOC_scripting Scripting

Lua functions

LUA_table.concat table.concat (Concatenates table items together into a string)
LUA_table.foreach table.foreach (Applies a function to each item in a table)
LUA_table.foreachi table.foreachi (Applies a function to each item in a numerically-keyed table)
LUA_table.getn table.getn (Returns the size of a numerically-keyed table)
LUA_table.insert table.insert (Inserts a new item into a numerically-keyed table)
LUA_table.maxn table.maxn (Returns the highest numeric key in the table)
LUA_table.remove table.remove (Removes an item from a numerically-keyed table)
LUA_table.setn table.setn (Sets the size of a table (obsolete))

(Help topic: lua=table.sort)

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