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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 ...

Lua functions

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

Topics

Lua base functions
Lua bc (big number) functions
Lua bit manipulation functions
Lua coroutine functions
Lua debug functions
Lua io functions
Lua LPEG library
Lua math functions
Lua os functions
Lua package functions
Lua PCRE regular expression functions
Lua script extensions
Lua string functions
Lua syntax
Lua table functions
Lua utilities
Scripting
Scripting callbacks - plugins

(Help topic: lua=table.sort)

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