Skip to content

Sorting Tables in Lua

The crux of the problem is that Lua tables are not implicitly ordered.

However, Lua does come with a table.sort function.

Table of Contents

Using table.sort

This function works very well on dictionaries with numbered keys.

Sorts table elements in a given order, in-place, from table[1] to table[n], where n is the length of the table (see |lua-length|).

Syntax

You cannot call sort as a method (i.e., my_table:sort()).
You must explicitly call table.sort(my_table)

table.sort({table} [, {rules}])

As an optional second argument, you can pass a function that determines your own sorting rules.

Making your own sorting rules

The second (optional) argument should take two table elements and returns a boolean value.

It must be a function that receives two table elements, and returns true when the first is less than the second: 

function(a, b) return a < b end

Example Table

As an example table, we'll use a table of players and their levels: 

local players = {
    {name = "Jeff", level = 10},
    {name = "Bob", level = 15},
    {name = "Jane", level = 12},
    {name = "Jill", level = 10},
}

Sorting tables in ascending order

Sort a table in ascending order by using the less-than (<) operator.

Now we can sort the table by comparing the level field in the second argument to table.sort: 

table.sort(players, function(a, b) return a.level < b.level end)
This sorts it in ascending order, because we're using the less-than < operator.

Sorting tables in descending order

Sort a table in descending order by using the greater-than operator (>), reversing the comparison.

To sort it in descending order, we can change the comparison to be greater-than (>): 

table.sort(players, function(a, b) return a.level > b.level end)