Lua - Sorting Arrays

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One of the most used functions in Lua is the sort function which is provided by the Lua library which tables a table as an argument and sorts the values that are present inside the table.

The sort function also takes one more argument with the table and that argument is a function which is known as the order function. This order function is used to provide the logic if we want to sort the elements of the table in a certain order.

The order function takes two arguments, and these two arguments must return true if the first argument should come first in the sorted array. If this function is not provided, sort uses the default less-than operation (corresponding to the `<´ operator).

Syntax

table.sort(x,f)

In the above syntax, the x identifier is used to represent the name of the table which entries we want to sort, and the f identifier is used to represent the order function which is not necessary to provide if you are okay with the default order of sorting.

Example - Print array values

Let's consider a simple example of a table where we have different strings stored in it and we are printing the values of the table using the generic for loop.

Consider the example shown below −

main.lua

-- initialize an array
t = { 'the', 'quick', 'brown', 'fox' }

-- loop over array and print its values
for i,v in ipairs(t) do print(v) end

Output

When we run the above program, we will get the following output −

the
quick
brown
fox

Now, let's consider the scenario that we want to sort the elements of the above table, for that we just need to use the sort function that the Lua library provides us.

Example - Sort and Print array values

Consider the example shown below −

main.lua

-- initialize an array
t = { 'the', 'quick', 'brown', 'fox' }

-- sort the array
table.sort(t)

-- loop over array and print its values
for i,v in ipairs(t) do print(v) end

Output

When we run the above program, we will get the following output −

brown
fox
quick
the

Now let's consider a more complex example, where we will try to use an order function.

Example - Custom Sorting

Consider the example shown below −

main.lua

-- initialize a multidimensional array
t = {
   { str = 42, dex = 10, wis = 100 },
   { str = 18, dex = 30, wis = 5 }
}
-- sort the array based on custom comparator
table.sort(t, function (k1, k2) return k1.str < k2.str end )

-- loop over array and print its values
for i,v in ipairs(t) do print(v.str,v.dex,v.wis) end

In the above example, the idea is to sort the values in the table based on the "str" field, and hence when I print the values of the fields, they will be in that order.

Output

When we run the above program, we will get the following output −

18  30 5
42  10 100