Lua - Table Sorting

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 - Sort Function

table.sort(x,f)

Where

• x− represents the name of the table which entries we want to sort.

• f− represents the order function which is not necessary to provide if you are okay with the default order of sorting.

Example - Sorting Numerically Indexed Table

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 a table
t = { "the", "quick", "brown", "fox" }

-- loop through table entries
for _,v in ipairs(t) do
   -- print entry 
   print(v) 
end

Output

When the above code is built and executed, it produces the following result −

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.

main.lua

-- initialize a table
t = { "the", "quick", "brown", "fox" }

-- sort the table in default ascending order
table.sort(t)

-- loop through table entries
for i,v in ipairs(t) do 
   -- print entry
   print(v) 
end

Output

When the above code is built and executed, it produces the following result −

brown
fox
quick
the

Example - Sorting Nested and Key-Value based Table

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

main.lua

-- initialize a table with nested tables
students = {
   { name = "Robert", rollNo = 1, marks = 100 },
   { name = "Julie", rollNo = 2, marks = 90 },
   { name = "Samson", rollNo = 4, marks = 95 },
}

-- define a function to sort entries by name
sortByName = function (entry1, entry2)
   return entry1.name < entry2.name
end

-- define a function to sort entries by roll no
sortByRollNo = function (entry1, entry2)
   return entry1.rollNo < entry2.rollNo
end

print("Sorted By Name")
-- sort table by name
table.sort(students, sortByName )

-- naviagate through table entries and print
for _,entry in ipairs(students) do 
   print(entry.name,entry.rollNo,entry.marks) 
end

print("Sorted By Roll No")
-- sort table by roll no
table.sort(students, sortByRollNo )
-- naviagate through table entries and print
for _,entry in ipairs(students) do 
   print(entry.name,entry.rollNo,entry.marks) 
end

Output

When the above code is built and executed, it produces the following result −

Sorted By Name
Julie	2	90
Robert	1	100
Samson	4	95
Sorted By Roll No
Robert	1	100
Julie	2	90
Samson	4	95