Functions in Lua

15 November 2021 - 4 mins read time
Tags: Lua

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Functions in Lua

Lua programs can use functions defined both in Lua and in C.

Declaration of function

function name_of_function(arg1, arg2)
    --> body of the function
    --> return statement
end

Some examples are given below

function printTable(atable)
  for i=1,#atable do
    print( "atable(" .. i .. ")=", atable[i] )
  end
end
-- let us not call this function
printTable({1,2,3,4,5})

Results:

atable(1)=	1
atable(2)=	2
atable(3)=	3
atable(4)=	4
atable(5)=	5

A function is allowed to return nothing (actually nil).

function foo(a,b) print(a,b) end

Let us call this function

foo(1,1)
foo("hello", " world")
foo("hello", 1 )

Results:

1	1
hello	 world
hello	1

Variable numbers of arguments

We can call a function with a different number of arguments. If we provide more than the required arguments, then Lua will drop the unnecessary arguments. If we provide lesser number of arguments, then Lua will set the missing arguments to nil. This is explained below.

foo() --> nil nil
foo(1) --> 1 nil
foo(1,2) --> 1 2
foo(1,2,3) --> 1,2

Results

nil	nil
1	nil
1	2
1	2

Default value of arguments

We can set the default value of an arguments as shown below.

function foo(a)
    local b = a or 3
    print( 'foo( ', a or ' ', ' )=', b )
end

Call:

foo(1)
foo( 'hello world' )
foo( )

Results:

foo( 	1	 )=	1
foo( 	hello world	 )=	hello world
foo( 	 	 )=	3

In the above function if a is not present then it has a default value of 3.

How multiple results works

Lua function can return

💎 no value
💎 single value
💎 multiple values

as shown below.

function foo0() end
function foo1() return 1 end
function foo2() return 1,2 end

🚀 When we call a function as a statement, Lua discards all results from the function.

🚀 When we call a function as an expression, Lua keeps only the first result.

For example, in the code given below function foo2 returns first entry only.

print("ans=", foo1()+foo2())

Results:

ans=2

🚀 We get all the results from a function when the call to the function is the last expression in a list of expressions.

For example, in the code block shown below the function foo2 will return box x and y, because the call to the function is last in the expression list.

x,y=foo2()
print('x=',x, 'y=',y)

Result:

x=	1	y=	2

Similarly, in the code block given below foo2 will return both the values because the call to the function is last in the expression list.

x,y,z=10,foo2() 
print('x=',x, ' y=', y, ' z=', z)

Result:

x=	10	 y=	1	 z=	2

💀 However, in the code given below the function foo2 will only return single value (that is, 1), because the call to the function is not the last in the list.

x,y,z=foo2(),10 --> x=1,y=10, z=nil

Results:

x=	1	 y=	10	 z=	nil

Try the following and inspect the results.

print(foo2(  ))
print('x,y =', foo2(  ))

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