Lua - Nested Loops

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Lua programming language allows to use one loop inside another loop. Following section shows few examples to illustrate the concept.

Syntax - Nested for loop

The syntax for a nested for loop statement in Lua is as follows −

for init,max/min value, increment
do
   for init,max/min value, increment
   do
      statement(s)
   end
   statement(s)
end

Example - Print Months and Days

The following program uses a nested for loop to print months and days mapping −

main.lua

months = {"Jan", "Feb", "Mar"}
days = {"Sun", "Mon", "Tue"}

for x=1, #months
do
   for y=1, #days
   do
      print(months[x], days[y])
   end
end

Output

When you build and run the above code, it produces the following result.

Jan	Sun
Jan	Mon
Jan	Tue
Feb	Sun
Feb	Mon
Feb	Tue
Mar	Sun
Mar	Mon
Mar	Tue

Syntax - Nested while loop

The syntax for a nested while loop statement in Lua programming language is as follows −

while(condition)
do
   while(condition)
   do
      statement(s)
   end
   statement(s)
end

Example - Print Prime Numbers

The following program uses a nested while loop to get prime numbers−

main.lua

i = 2
while i < 25
do
   j = 2
   while j <= (i/j)
   do
      if(i%j == 0)
      then
         break
      end
      j = j + 1
   end
   if(j > (i/j))
   then
      print(i, " is prime")
   end
   i = i + 1
end

Output

When you build and run the above code, it produces the following result.

2	 is prime
3	 is prime
5	 is prime
7	 is prime
11	 is prime
13	 is prime
17	 is prime
19	 is prime
23	 is prime

Syntax - Nested repeat...until loop

The syntax for a nested repeat...until loop statement in Lua programming language is as follows −

repeat
   statement(s)
   repeat
      statement(s)
   until( condition )
until( condition )

Example - Get Prime Numbers

The following program uses a nested repeated until loop to get prime numbers−

main.lua

i = 2
repeat 
do
   j = 2
   repeat 
   do
      if(i%j == 0)
      then
         break
      end
      j = j + 1
   end
   until j > (i/j)
   if(j > (i/j))
   then
      print(i, " is prime")
   end
   i = i + 1
end
until i > 25

Output

When you build and run the above code, it produces the following result.

2	 is prime
3	 is prime
5	 is prime
7	 is prime
11	 is prime
13	 is prime
17	 is prime
19	 is prime
23	 is prime

A final note on loop nesting is that you can put any type of loop inside of any other type of loop. For example, a for loop can be inside a while loop or vice versa.