The best way to iterate over tables in Lua

In the Lua programming language, an array known as a desk. A desk is utilized in Lua to retailer knowledge. If you are storing a variety of knowledge in a structured approach, it is helpful to know your choices for retrieving that knowledge while you want it.

Creating a desk in Lua

To create a desk in Lua, you instantiate the desk with an arbitrary identify:

mytable = {}

There are other ways you possibly can construction your knowledge in a desk. You may fill it with values, basically creating an inventory (known as a checklist in some languages):

mytable = { 'zombie', 'apocalypse' }

Or you may create an related array (known as a map or dictionary in some languages). You can add arbitrary keys to the desk utilizing dot notation. You may also add a price to that key the identical approach you add a price to a variable:

myarray = {}
myarray.baz = 'completely happy'
myarray.qux = 'halloween'

You can add verification with the assert() operate:

assert(myarray.baz == 'completely happy', 'sudden worth in myarray.baz')
assert(myarray.qux == 'halloween', 'sudden worth in myarray.qux')

You now have two tables: a list-style mytable and an associative array-style myarray.

Iterating over a desk with pairs

Lua’s pairs() operate extracts key and worth pairs from a desk.

print('pairs of myarray:')
for ok,v in pairs(myarray) do
  print(ok,v)
finish

Here’s the output:

pairs of myarray:
baz     completely happy
qux     halloween

If there aren’t any keys in a desk, Lua makes use of an index. For occasion, the mytable desk comprises the values zombie and apocalypse. It comprises no keys, however Lua can improvise:

print('pairs of mytable:')
for ok,v in pairs(mytable) do
  print(ok,v)
finish

Here’s the output:

1   zombie
2   apocalypse

Iterating over a desk with ipairs

To account for the truth that tables with out keys are frequent, Lua additionally offers the ipairs operate. This operate extracts the index and the worth:

print('ipairs of mytable:')
for i,v in ipairs(mytable) do
  print(i,v)
finish

The output is, on this case, the identical because the output of pairs:

1   zombie
2   apocalypse

However, watch what occurs while you add a key and worth pair to mytable:

mytable.shock = 'this worth has a key'
print('ipairs of mytable:')
for i,v in ipairs(mytable) do
  print(i,v)
finish

Lua ignores the important thing and worth as a result of ipairs retrieves solely listed entries:

1   zombie
2   apocalypse

The key and worth pair, nevertheless, have been saved within the desk:

print('pairs of mytable:')
for ok,v in ipairs(mytable) do
  print(ok,v)
finish

The output:

1          zombie
2          apocalypse
shock   this worth has a key

Retrieving arbitrary values

You do not need to iterate over a desk to get knowledge out of it. You can name arbitrary knowledge by both index or key:

print('name by index:')

print(mytable[2])

print(mytable[1])

print(myarray[2])

print(myarray[1])
print('name by key:')
print(myarray['qux'])
print(myarray['baz'])
print(mytable['surprise'])

The output:

name by index:

apocalypse

zombie

nil

nil
name by key:
halloween
completely happy
this worth has a key

Data constructions

Sometimes utilizing a Lua desk makes much more sense than making an attempt to maintain observe of dozens of particular person variables. Once you perceive how you can construction and retrieve knowledge in a language, you are empowered to generate complicated knowledge in an organized and protected approach.