The pairs and ipairs functions allow us to iterate over
tables.
I'll demonstrate by defining a table with three key-value pairs:
> t = {key1 = true, key2 = false, key3 = true}
We can use an iterator with a special form of a for loop using
the in keyword, like this. I'll print out all the key-value
pairs in the table.
> for k, v in pairs(t) do print(k, v) end
key1 true
key3 true
key2 false
The first loop variable - which is k here - gets the keys
from the table.
The second - which is v here - gets the values.
Tables don't store any order information about keys, and we don't get any guarantees about the order of iteration.
I can also use a single loop variable if I only care about the keys:
> for k in pairs(t) do print(k) end
key1
key3
key2
If I only care about the values, then I do need to use two
loop variables, but, as a convention to other programmers,
I can set an underscore (_) as the first loop variable.
This indicates that I don't really care about the value stored
in the underscore variable.
> for _, v in pairs(t) do print(v) end
true
true
false
The ipairs function works with arrays, so I'll define an
array with some Fibonacci numbers in it.
> arr = {1, 1, 2, 3, 5, 8}
The syntax to use ipairs is the same as pairs, like this:
> for k, v in ipairs(arr) do print(k, v) end
1 1
2 1
3 2
4 3
5 5
6 8
The difference is that ipairs expects its input table to
be an array, and that it iterates over keys in order starting
from 1.
To summarize, use ipairs to iterate over an array that needs
to be processed in order; otherwise use pairs.