Just as you are able to find the length of a string with the # operator, you can find the length of an array as well. This is because the # operator gives back the size of a table. We can use this operator to loop through an array that has a dynamic size, for example, such as the following code snippet:

arr = { "a", "b", "c", "d", "e", "f", "g" }

length = #arr 
print ("array length: " .. length)

for i=1,#arr do
  print (arr[i])
end

The length operator # will only count array elements starting from index 1. This means if you use index 0, it will not be counted towards the number of elements in the array:

arr = { }

arr[0] = "x" -- not counted towards length
arr[1] = "y"
arr[2] = "z"

length = #arr -- length = 2!
print ("array length: " .. length)

Trying to find the length of a sparse array is tricky. The # considers an array over if it finds two nil values one after the other. For example, in this code, the length of the array is incorrectly reported as 2:

arr = { }

arr[1] = "x"
arr[2] = "y"
-- Skipping 3 & 4, at least 2 nils after each other end the array
arr[5] = "z" -- not counted towards length
arr[6] = "w" -- not counted towards length

length = #arr -- length = 2, which is WRONG!
print ("array length: " .. length)

Because of this unintuitive behavior, using the # operator to find the length of an array is considered to be unreliable. A better way of finding the length of an array will be covered later in this chapter, in the Iterating section.