How can I check if a lua table contains only sequential numeric indices?

Question

How can I write a function that determines whether it's table argument is a true array?

isArray({1, 2, 4, 8, 16}) -> true
isArray({1, "two", 3, 4, 5}) -> true
isArray({1, [3]="two", [2]=3, 4, 5}) -> true
isArray({1, dictionaryKey = "not an array", 3, 4, 5}) -> false

I can't see any way of finding out if the numeric keys are the only keys.

Answer 1

EDIT: Here's a new way to test for arrays that I discovered just recently. For each element returned by pairs, it simply checks that the nth item on it is not nil. As far as I know, this is the fastest and most elegant way to test for array-ness.

local function isArray(t)
  local i = 0
  for _ in pairs(t) do
    i = i + 1
    if t[i] == nil then return false end
  end
  return true
end

Answer 2

ipairs iterates over indices 1..n, where n+1 is the first integer index with a nil value
pairs iterates over all keys.
if there are more keys than there are sequential indices, then it cannot be an array.

So all you have to do is see if the number of elements in pairs(table) is equal to the number of elements in ipairs(table)
the code can be written as follows:

function isArray(tbl)
    local numKeys = 0
    for _, _ in pairs(tbl) do
        numKeys = numKeys+1
    end
    local numIndices = 0
    for _, _ in ipairs(tbl) do
        numIndices = numIndices+1
    end
    return numKeys == numIndices
end

I'm pretty new to Lua, so there might be some builtin function to reduce the numKeys and numIndices calculations to simple function calls.

Answer 3

By "true array", I suppose you mean a table whose keys are only numbers. To do this, check the type of every key of your table. Try this :

function isArray(array)
    for k, _ in pairs(array) do
        if type(k) ~= "number" then
            return false
        end
    end
    return true --Found nothing but numbers !
end

Answer 4

Note: as @eric points out, pairs is not defined to iterate in a specific order. Hence this is no valid answer.

The following should be sufficient; it checks that the keys are sequential from 1 until the end:

local function isArray(array)
  local n = 1
  for k, _ in pairs(array) do
    if k ~= n then return false end
    n = n + 1
  end

  return true
end