How do I get the number of keys in a hash table in Lua?

Question

myTable = {}
myTable["foo"] = 12
myTable["bar"] = "blah"
print(#myTable) -- this prints 0

Do I actually have to iterate through the items in the table to get the number of keys?

numItems = 0
for k,v in pairs(myTable) do
    numItems = numItems + 1
end
print(numItems) -- this prints 2

Answer 1

I experimented with both the # operator and table.getn(). I thought table.getn() would do what you wanted but as it turns out it's returning the same value as #, namely 0. It appears that dictionaries insert nil placeholders as necessary.

Looping over the keys and counting them seems like the only way to get the dictionary size.

Answer 2

Anything helpful here? http://lua-users.org/wiki/TablesTutorial

Answer 3

The length of a table t is defined to be any integer index n such that t[n] is not nil and t[n+1] is nil; moreover, if t[1] is nil, n can be zero. For a regular array, with non-nil values from 1 to a given n, its length is exactly that n, the index of its last value. If the array has "holes" (that is, nil values between other non-nil values), then #t can be any of the indices that directly precedes a nil value (that is, it may consider any such nil value as the end of the array). so only way to get length is iterate over it.

Answer 4

I have the same issue and doing a loop to get the count is not acceptable in my circumstance. This is what I'm currently doing, which I still consider inelegant, but for my situation is better than a loop within a loop:

local t = {}
for i=1,10 do
  t["k"..i] = "v"..i
  t.count = (t.count or 0) + 1
end
print("t.count = " .. t.count)
assertEquals(t.count,10)

I would love if someone could show us a better way