How to get number of entries in a Lua table?

The easiest way that I know of to get the number of entries in a table is with '#'. #tableName gets the number of entries as long as they are numbered:

tbl={
    [1]
    [2]
    [3]
    [4]
    [5]
}
print(#tbl)--prints the highest number in the table: 5

Sadly, if they are not numbered, it won't work.

You can set up a meta-table to track the number of entries, this may be faster than iteration if this information is a needed frequently.

You already have the solution in the question -- the only way is to iterate the whole table with pairs(..).

function tablelength(T)
  local count = 0
  for _ in pairs(T) do count = count + 1 end
  return count
end

Also, notice that the "#" operator's definition is a bit more complicated than that. Let me illustrate that by taking this table:

t = {1,2,3}
t[5] = 1
t[9] = 1

According to the manual, any of 3, 5 and 9 are valid results for #t. The only sane way to use it is with arrays of one contiguous part without nil values.