How to get number of entries in a Lua table?

Question

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.

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.

There's one way, but it might be disappointing: use an additional variable (or one of the table's field) for storing the count, and increase it every time you make an insertion.

count = 0
tbl = {}

tbl["test"] = 47
count = count + 1

tbl[1] = 48
count = count + 1

print(count)   -- prints "2"

There's no other way, the # operator will only work on array-like tables with consecutive keys.

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 could use penlight library. This has a function size which gives the actual size of the table.

It has implemented many of the function that we may need while programming and missing in Lua.

Here is the sample for using it.

> tablex = require "pl.tablex"
> a = {}
> a[2] = 2
> a[3] = 3 
> a['blah'] = 24

> #a
0

> tablex.size(a)
3

function GetTableLng(tbl)
  local getN = 0
  for n in pairs(tbl) do 
    getN = getN + 1 
  end
  return getN
end

You're right. There are no other way to get length of table