Question

Okay, so I’ve got a strange problem with the following Lua code:

function quantizeNumber(i, step)
    local d = i / step
    d = round(d, 0)
    return d*step
end

bar = {1, 2, 3, 4, 5}

local objects = {}
local foo = #bar * 3
for i=1, #foo do
    objects[i] = bar[quantizeNumber(i, 3)]
end
print(#fontObjects)

After this code has been run, the length of objects should be 15, right? But no, it’s 4. How is this working and what am I missing?

Thanks, Elliot Bonneville.

Answer 1

Yes it is 4.

From the Lua reference manual:

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).

Let’s modify the code to see what is in the table:

local objects = {}
local foo = #bar * 3
for i=1, foo do
    objects[i] = bar[quantizeNumber(i, 3)]
    print("At " .. i .. " the value is " .. (objects[i] and objects[i] or "nil"))
end
print(objects)
print(#objects)

When you run this you see that objects[4] is 3 but objects[5] is nil. Here is the output:

$ lua quantize.lua 
At 1 the value is nil
At 2 the value is 3
At 3 the value is 3
At 4 the value is 3
At 5 the value is nil
At 6 the value is nil
At 7 the value is nil
At 8 the value is nil
At 9 the value is nil
At 10 the value is nil
At 11 the value is nil
At 12 the value is nil
At 13 the value is nil
At 14 the value is nil
At 15 the value is nil
table: 0x1001065f0
4

It is true that you filled in 15 slots of the table. However the # operator on tables, as defined by the reference manual, does not care about this. It simply looks for an index where the value is not nil, and whose following index is nil.

In this case, the index that satisfies this condition is 4.

That is why the answer is 4. It’s just the way Lua is.

The nil can be seen as representing the end of an array. It’s kind of like in C how a zero byte in the middle of a character array is actually the end of a string and the “string” is only those characters before it.

If your intent was to produce the table 1,1,1,2,2,2,3,3,3,4,4,4,5,5,5 then you will need to rewrite your quantize function as follows:

function quantizeNumber(i, step)
    return math.ceil(i / step)
end