I should point out that the behavior is unintuitive even when compared to those other expanding constructs. Table.create is intended to be a capacity-reserving constructor The implementation is a balance between performance and consistency. If you really need to track the length of an array with holes, you should probably store it yourself instead of using #, as the behavior is somewhat undefined. It will have compatibility issues, and I’m not sure we’ll be able to deploy it safely (this is because it would change the semantics of # operator on tables with holes) I have a dream to fix this but it involves doing a deep semantical change to how Lua tables/arrays behave with length. To make sure the extra overhead of # didn’t “pollute” the benchmark numbers. To a loop like this: for i=1,N do t = i end in one of our benchmarks that tested array assignment performance, we had to change a loop like this for i=1,N do t = i end I’ll spare the details but it can take time, e.g. Computing the length is not free - it’s very cheap for “proper” arrays with exact size but gets expensive when arrays get reallocated. If the table doesn’t reallocate then it’s indeed faster to “cache” the table length as you do right now. There is one thing you can do as a programmer to avoid having to deal with this mess - don’t put nil in arrays. But the alternatives are worse for performance. I get it, it’s unintuitive, I didn’t even believe this was the behaviour when I was first told. Thanks to clever al gorithms, this only takes O( log n) time to find, which is much faster for large tables (log_10(1000000) = 6, for instance.)

What Lua does instead is declare that #t is allowed to find any natural i such that t is non-nil and t is nil. This is what is called an O(n) algorithm - the time taken to run it grows linearly with the number of elements in the table. For the first one, you’d effectively have to write: function getLength(t) (or zero, if t is zero)īut the problem with these is that they’re slow to compute for tables with a lot in them. The lowest natural index i such that t is nil.The highest natural (integer > 0) index i such that t is non-nil.If you implemented one, people could reasonably guess the other): So what is the length of a table like this? Here are a couple of sane possible answers (and it’s already a problem that there are a couple.

As a Roblox developer, it is currently too hard to understand why this code does what it does: print(# is zero!)

Lua table insert associative array