• Subject: The many lengths of Lua
• From: "Soni L." <fakedme@...>
• Date: Wed, 14 Sep 2016 16:00:03 -0300

Lua has many lengths. From string lengths to table lengths, from number lengths to sequence lengths, from sequence lengths to proper sequence lengths. They are the many lengths of Lua.
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The # operator returns string lengths and table lengths. It is the standard length operator, and it's what you usually use to get the length of an object.
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The string.len() function returns string lengths. It typechecks the argument to make sure it's a string, but otherwise returns the same value as #.
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There are various types of table length. Some of them are sequences, some of them are not. Some have nothing to do with length, but rather with count.
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The simplest length is the one provided by ipairs(). It only iterates the "proper sequence" part of a table. That is, it iterates the set of contiguous positive integer keys of a table.[1] When in doubt, this is the length you should rely on. Don't do `for i=1,#t`, but instead use `for i,v in ipairs(t)`.
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Another simple length is the one provided by pairs(). This is actually a count. If for every iteration of pairs() you increment a counter, you'll end up with the number of keys in a table. It is rarely used, but can be useful sometimes.
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If you want a manual table length, the simplest way to do it is probably to just use an `n` field. While Lua supports this usage, the standard library doesn't, so you have to deal with it manually. While the standard library doesn't natively support the `n` field, some functions, such as table.pack(), may emit it.
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Another length option is the highest key of a table. You can get this length by combining pairs() and math.max(). It can be useful in some niche applications but it's quite slow, so consider a manual length (see previous paragraph) instead.
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By combining pairs() with type(), you can get the number of non-integer keys in a table. While this count does exist, I have never seen it used in practice.
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The length operator, #, can also be applied to tables. If your table has a positive integer key, and there's a smaller possitive integer that is not a key (i.e. the value associated with it is `nil`), then this shouldn't be used. When using a table without manual length, this operator is usually faster than any other method[2], but it does have the aforementioned drawback. This table length is the only table length that is unspecified for some tables.
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Finally, you can also use your own length algorithm. Use this if you want fast runtime, but the drawbacks of the length operator make it unsuitable for your use-case.
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And these are the many lengths of Lua!

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[1] - I'm not sure how many people know this, but this property (stopping on first `nil`) is actually described in the manual. That is, ipairs() on a table with "holes" is actually well-defined. [2] - The Lua manual doesn't guarantee O(log n) time complexity for the length operator. If you want guaranteed O(log n) runtime, use your own length algorithm. This means # could have O(n) time complexity (i.e. equivalent to ipairs()), or even O(m) where m = the number of keys in the table (i.e. equivalent to pairs() + math.max()).
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PS: Sorry for the wall of text.

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