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> Tables as a single data structure are not ideal for many tasks. Especially the dualism between tables that are indexed by sequential numbers and tables acting

> Tables as a single data structure are not ideal for many tasks. Especially the dualism between tables that are indexed by sequential numbers and tables acting as maps. It's a lot more kind to your users to give up with the solely academical ideal of "just one data structure", and give programmers at least an ordered and a map data type, or at least come up with something better than that. Lisp attempt at doing most things with lists was much more successful and clean. I have a different view. Lua tables are the foundation for just about any conceivable data structure in CS. While you can do a lot with raw tables in Lua, you can also use them to build all kinds of things on top of them, like b-trees. Or an object system (which is a popular hobby among Lua aficionados). There's a limit to what can and should be built into any base language. Trying to wedge in more container types can cause a bit of difficulty, in my opinion. Python is the classic example here, where you have tuples using parens, arrays using square brackets, and dictionaries using curly brackets. At some level that is clever, but at another level it is a cause of confusion.

sulam 8y ago
Sorry, but tables are awful in practice. I have to iterate all the elements to get a count? This is especially egregious when I’m using one like a list/array and need to know how far to iterate. Maybe there’s a better way I don’t know?
- Twisol 8y ago
  If you need something quick-and-dirty, you can maintain an extra `["length"]` field on your table to cache that information. If you want something more polished, use a proxy table with custom methods/metamethods mediating access to your array so that the length is properly kept up to date. This isn't terribly different from the situation in C with arrays. Tables are a basic mechanism for implementing any data structure. That is a qualitatively different statement from saying that tables can replace any data structure. If you need semantics different from what tables immediately provide, you'll need to implement the desired data structure yourself -- but Lua gives you good tools to do that. > Maybe there’s a better way I don’t know? You can use `for i, v in ipairs(arr) do ... end` to iterate over an array without worrying about "how far" to iterate. The `ipairs` iterator will determine when it hits the end of the array.
  - sulam 8y ago
    Yeah, this is fine until I want the last element of an array. Then I have to iterate it to find it. And writing a loop to get the last element of an array is crazy-town.
    - nikki93 8y ago
      Have you actually used Lua? You don't need to iterate to get the last element of an array.
      - sulam 8y ago
        I have! I’m not an expert at it, though.
- gvx 8y ago
  There's the length operator (https://www.lua.org/manual/5.3/manual.html#3.4.7 https://www.lua.org/manual/5.3/manual.html#3.4.7): #sometable returns the length of the sometable if it is a sequence, and has an upper bound of O(log(N)).
adwn 8y ago
This perceived simplicity of only having a single non-scalar datatype comes at the cost of increased complexity when you have to stuff a square peg into a round hole, i.e., when you approximate other data structures with tables. My favorite example are arrays in Lua: They are hashtables, but not quite (in the implementation, every hashtable has a separate array region). They look like hashtables, but not quite (there's syntactic sugar to help dealing with them). You can use hashtable-functions on them, but they don't quite work (which is why there are special functions for when you want to treat a hashtable as an array). Hashtables are great for unordered maps and sets, but they're a crappy replacement for ordered sequences. > Python is the classic example here, where you have tuples using parens, arrays using square brackets, and dictionaries using curly brackets. At some level that is clever, but at another level it is a cause of confusion. If that confuses you, then the source of the confusion is you not having a clear understanding of the intended semantics of your code or datastructure. Mixing all of that into a single type won't help here.
vvanders 8y ago
Yeah, came here to say the same thing. Tables + metatables mean that pretty much any structure you want is available. Single Inheritance + Interfaces, Multiple Inheritance, Component Based Design are all things I've seen in the wild on shipped products with Lua. It's a fantastic 'orchestration' language. Coroutines were also invaluable. We used them extensively to script AI behaviors. Our designers(yes, not developers) would write them with "go to X location", yield, "search for players", yield, etc. Was a really compact way to build a FSM that they could understand.

sulam 8y ago

Sorry, but tables are awful in practice. I have to iterate all the elements to get a count? This is especially egregious when I’m using one like a list/array and need to know how far to iterate. Maybe there’s a better way I don’t know?

Twisol 8y ago
If you need something quick-and-dirty, you can maintain an extra `["length"]` field on your table to cache that information. If you want something more polished, use a proxy table with custom methods/metamethods mediating access to your array so that the length is properly kept up to date. This isn't terribly different from the situation in C with arrays. Tables are a basic mechanism for implementing any data structure. That is a qualitatively different statement from saying that tables can replace any data structure. If you need semantics different from what tables immediately provide, you'll need to implement the desired data structure yourself -- but Lua gives you good tools to do that. > Maybe there’s a better way I don’t know? You can use `for i, v in ipairs(arr) do ... end` to iterate over an array without worrying about "how far" to iterate. The `ipairs` iterator will determine when it hits the end of the array.
- sulam 8y ago
  Yeah, this is fine until I want the last element of an array. Then I have to iterate it to find it. And writing a loop to get the last element of an array is crazy-town.
  - nikki93 8y ago
    Have you actually used Lua? You don't need to iterate to get the last element of an array.
    - sulam 8y ago
      I have! I’m not an expert at it, though.
gvx 8y ago
There's the length operator (https://www.lua.org/manual/5.3/manual.html#3.4.7 https://www.lua.org/manual/5.3/manual.html#3.4.7): #sometable returns the length of the sometable if it is a sequence, and has an upper bound of O(log(N)).

adwn 8y ago

This perceived simplicity of only having a single non-scalar datatype comes at the cost of increased complexity when you have to stuff a square peg into a round hole, i.e., when you approximate other data structures with tables. My favorite example are arrays in Lua: They are hashtables, but not quite (in the implementation, every hashtable has a separate array region). They look like hashtables, but not quite (there's syntactic sugar to help dealing with them). You can use hashtable-functions on them, but they don't quite work (which is why there are special functions for when you want to treat a hashtable as an array). Hashtables are great for unordered maps and sets, but they're a crappy replacement for ordered sequences. > Python is the classic example here, where you have tuples using parens, arrays using square brackets, and dictionaries using curly brackets. At some level that is clever, but at another level it is a cause of confusion. If that confuses you, then the source of the confusion is you not having a clear understanding of the intended semantics of your code or datastructure. Mixing all of that into a single type won't help here.

vvanders 8y ago

Yeah, came here to say the same thing. Tables + metatables mean that pretty much any structure you want is available. Single Inheritance + Interfaces, Multiple Inheritance, Component Based Design are all things I've seen in the wild on shipped products with Lua. It's a fantastic 'orchestration' language. Coroutines were also invaluable. We used them extensively to script AI behaviors. Our designers(yes, not developers) would write them with "go to X location", yield, "search for players", yield, etc. Was a really compact way to build a FSM that they could understand.