A better way to assign multiple return values to table keys in Lua

Question

Assume I have a function that returns multiple values. I happen to be working with LÖVE's (Image):getDimensions. This returns two values, which I know to be width,height. I want to assign them to a new table, as an array. I would like named (string) keys. So for instance, I would like to assign the return values of the getDimensions() function to a new table with keys width and height, respectively.

I know the following works...

image = {}
image.data = love.graphics.newImage('myimage.png')
image.size = {}
image.size.width, image.size.height = image.data:getDimensions()

I'm wondering if there is any sort of syntactic sugar I can use, or any use of standard library functions that will allow a syntax more along the lines of...

image.size = { width, height = image.data:getDimensions() }

I know the above line does not work, along with many variations I've tried, including various attempts to use unpack(). I'm new to Lua (~2 days in now), so maybe there is another standard function or best practice that I'm unaware of that will associate a table of keys to an array-like table. Thanks!

You could do image.size = table.pack(image.data:getDimensions()), but that would create image.size[1] and image.size[2], i.e. make image.size a table with integer keys. That is probably not what you want. — Marc Balmer
– Marc Balmer, Commented Jan 1, 2017 at 21:25
@MarcBalmer Thank you Marc, but that is what I'm trying to avoid. I want to be able to reference the elements by a specific name later, such as width and height. It seems I wouldn't even need to use table.pack(), as I could just wrap the function call in curly brackets (which might implicitly do the same thing as table.pack()), like so image.size = { image.data:getDimensions() }. I'm currently thinking what I actually want would be accomplished with a class-like construct using a helper function, and a __metatable with a definition for __newindex. — Eric Lease
– Eric Lease, Commented Jan 1, 2017 at 21:42

Egor Skriptunoff · Accepted Answer · 2017-01-02 15:21:37Z

4

You can write your own functions:

local function set_fields(tab, fields, ...)
   -- fields is an array of field names
   -- (use empty string to skip value at corresponging position)
   local values = {...}
   for k, field in ipairs(fields) do
      if field ~= "" then 
         tab[field] = values[k]
      end
   end
   return tab
end

local function get_fields(tab, fields)
   local values = {}
   for k, field in ipairs(fields) do
      values[k] = tab[field]
   end
   return (table.unpack or unpack)(values, 1, #fields)
end

Usage example #1:

image.size = set_fields({}, {"width", "height"}, image.data:getDimensions())

Usage example #2:
Swap the values on-the-fly!

local function get_weekdays()
   return "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"
end
-- we want to save returned values in different order
local weekdays = set_fields({}, {7,1,2,3,4,5,6}, get_weekdays())
-- now weekdays contains {"Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"}

Usage example #3:

local function get_coords_x_y_z()
   return 111, 222, 333  -- x, y, z of the point
end
-- we want to get the projection of the point on the ground plane
local projection = {y = 0}
-- projection.y will be preserved, projection.x and projection.z will be modified
set_fields(projection, {"x", "", "z"}, get_coords_x_y_z())
-- now projection contains {x = 111, y = 0, z = 333}

Usage example #4:
If require("some_module") returns a module with plenty of functions inside, but you need only a few of them:

local bnot, band, bor = get_fields(require("bit"), {"bnot", "band", "bor"})

edited Jan 2, 2017 at 15:21

answered Jan 1, 2017 at 21:51

Egor Skriptunoff

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1 Comment

Eric Lease Over a year ago

Thank you Egor. I had been considering this approach, and like it's general applicability.

Eric Lease · Accepted Answer · 2017-01-02 03:12:58Z

0

Using a class construct, I have come up with the following...

Size = {}    
Size.mt = {}
Size.prototype = { width = 0, height = 0 }

function Size.new (dimensions)
  local size = setmetatable({}, Size.mt)

  if dimensions ~= nil and type(dimensions) == 'table' then
    for k,v in pairs(dimensions) do    
      size[k] = v
    end
  end

  return size
end

Size.mt.__index = function (t, k)
  if k == 'width' or k == 'height' then
    rawset(t, k, Size.prototype[k])

    return t[k]
  else
    return nil
  end
end

Size.mt.__newindex = function (t, k, v)
  if k == 1 or k == 'width' then
    rawset(t, 'width', v)
  elseif k == 2 or k == 'height' then
    rawset(t, 'height', v)
  end
end

Then I can initialize a Size object in a number of ways

Using multiple return values:
- image.size = Size.new{image.data:getDimensions()}
- image.size = Size.new(table.pack(image.data:getDimensions())
Using default values:
- image.size = Size.new()
- image.size = Size.new{}
- image.size = Size.new({})
Using mixed array and hash tables:
- image.size = Size.new({height=20, width=30})
- image.size = Size.new({height=20, 30})

There are pros and cons to this approach vs. Egor's (utility function), which is what I was considering doing if there wasn't a simple syntax trick or an existing function that I was unaware of.

Pros:

(personal) learning experience with OO constructs in Lua
I can limit the number of actual keys on the table, while allowing 'synonyms' for those keys to be added by expanding the accepted values in the if/else logic of __index and __newindex
Explicit definition of fields in the table, without needing to worry about syncing a table of keys with a table of values (as with a general purpose utility function)

Cons

would need to repeat this pattern for each data structure where I wanted this behavior
costly, a lot of overhead for what amounts to a very small difference to the consumer

I'm sure I can make this approach more robust in time, but I would appreciate any feedback.

edited Jan 2, 2017 at 3:12

answered Jan 1, 2017 at 22:54

Eric Lease

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2 Comments

Egor Skriptunoff Over a year ago

This is OK: Size.new({height=20, 30}), but the opposite Size.new({width=30, 20}) is not OK.

Eric Lease Over a year ago

@EgorSkriptunoff good point. I could argue for it being desired behavior, or some additional logic along the lines of if rawget('width', 1) ~= nil could be added to if k == 1 ... in __newindex. This would then have to be extended to checking rawget('height', 2) when k == 2, and then continue to discard any additional array keys. This method doesn't scale very well with more desired fields in the table. Although I would argue it is poor use of the class, passing mixed tables becomes less user friendly as well I'm liking the general purpose utility functions more and more :)

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