openlang
Language design and syntax

lua: collecting a table’s keys into another table

The conversation continues 2 ↓

I'd like to find out how to obtain all the keys from a lua table. For instance, suppose this is my table:

entries = {}
entries[2] = 'c'
entries[4] = 'd'
entries[7] = 'h'

What I want back is a table resembling this one:

indices = {2, 4, 7}

The conversation continues 2

Follow the reasoning
  1. local indices={}
local count=0
for index, value in pairs(entries) do
  count=count+1
  indices[count]=index
end

    Note that indices has no guaranteed ordering. To obtain the keys sorted, apply a sort to indices using table.sort(indices).

  2. local function collect_indices(source)
  local result={}
  for index,_ in pairs(source) do
    table.insert(result, index)
  end
  return result
end

Other sides of the same question

  1. Language design and syntax

    Choosing enum entries randomly with GDScript in Godot 3.3 without listing each one?

    Here's the enum declaration I'm using in GDScript: I'd like to choose an enum variant randomly without listing every possible…

    Replies 2gdscript
  2. Language design and syntax

    Nim remainder syntax

    How is modulus expressed in Nim? cell % 7 == 0 gives undeclared identifier: '%' Neither a Google search nor looking through S…

    Replies 2nim
  3. Language design and syntax

    Returning results from an APL function

    How do I make my function return something once execution has finished? For instance, I have: ROUTINE N ? N ⍴ 12 //That gener…

    Replies 2apl