Lua - Choose a random value from a range (or table) excluding the values of a (or another) table

Question

A range, 1, 2, 3, 4, 5, 6, 7, 8 (it can populate a Lua table if it makes it easier)
table = {1, 4, 3}

The possible random choice should be among 2, 5, 6, 7, 8.

In Python I have used this to get it:

possibleChoices = random.choice([i for i in range(9) if i not in table])

Any ideas how to achieve the same in Lua?

Answer 1

Lua has a very minimal library, so you will have to write your own functions to do some tasks that are automatically provided in many other languages.

A good way to go about this is to write small functions that solve part of your problem, and to incorporate those into a final solution. Here it would be nice to have a range of numbers, with certain of those numbers excluded, from which to randomly draw a number. A range can be obtained by using a range function:

-- Returns a sequence containing the range [a, b].
function range (a, b)
  local r = {}
  for i = a, b do
    r[#r + 1] = i
  end
  return r
end

To get a sequence with some numbers excluded, a seq_diff function can be written; this version makes use of a member function:

-- Returns true if x is a value in the table t.
function member (x, t)
  for k, v in pairs(t) do
    if v == x then
      return true
    end
  end
  return false
end

-- Returns the sequence u - v.
function seq_diff (u, v)
  local result = {}
  for _, x in ipairs(u) do
    if not member(x, v) then
      result[#result + 1] = x
    end
  end
  return result
end

Finally these smaller functions can be combined into a solution:

-- Returns a random number from the range [a, b],
-- excluding numbers in the sequence seq.
function random_from_diff_range (a, b, seq)
  local selections = seq_diff(range(a, b), seq)
  return selections[math.random(#selections)]
end

Sample interaction:

> for i = 1, 20 do
>> print(random_from_diff_range(1, 8, {1, 4, 3}))
>> end
8
6
8
5
5
8
6
7
8
5
2
5
5
7
2
8
7
2
6
5