Programming in Lua 4th edition. Chapter 6: Functions

Exercise 6.4.lua

-- Exercise 6.4: Write a function to shuffle a given list. Make sure that all permutations are equally probable. 
function q4(...)
  items = table.pack(...)
  itemsr = {}
  for i=1,#items do
    math.randomseed(os.time())
    pick = math.random(#items)
    print("Picked item:", pick)
    table.insert(itemsr,items[pick])
    table.remove(items, pick)
  end
  return table.unpack(itemsr)
end

Exercise 6.5.lua

-- Write a funcion that takes an array and prints all combinations of the elements in the array. 
-- (Hint: you can use recursive formula for combination: C(n,m)=C(n-1,m-1)+C(n-1,m). To generate
-- all C(n,m) combinations of n elements in groups of size m, you fist add the firs element to
-- the result and then generate all C(n-1,m-1) combinations of the remaining elements in the
-- remaining slots; then you remove the first element from the result and then generate all
-- C(n-1,m) combinations of the remaining elements in the free slots. When n is smaller than m,
-- there are no combinations. When m is zero, there is only one combination, which uses no
-- elements.)

--
-- User: valtoni
-- Date: 31/10/18
-- Time: 12:12
-- To change this template use File | Settings | File Templates.
--

function combs(items, x, y)
    local m, n = x, y
    if m == nil then
        m = #items - 1
    end
    if n == nil then
        n = #items
        print("FIRST LEVEL")
    end
    if m * n == 0 then
        print("EXITED (NO LEVEL ANYMORE)")
        return {{}}
    end
    local ret, old = {}, combs(items, m-1, n-1)
    for i = 1, n do
        for k, v in ipairs(old) do
            ret[#ret+1] = {i, map(incr(i), table.unpack(v))}
        end
    end

    retplain={{}}
    if x == nil then
        for i=1,#ret do
            item = ret[i]
            plain = {}
            for j=1,#item do
                table.insert(plain, items[item[j]])
            end
            table.insert(retplain, plain)
        end
        print("FIRST LEVEL")
        return retplain
    else
        print("LEVEL", x, y)
        return ret
    end
end

function map(f, a, ...)
    if a then
        return f(a), map(f, ...)
    end
end

function incr(k)
    return
    function(a)
        return k > a and a or a+1
    end
end

r=combs({"banana", "avocado", "pinneaple", "apple"})

for i=1,#r do
    print(table.unpack(r[i]))
end