无状态迭代器

无论对与 ipairs代表无状态的迭代器。无状态迭代器仅使用泛型 for 循环的控制变量和不变状态来计算迭代值。

对迭代器

我们可以使用 next 函数实现无状态 pairs 迭代器。

-- generator function which initializes the generic for loop
local function pairs(t)
  -- next is the iterator function
  -- t is the invariant state
  -- control variable is nil
  return next, t, nil
end

Ipairs 迭代器

我们可以在两个独立的函数中实现无状态 ipairs 迭代器。

-- function which performs the actual iteration
local function ipairs_iter(t, i)
  local i = i + 1  -- next index in the sequence (i is the control variable)
  local v = t[i]   -- next value (t is the invariant state)
  if v ~= nil then
    return i, v    -- index, value
  end
  return nil       -- no more values (termination)
end

-- generator function which initializes the generic for loop
local function ipairs(t)
  -- ipairs_iter is the iterator function
  -- t is the invariant state (table to be iterated)
  -- 0 is the control variable (first index)
  return ipairs_iter, t, 0
end

角色迭代器

我们可以通过履行通用 for 循环的契约来创建新的无状态迭代器。

-- function which performs the actual iteration
local function chars_iter(s, i)
  if i < #s then
    i = i + 1
    return i, s:sub(i, i)
  end
end

-- generator function which initializes the generic for loop
local function chars(s)
  return chars_iter, s, 0
end

-- used like pairs and ipairs
for i, c in chars 'abcde' do
    print(i, c) --> 1 a, 2 b, 3 c, 4 f, 5 e
end

Prime Number 迭代器

这是无状态迭代器的一个更简单的例子。

-- prime numbers iterator
local incr = {4, 1, 2, 0, 2}
function primes(s, p, d)
   s, p, d = s or math.huge, p and p + incr[p % 6] or 2, 1
   while p <= s do
      repeat
         d = d + incr[d % 6]
         if d*d > p then return p end
      until p % d == 0
      p, d = p + incr[p % 6], 1
   end
end

-- print all prime numbers <= 100
for p in primes, 100 do  -- passing in the iterator (do not call the iterator here)
   print(p)  -->  2  3  5  7  11 ... 97
end

-- print all primes in endless loop
for p in primes do  -- please note: "in primes", not "in primes()"
   print(p)
end