如何获取Lua中哈希表中的键数？

Question

myTable = {}
myTable["foo"] = 12
myTable["bar"] = "blah"
print(#myTable) -- this prints 0

我是否真的必须遍历表中的项目才能获得密钥数量？

numItems = 0
for k,v in pairs(myTable) do
    numItems = numItems + 1
end
print(numItems) -- this prints 2

Answer 1

我尝试了#运算符和table.getn().我认为table.getn()可以做你想要的,但事实证明它返回与#相同的值,即0.看起来字典会根据需要插入nil占位符.

循环键并计算它们似乎是获得字典大小的唯一方法.

Answer 2

表t的长度被定义为任何整数索引n,使得t [n]不是nil且t [n + 1]是nil; 此外,如果t [1]为零,则n可以为零.对于常规数组,非n值从1到给定n,其长度恰好是n,即其最后一个值的索引.如果数组具有"空洞"(即,其他非零值之间的nil值),那么#t可以是直接在nil值之前的任何索引(也就是说,它可以将任何这样的nil值视为结束的数组).因此,获取长度的唯一方法是迭代它.

Answer 3

除了手动迭代键之外，通过元方法自动跟踪它也很简单。考虑到您可能不想跟踪您创建的每个表，您可以编写一个函数来将任何表转换为键可数对象。以下内容并不完美，但我认为它可以说明这一点：

function CountedTable(x)
  assert(type(x) == 'table', 'bad parameter #1: must be table')

  local mt = {}
  -- `keys`  will represent the number of non integral indexes
  -- `indxs` will represent the number of integral indexes
  -- `all`   will represent the number of both 
  local keys, indxs, all = 0, 0, 0

  -- Do an initial count of current assets in table. 
  for k, v in pairs(x) do
    if (type(k) == 'number') and (k == math.floor(k)) then indxs = indxs + 1
    else keys = keys + 1 end

    all = all + 1
  end

  -- By using `__nexindex`, any time a new key is added, it will automatically be
  -- tracked.
  mt.__newindex = function(t, k, v)
    if (type(k) == 'number') and (k == math.floor(k)) then indxs = indxs + 1
    else keys = keys + 1 end

    all = all + 1
    t[k] = v
  end

  -- This allows us to have fields to access these datacounts, but won't count as
  -- actual keys or indexes.
  mt.__index = function(t, k)
    if k == 'keyCount' then return keys 
    elseif k == 'indexCount' then return indxs 
    elseif k == 'totalCount' then return all end
  end

  return setmetatable(x, mt)
end

使用此功能的示例包括：

-- Note `36.35433` would NOT be counted as an integral index.
local foo = CountedTable { 1, 2, 3, 4, [36.35433] = 36.35433, [54] = 54 }
local bar = CountedTable { x = 23, y = 43, z = 334, [true] = true }
local foobar = CountedTable { 1, 2, 3, x = 'x', [true] = true, [64] = 64 }

print(foo.indexCount)    --> 5
print(bar.keyCount)      --> 4
print(foobar.totalCount) --> 6

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希望这有帮助！:)